3rd Workshop on Petroleum Geomechanics Testing

Core Acquisition, Handling and Sampling to

Ensure Quality Tests

The Commons Hotel,

University of Minnesota, Minneapolis, USA

May 31, 2014

Organized by the ISRM Petroleum Geomechanics Commission and the Society of Core Analysts

Russ Ewy, John Shafer, Tony Addis, Maurice Dusseault (PGC Chair)

And by the American Rock Mechanics Association

Joe Labuz (Symposium Chair), Antonio Bobet (President), Peter Smeallie (Executive Director)

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PRESENTATIONS and Additional Contents

CORE ACQUISITION, HANDLING AND PRESERVATION

The value of core – Coring technology Lee Whitebay, Whitebay & Associates

Core damage – Mitigation and evaluation Tom Fate, Chevron ETC

Core acquisition, handling and preservation Mike Gay, ExxonMobil URC

SHALE HANDLING AND PRESERVATION, AND UNCONVENTIONALS

Shale handling, preservation and sampling Russ Ewy, Chevron ETC

Geomechanical characterization and thermo-hydro-mechanical testing of shales

Alessio Ferrari, Valentina Favero, Alberto Minardi, Lyesse Laloui, EPFL

Recent experience in coring and core characterization for unconventional resources Jon Wallace, Hess, Matt Honarpour, Hess (retired)

Shale characterization and test protocols Jørn Stenebråten, Sintef

SIDEWALL CORES

Rock mechanics testing of rotary sidewall core: Application to reservoir description Eva Krauss, Core Laboratories

DEALING WITH CORE AND SAMPLE DAMAGE

Consequences of core damage Sander Hol, Shell

Geomechanical test protocol stages to address sample-to-sample variation assessment John Dudley, Shell

Use of synthetic rocks for quantification of coring induced rock alteration Rune Holt, Sintef / NTNU

Wave velocities in stressed shales and sands: How can we obtain representative measurements in the laboratory?

Rune Holt, Erling Fjaer, Andreas Bauer, Jørn Stenebråten, Sintef / NTNU

QUANTIFYING HETEROGENEITY ON CORE

The scratch test: A high resolution log of rock strength with application to geomechanic and petrophysic

Christophe Germay, T. Lhomme, T. Richard, EPSLOG SA

Rapid heterogeneity assessment using the impulse hammer

Andy Rathbun, Steve Carlson, Russ Ewy, Paul Hagin, Craig Bovberg, Chevron ETC, Greg Boitnott, New England Research

Enhanced rock strength profiling, combining triaxial compressive strength, non-destructive core scratching and index tests

Abbas Khaksar, Feng Gui, Baker Hughes, Christophe Germay, Thomas Richard, EPSLOG SA

Using continuous profiles of core properties to map heterogeneity and improve geologic core descriptions

Robert Suarez-Rivera, Schlumberger-TerraTek

The Value of Core – Coring Technology

Lee Whitebay

Principal Consultant

Whitebay & Associates, LLC

Core is cut to acquire basic and advanced engineering and geologic data. The data may be used to predict the outcome of a treatment such as fracturing or acidizing, and it is routinely used to forecast the value of assets. The challenge facing analysts and end-users lies in ensuring the samples measured truly represent the in-situ formations. The one constant is that every coring and core analysis program will be different and must be approached with and open mind.

Coring is an expensive operation, so it is essential that all parties requesting data participate in the planning process. The first step is to identify the stakeholders, geologists, engineers, scientists, drillers, and service providers. Then goals need to be set and prioritized. The logistics of the drilling operation need to be explored. Can the core handling materials requested by the laboratory be stored and used on location? Will measurements be made on-site? How will the core be transported to the Laboratory? You get the idea. There is a lot to be discussed and agreed upon before reaching core point. Open and clear communications are essential, and information must be directed to the correct people at the right time. Once there is a plan it is important to document all changes. Plans may call for the perfect coring fluid, but drillers need the latitude to alter mud to drill safely. Those changes need to be known to the laboratory, so it can be determined if any analytical alterations are needed.

So, starting with the coring process it is important to understand how coring tools work and what happens when a core is cut. All coring systems with the exception of sidewall cores are variations of the double tube conventional core barrel. The inner barrel is suspended on a bearing assembly while the outer barrel and core bit are rotated. Normally, we rotate the entire string from the surface, but coring can be conducted with a downhole motor. The drilling fluid of our choice is pumped down the drill string flowing between the inner and outer barrels and over the face of the bit, carrying away cuttings. Over the decades coring service providers have offered a wide array of disposable inner core barrels and liners to simplify core handling and preservation tasks. They developed barrels with full-closure core catchers for unconsolidated material and sponge lined systems to trap expulsed oil. The acceptance of wire-line coring systems by the oil industry was a long time in coming, but they are now well received. Where a conventional barrel must be brought to the surface to recover core a wire-line barrel stays on bottom, and only the full inner barrel is retrieved to the surface. Wire-line coring is fast and efficient; however the core diameter and length per barrel are less than most conventional cores. A recent addition to wire-line technology has been the development of a pseudo pressure core barrel. The new system traps gas within a chamber in the tool as the inner barrel is brought to the surface. Combining this data with gas desorption measurements, on the surface, provide a clearer understanding of tight gas reservoirs. For those not working on extremely tight reservoirs it is important to understand the concept of “flushing ahead of the bit”. Research in the 1990’s

conclusively showed that drilling fluid filtrate invaded below and ahead of the core bit. This means that every core is going to experience some amount of flushing with mud filtrate. The faster the core is cut the less invasion takes place, but it is next to impossible to eliminate filtrate invasion. This brings us back to the design of the coring fluid. Choose a fluid compatible with your analytical needs and add a tracer if you wish to quantify filtrate invasion. The other fact of note is that all core will experience some amount of anelastic strain recovery. The act of cutting the core drastically changes the stress state on that rock, and just like the North American Shield rebounding after glaciation, your core is going to seek a new equilibrium state. Common expressions of stress relief are petal-centerline fractures and disking. More subtle changes have been detected by instrumenting fresh core and recording the rock expansion. Sonic velocity measurements have been used to detect oriented micro-cracks in whole core. For the most part the oil and gas industry ignore these effects. Loading samples to in-situ conditions closes micro-cracks and yields accurate porosity and permeability values. However, since rock strength is largely controlled by discontinuities within the rock mass, potential ASR effects should be evaluated prior to rock strength testing. Understanding the coring process and knowing what sample properties are essential for your analytical program will lead to choosing the most appropriate coring and core handling procedures for your project.

Core Damage- Mitigation and Evaluation

48th US Rock Mechanics Symposium 3rd Workshop on Petroleum Geomechanics Testing

Tom Fate, Chevron, ETC, Houston, Texas Core Damage- Mitigation and Evaluation Geomechanical test result quality is dependent on the quality of the core being tested. Core material that is fractured (micro or macro) can impact the test result quality. The core condition can reduce the size and amount of samples available for testing. There are many ways the core can be damaged during coring, processing and transportation to the lab. The core can be damaged while it is being acquired due to the application of excess weight on bit, fluid invasion (filtrate or whole mud) and disruption in the inner barrel as the core column length increases. The core can be damaged while tripping the core barrel out of the hole due to excess and rapid gas expansion or stress relief. Once on the surface the core is subject to damage due to the laydown process and processing procedures. To prevent damage to the cores the coring program needs to be well designed and implemented. The proper coring equipment is required, including core barrel type and length, bit selection and coring parameters. The type of coring equipment needs to be determined, wireline cores are popular due to the time savings on the rig, but the cores are usually smaller than conventional cores and they are usually brought to the surface rapidly. The implications of wireline coring should be evaluated prior to designing the coring program. All involved parties to the core analysis should be involved with the core program development to ensure a good quality core is delivered to the lab. Once in the lab the core is usually cat scanned in 3 foot sections, these cat scans can be used to evaluate the core condition. There are many attributes seen in the cat scans that can help understand the cause of the damage and indicate methods to prevent such damage in future cores.

Core Acquisition, Handling and Preservation

48th US Rock Mechanics Symposium 3rd Workshop on Petroleum Geomechanics Testing

Mike Gay, ExxonMobil Upstream Research Co. Core Acquisition and Handling The two factors that will define the tools, handling and stabilization/preservation for the coring program are the lithology or type of rock being cored and the type of core analysis tests to be performed. These influence the coring objectives. Once identified, we are able to select the necessary coring fluids, tools, core handling equipment and the safest (for core integrity) transportation system to deliver the core to the laboratory. Throughout the program, personnel safety is the primary and highest objective. The starting point is the decision to collect core material from the well. There is usually an objective for the core that could span several different disciplines such as reservoir evaluation, drilling and completions, geomechanics and many others. Knowing the uses for the core from these groups is necessary to adequately design the coring program so that full use of the core can be realized. Once we know the objectives we can start defining the optimum coring fluids, tools, handling and transportation for the program. With the analytical objectives in mind we would determine the proper stabilization and/or preservation methods to employ to ensure the core remains intact and unaltered from the well until it is finally sampled in the laboratory, and possibly beyond. Different lithologies and rock types, consolidated, unconsolidated, fractured, laminated, etc., will somewhat dictate the stabilization route. The level of core analysis, geomechanical tests, routine core analysis or special core analysis, will also have a large bearing on the preservation method used. Coring tools are chosen based on the lithology and type of rock being cored, the diameter of wellbore, desired diameter of the core and the coring company. The coring tools may be conventionally run on drill pipe or using a wireline system. These include the coring bits, core catchers, core barrels (inner and outer) and other core handling equipment (lay-down cradles, saws, etc.). PDC core bits are available for almost all rock types, reserving natural and man-made diamond for the hardest or most brittle rocks. Core catchers should be chosen based on the type of rock and how well it can maintain its shape after being cut. The inner barrel material, Aluminum, fiberglass, PVC or steel, must meet the downhole conditions for temperature and gas/fluid composition (H2S?). The length of the inner barrel string is dependent on the type of core. Unconsolidated core may not have the compressive strength to support the weight of greater than 30 feet or less. Coring parameters should be tailored to meet the specific lithology and rock type. Generally speaking, the lower the weight on bit required to achieve a reasonable rate of penetration, the better the quality of the core. Coring fluid rates should be consistent with and a balance between the rock type and necessary cuttings removal. Coring fluids should be tailored to the specific lithology, rock type and core analysis program. They should be optimized to meet the coring objectives, i.e. preserving wettability or reducing filtrate invasion. There are special considerations to take into account when coring is for special core analysis projects. Shale and claystone rocks present special needs as well. ‘Trip Out of the Hole’ rates should be designed to allow the core to degas in a non-disruptive manner. Gas coming out of solution can cause both consolidated and unconsolidated rocks to separate and create breaks and gaps. The critical time to avoid a rapid tripping rate is just below and above the gas bubble point depth in the well.

The use of lay-down cradles is of utmost importance when removing the core from the drill floor to the processing area. Even though the inner barrels are strong and full of rock, over the length of the barrel a flexing of the barrel will occur. Failure to use a lay-down cradle will result in core damage due to flexing or bending of the inner core barrel and rock. Whether the coring company is providing the handling and processing or there is a core laboratory company providing the service, there should be a seamless but definite hand-off of the core handling tasks. The coring company engineers should maintain responsibility of the coring tools in the rig critical path and concentrate on make up or break down of the coring tools. Core handling and processing should be done by another dedicated team so that neither rig time nor core quality are impacted. Core handling and processing, whether a simple stabilization packaging, a full blown SCAL preservation or any level in-between, should be executed in an expeditious and safe operation. Once the inner core barrel is located in the processing area it should be marked with orientation striping (red on the right (looking from deep to shallow), black (or other color) on the left, core ID’s, core depths. More detailed markings of 1 foot or 1 meter could be done at this time as well. Some inner barrels allow the removal of the top half to expose the core for photos, descriptions or sampling. Other inspections may be performed at this time, i.e. Spectral or Total core gamma. The long barrel can be segmented into 3 foot, 1 meter or other lengths and stabilized and/or preserved using one of many different procedures specific for the lithology and rock type (consolidated, unconsolidated, fractured, shale or other). Once the core is stabilized and preserved, it should be put into multi-section core shipping containers. These containers must have shock absorbing and motion limiting insulations or liners (i.e. PE Foam sheets) to prevent shipping damage from occurring. The inclusion of shock sensors, tilt sensors and temperature loggers will help reveal potential damage to the core. New coring tools are now available to recover core at a reduced reservoir pressure but with all of the gas and fluids intact. These tools may be run either on wireline or conventionally on drill pipe. Core sizes range from 3” to 4”. Total gas volumes can be measured on the surface with more robust analyses possible, including gas composition and fluid analyses. Core Preservation & Sampling Once the cores are out of the ground they should be handled with care. The core should be in as close to in-situ shape as possible. Once it has had preliminary treatment in the processing area it is ready to be packaged, containerized and shipped to the laboratory or storage facility. At the minimum, the core should be stabilized inside some container to prevent mechanical damage to occur during shipment. This could be as simple as laying in a cardboard box with a sheet of foam rubber or as robust as inserting shims between the inside of the inner core barrel and the outside of the core to prevent rolling. This type of stabilization will work for consolidated core where basic routine core analysis or some types of geomechanical tests will be the end product. For core that is of other rock types or will be the subject of water saturation, SCAL testing or shale/claystone geomechanical tests, stabilization will be included in a preservation process. The preservation objective is to further limit the exposure of the core to environmental variables which would alter the natural properties and characteristics of the rock, including fluid saturations, shape and wettability. In cases where there will be an extended time between acquisition (for core or plugs) and analysis, it is very important to identify the correct preservation system to use to prevent the decay of initial properties. Certain lithologies, i.e. shale or claystone, will require preservation techniques using specific processes in order to prevent damage prior to sampling. Other reasons for preservation would be the complete immobility of the core inside the container when shipping very long distances or over rugged terrain. Several types of preservation processes are available and many may be used across different lithologies and rock types. The type of preservation done should be chosen with the rock, transportation time, and

level of analyses in mind. Table 1 lists stabilization and preservation systems. This list is representative of the types of processes available but not exhaustive. Following stabilization and preservation of the core, it should be loaded into multi-section core shipping containers. These containers should have ample packing material to isolate the core sections and prevent them from rolling around inside the container. Shipping indicators is often included inside the shipping containers to alert the lab of possible mishandling during shipment.

There are several different types of well site analyses that can be performed prior to packaging the core. These would include the acquisition of core plugs for various reasons, i.e. water saturation, mud tracer invasion, special preservation for later core analysis tests, etc. Additionally the tests could include photography, core gamma, core description, scratch testing, etc.

In summary, the entire coring program should be based on the lithology and type of rock that is being cored and the level of testing that the core will receive. When properly planned, the types of coring tools to be used, the type of stabilization and/or preservation used and the method of coring (wireline or drill string) will result in the successful completion of the core acquisition, stabilization and preservation and the transportation of the best quality core to be used for the desired core analysis program.

Table 1 Stabilization and Preservation Systems

Stabilization Preservation / Stabilization Basic Core Containerized Core Basic Core

Foam rubber sheets Epoxy Resin Cardboard/Plastic Box Rags or paper packing Expanding Foam Submerged in fluid PVC or plastic rods/wedges Heat Seal Bags Plastic Wax Dipped Bubble wrap Gypsum Freezing / Chilling

Shale Handling, Preservation and Sampling

Russ Ewy

Chevron Energy Technology Co.

3rd Workshop on Petroleum Geomechanics Testing, 31 May 2014, Minneapolis

Proper shale (claystone) handling and preservation can be summed up by one overriding goal: Do not allow the native water content to increase, or to decrease, unless you are doing so on purpose and under controlled conditions. Mechanical handling damage to shales can occur, but by far the most common and damaging mistake is to let the shale dry out, or to inadvertently over-hydrate it. The RULE OF THUMB is pretty simple; minimize exposure to air, and do not contact with water or brine unless the shale is under stress.

The preferred coring fluid for shales is any non-aqueous (non-polar) fluid. If this is not possible, one should choose a fluid that will minimize swelling, such as a KCl fluid or a high-concentration NaCl fluid. Our studies on various shales show that CaCl2 fluids will cause most shales to swell, as will a low-concentration NaCl fluid [6, 7]. Some have reported shale shrinkage from KCl fluids; we have only seen this for one shale [6]. Note that KCl is likely to change the dominant cation adsorbed on smectites to potassium [7]. If one wishes to avoid this then the preferred fluid (other than non-aqueous) would be high-concentration NaCl for most shales. A non-aqueous coring fluid is always preferred over any aqueous coring fluid. A non-aqueous coring fluid with an emulsified internal brine phase must have 1) stable emulsion under all downhole and surface conditions and 2) sufficient salt in the internal brine phase to prevent osmotic transfer of water to the shale.

Once on surface the mud should be drained from the core barrel, whether or not it is an aqueous or non-aqueous fluid. The urgency is less with a non-aqueous fluid. One of the best ways to keep a shale preserved (native water content) and at the same time avoid mechanical damage is to then fill the annulus around the core, while still in the inner core barrel, with some sort of resin or expanding foam. None of the common resins/foams seem to damage the water content of the shale. If the core must be removed from the barrel then it should be wiped free of mud and then immediately wrapped and dipped, or sealed in some completely airtight wrapper with minimal contained air. Wax (or similar) dip also provides some degree of mechanical support for shipping, although careful packaging is still required. Slabbing of shales should be avoided if at all possible, because it usually leads to excessive air exposure. Under no circumstances should a slabbed shale be sprayed with water (or acid!).

The RULE OF THUMB is pretty simple; minimize exposure to air, and do not contact with water or brine unless the shale is under stress. Thus, contact with a hydrocarbon while handling shale core is not a problem. Filling the core barrel with hydrocarbon and sealing the end caps does keep the internal water content constant; however, the core usually rattles around in the barrel when shipping, which can result in mechanical damage. Shales should never be frozen during storage or transport, or to stabilize for plugging.

The RULE OF THUMB applies in the lab when the core is opened for sampling. It is good practice to keep sections not being worked on completely sealed up airtight, or sitting completely under hydrocarbon in a sealed container. The container must be sealed. Water vapor can move back and forth through the hydrocarbon, between the shale core and the air. A piece of shale core that is being sampled should be kept wet with hydrocarbon. This minimizes water transfer between the shale and the surrounding air. Hydrocarbon should be used for the plugging fluid. Air can be used if necessary but it will tend to dry out the samples. The RULE OF THUMB calls for air exposure to be minimized. The RULE OF THUMB also clearly disallows use of water or brine. Many hydrocarbons are acceptable; decane has the advantage that it quickly evaporates off the samples and also leaves no film or residue. Some hydrocarbons leave a residue after evaporation. A well-preserved shale core will be fully-saturated [3, 7], and therefore it is very unlikely that hydrocarbon will enter the shale pore space. If some invasion does occur, the hydrocarbon can be evaporated off in a controlled manner without changing the water content (more on this later). The presence of a hydrocarbon residue can cause issues with certain tests.

The RULE OF THUMB applies to all samples cut from a shale core. They should be stored in one of three possible ways [1, 3, 4]: 1) under hydrocarbon in a sealed container, 2) completely sealed up in an airtight manner with only a very small volume of air, or 3) in a vacuum desiccator of controlled relative humidity (RH). Shale samples should not be stored in any brine, not even a simulated pore fluid. If they are not under stress, they will swell when exposed to a simulated pore fluid [6, 7].

The RULE OF THUMB is pretty simple; minimize exposure to air, and do not contact with water or brine unless the shale is under stress. So, why is it acceptable to put a shale in a vacuum desiccator with controlled RH? It is OK because it is not being exposed to air of uncontrolled or unknown RH. The water content of any particular shale, in contact only with air, is controlled by RH. If a shale is placed in a RH higher than its ‘native state’ then it will gain water; if it is placed in a RH lower than its native state then it will lose water [7]. The advantages of a vacuum desiccator of controlled RH is that water moves into or out of the shale only by vapor phase, air will not get trapped in the shale, and any hydrocarbon on the sample will evaporate [1, 4]. A vacuum desiccator can be used to maintain shale water content. Also, it is the only known means of increasing or decreasing the water content of a shale without causing damage. Even then, significant changes of water content, either up or down, relative to the native state can result in irreversible changes to the shale. Drying a shale is generally more damaging than controlled hydration (vacuum desiccator) of a preserved shale.

Why does a shale swell if it is contacted by simulated pore fluid? Downhole, before it was cored, the activity (free energy) of the pore water is being influenced not only by its salinity but also by the nature of the water that is in between clay interlayers. The interlayer water is usually only a small fraction of a shale’s total pore water, but it usually has a low activity (low escaping tendency) because it is tightly held by clay surfaces, and also due to ion concentrations in the interlayer water. This leads to an overall activity of shale pore water that is much lower than the activity of the brine in the main shale pores. In-situ, for a shale to gain water it needs to pull water from neighboring shale, which wants the water just as badly. If a shale is placed in simulated pore fluid then it has access to all the water it wants – it does not have to pull the water from neighboring shale. This is one explanation for shale swelling due to brine contact. There is one additional contributor which occurs in many situations in the lab, which is the release of capillary tension [7].

The pore water in a shale (even a fully-preserved shale), under zero confining stress, is almost always in a state of tension [3, 7]. While this does not fully explain the low escaping tendencies (low RH) sometimes measured on native-state shales, it is a significant component in many cases [3, 7]. Shales slightly expand when they are cored and brought to surface. Due to the extremely low permeability, the fluid content does not change even though the shale pore space expands. The amount of pore pressure change can be estimated using the theory of undrained poroelasticity [7]. This reveals the presence of negative pore water pressure even in fully-preserved, fully-saturated shales; negative pore water pressure has also been confirmed experimentally [3].

Contacting a shale with water or brine releases the capillary tension [7]. For a shale that is not fully-saturated, it is even worse. The liquid entering the shale traps air in the shale, compresses the air, and essentially ‘blows the shale apart’. Tests should never be performed on unpreserved shale samples, as there are many possible artefacts due to partial saturation. Unstressed shale samples, even if fully-preserved, should always be considered to have negative pore pressure within them.

Measurements of shale swelling vs. confining stress reveal that confining stress can inhibit swelling [6]. Part of this is probably due to removing the capillary tension, but it is also true that stress prevents the clays from expanding. Measurements show that, when an unstressed shale swells, much of the swelling occurs between clay layers [7].

Some lab tests require that a shale be contacted with brine. For such tests, it is always best to have the shale under stress before brine contact. The choice of brine can affect how much stress is required to avoid swelling. The choice of brine also affects the likelihood of developing osmotic pressures [2, 6]. This is important to consider when using brine as an external pore fluid, contacting the shale pore space. Lab tests clearly show that the hydraulic pressure in a brine external to a shale can be significantly different than the hydraulic pressure within the shale pore space [2, 6]. Therefore, the shale may have a pore pressure different than assumed. To minimize such osmotic pressure effects, the brine should be chosen to have a water activity that closely matches the activity of the shale pore water (when the shale is under stress).

Brines can also alter shale strength [5]. Significant exposure is probably required for this to occur, and it is also possible that significant weakening requires significant swelling (water content increase) to occur. If a fully-saturated shale (under stress) is contacted only at one end with an external brine pore fluid in order to aid in the measurement of pore pressure, the strength alteration would hopefully be minimal.

Proper shale (claystone) handling and preservation can be summed up by one overriding goal: Do not allow the native water content to increase, or to decrease, unless you are doing so on purpose and under controlled conditions. The RULE OF THUMB is pretty simple; minimize exposure to air, and do not contact with water or brine unless the shale is under stress.

REFERENCES

1. Steiger, R.P. and Leung, P.K. 1991. Consolidated undrained triaxial test procedure for shales. Proc., 32nd US Rock Mechanics Symposium–Rock Mechanics as a Multidisciplinary Science, Norman, Oklahoma, USA, 10–12 July, 637–646.

2. Ewy, R.T. and Stankovich, R.J. 2000. Pore pressure change due to shale-fluid interaction: Measurements under simulated wellbore conditions. Proc., Pacific Rocks 2000, 4th North American Rock Mechanics Symposium, Seattle, Washington, USA, 31 July—3 August, 147–154.

3. Ewy, R.T., Daniels, E.J., and Stankovich, R.J. 2001. Behavior of a reactive shale from 12000 feet depth. In Rock Mechanics in the National Interest: Proceedings of the 38th US Rock Mechanics Symposium, 77–84. Rotterdam, The Netherlands: A.A. Balkema.

4. Ewy, R.T., R.J. Stankovich, and C.A. Bovberg 2003. Mechanical behavior of some clays and shales from 200m to 3800m depth. In Proceedings ‘Soil Rock America’ 39th U.S. Rock Mechanics Symp. / 12th Panamerican Conf. on Soil Mech. & Geotech. Eng., Cambridge, MA, 22-26 June 2003, eds. P. Culligan, H. Einstein and A. Whittle, 445-452 (paper 570). Essen: Verlag Gluckauf.

5. Ewy, R.T., Bovberg, C.A., and Stankovich, R.J. 2008. Shale triaxial strength alteration due to brine exposure. Paper ARMA 08-304 presented at San Francisco 2008, 42nd U.S. Rock Mechanics Symposium, 2nd U.S.-Canada Rock Mechanics Symposium, San Francisco, California, USA, 29 June–2 July.

6. Ewy R.T., Stankovic R.J. 2010. Shale swelling, osmosis and acoustic changes measured under simulated downhole conditions. SPE Drilling & Completions 25:177-186.

7. Ewy, R.T. 2014. Shale swelling/shrinkage and water content change due to imposed suction and due to direct brine contact. Acta Geotechnica, in press (available online), DOI 10.1007/s11440-013-0297-5.

Petroleum Geomechanics Testing Workshop, 31 May 2014, Minneapolis

Geomechanical characterization and Thermo-Hydro-Mechanical

testing of shales

Alessio Ferrari, Valentina Favero, Alberto Minardi, Lyesse Laloui

Ecole Polytechnique Fédérale de Lausanne (EPFL), School of Architecture, Civil and Environmental Engineering (ENAC), Laboratory for Soil Mechanics (LMS), EPFL-ENAC-

LMS, Station 18, CH-1015 Lausanne, Switzerland. alessio.ferrari@epfl.ch

Shales are among the most considered geomaterials in current energy-related geomechanical investigations. Due to the peculiar hydro-mechanical features that characterize shales respect to other geomaterials, a good preservation and handling of the shale cores are essential requirements for a sounded experimental characterization of the materials. The presentation addresses the workflow and the experimental procedures which have been set up at the EPFL for a systematic identification and characterization of shale cores in different saturation conditions. The following issues are discussed: (i) the geotechnical identification procedures, (ii) the consequences of the partial saturation on the test results and (iii) the sample preparation and the selection of the stress paths for hydro-mechanical testing.

Geotechnical characterization includes the definition of the index properties of the material, such as particle density, bulk density, water content, Atterberg limits, grain size distribution, pore size distribution and suction measurement. Well established protocols for soil testing have been adapted with specific reference to shale testing [1].

Changes in shale water content can have strong influence on the hydro-mechanical behaviour of the considered material. There are two important consequences that must be taken into account: the significance of considering the right water content when assessing material parameters especially for field application and the impact of sample preservation for laboratory testing. A complete procedure is presented to assess the water retention behaviour, relating the changes in water content with the variation of pore water potential and the consequent changes in porosity [2]. The set-ups developed at the EPFL for thermo-hydro-mechanical (THM) testing are presented, focusing on the possible stress paths for the assessment of the coupled THM behaviours of shales. The set-ups include controlled suction and temperature oedometric and triaxial cells. A regular shape of the specimen plays a key role in order to achieve a good contact between the sample and the confining ring in 1D compression test. An ad-hoc procedure has been developed for specimen preparation concerning hydro-mechanical testing. The issue related to the change in

the degree of saturation during the specimen preparation must be carefully taken into account when dealing with shales, especially in the context of unsaturated shale cores testing; such variation in the degree of saturation should be carefully assessed, minimized and when possible corrected. Results showing the importance of water potential and the influence of partial saturation on the mechanical behaviour of shales are presented. Finally a procedure is presented which aims at back calculating the in-situ porosity by the analysis and combination of the hydro-mechanical test results and the information gathered on the water retention behaviour. References [1] Ferrari A. and Laloui L. (2012). Advances in the Testing of the Hydro-mechanical Behaviour of Shales, In: Multiphysical Testing of Soils and Shales, Springer Series in Geomechanics and Geoengineering, p. 57-68. [2] Ferrari A., Favero V., Marschall P. and Laloui L. An insight in the retention properties of shales, submitted to International Journal of Rock Mechanics and Mining Sciences.

Shale characterization and test protocols

- REPRINTED FROM THE 1ST WORKSHOP, 2012

Jørn Stenebråten

SINTEF Petroleum Research

Department of Formation Physics and Well Integrity

The characterization and test protocols described focuses on in-situ saturated shales.

An initial evaluation of a shale core using CT scan often reveals information that affects sample preparation and testing, such as open fractures, inclusions or general heterogeneities, all which can affect the success of preparing samples for testing, as well as the quality of the sought test results. The condition of the core also determines the types of test (and sample size) that can be utilized; on a heavily fractured shale the focus may have to be on index type of tests, whereas on an intact core larger samples can generally be extracted for general mechanical characterization tests such as triaxial compression, indirect tensile strength, or hollow cylinder tests. During core handling, sample preparation and storage prior to testing it is important to not alter the present saturation of the sample, as this may trigger non-reversible processes altering the static and dynamic properties of the material.

Following the localization of regions of interest, comes the general characterization of the shale assessing the mineralogy, relevant petrophysical parameters and pore water composition.

A complete fluid saturation can not be expected at atmospheric conditions of a shale core retrieved from in-situ temperature and stress, however, it is important to re-establish this as good as possible prior to mechanical or dynamic characterization, both to reduce capillary suction forces and to create contact between the pore water and the fluid used in the pore pressure measurement loop, in case of tests performed in a pressure vessel. Before contacting a shale sample with an aqueous fluid it is vital to have the sample instrumented and stressed to monitor the response of contact with the aqueous fluid.

For tests performed under a confining pressure it is required to allow sufficient time for stabilizing at the consolidation conditions, and to use the consolidation response to estimate proper strain or load rates based on the drainage conditions and the sample properties.

Recent Experience in Coring and Core Characterization for Unconventional Resources

Authors: Jon Wallace and Matt Honarpour*, Hess Corporation *retired

Abstract:

Core quality is especially challenging for unconventional resources due to highly heterogeneous poro-mechanical properties. Some recent experience in improved coring/preservation techniques will be presented, along with techniques and results for detailed characterization of the whole core prior to conducting routine and special core analysis.

Rock Mechanics Testing of Rotary Sidewall Core: Application to Reservoir Description

Eva Krauss, Core Laboratories

Rotary sidewall coring is often done when conventional coring is not available or when

retrieving core is no longer available. Sidewall coring encompasses several different methods of

sample retrieval; percussion sidewall and rotary sidewall. Percussion sidewall coring uses an

explosive charge to push a sampling cylinder into the formation and retrieve the sample. Rotary

sidewall coring is very similar a traditional coring bit in that the bit is coated with diamond bits

and rotates into the formation to drill a plug from the side of the wellbore.

For many tests including the tri-axial test, vertical samples are needed to determine compressive

strength, young’s modulus, and Poisson’s Ratio. The vertical samples chosen for rock mechanics

testing no ideally have a 2:1 ration, meaning for example, a two inch length and a one inch

diameter. When obtaining sidewall cores, the plugs will be retrieved in a horizontal configuration

in relation to the wellbore and obtaining a 2:1 ratio is often difficult, therefore these samples

might not be representative of the reservoir.

For application in the oil and gas industry, rock mechanics testing is often done to obtain

hydraulic fracture design parameters, determine stability of a wellbore, and to understand

production/depletion characteristics of the reservoir. This presentation will review the areas of

rock mechanics testing which can utilize horizontal rotary sidewall core samples and the types of

formations where testing of rotary sidewall core can be used to determine mechanical properties

of a reservoir.

SANDER HOL, A DISCUSSION ON

CONSEQUENCES OF CORE DAMAGE

Alteration of physical properties of rock material as a result of core handling is regarded a major

problem within the Geomechanics community. Damage could lead to massive uncertainty in predictions

of in-situ behavior of reservoirs and seals, and seems difficult to minimize or constrain. This contribution

forms the introduction of a discussion amongst participants of the ISRM-SCA-ARMA Workshop on

Petroleum Geomechanics Testing: Core Acquisition, Handling and Sampling to Ensure Quality Tests, with

focus on the consequences of damage in rock material on Geomechanical test results.

For the purpose of Geomechanical testing, samples are often macroscopically subdivided in either rock

type (sandstone, carbonate, shale), or mechanical state (unconsolidated, weakly consolidated, or

consolidated). This guides handling the core, as well as the design of the experimental procedure. In the

discussion introduced here, we consider damage by subdividing core material into the abovementioned

macroscopic categories, i.e. unconsolidated, weakly consolidated, and consolidated, but include the

case of de-consolidation as a result of de-stressing and cooling by (geological) exhumation. The latter

affects the state of the material in-situ, and not during extraction and coring. We relate observed

damage to either one of the three types “mechanical”, “thermal” and “chemical” core damage, and go

on to consider changes in physical properties caused by this, and estimate their magnitude. By isolating

and categorizing damage in this manner, we may finally arrive at an answer to the central question: How

much variation in magnitude of the physical properties is acceptable for what types of material, and

how can we minimize, or correct for, the consequences?

Geomechanical test protocol stages that help address sample-to-sample variation assessment - REPRINTED FROM THE 1ST WORKSHOP, 2012 JWDudley, Shell Int. E&P

Abstract: Geomechanical characterization often requires multiple different tests (e.g., uniaxial-strain, triaxial and isotropic compression) on a suite of ideally similar plugs. Even plugs that seem quite similar before testing (from visual/CT inspection, bulk density, etc.), however, may not all act mechanically similar for a variety of reasons. A suite of triaxial compression tests, for instance, may show one or two tests that are unusually strong/stiff (or weak/soft) relative to the other tests. Often only the triaxial loading stage stress-strain data are reported, and it is difficult to objectively compare the various results to understand the difference. Therefore when testing such a suite of plugs it is useful to perform representative measurements under identical conditions during each test for just such a relative comparison. Adding these specific measurement stages to the test protocols of the suite provides a mechanical measurement on each plug that can be helpful in understanding the aggregate test suite results.

A confining pressure unload-reload cycle for determining the bulk modulus can often serve this purpose well. For example, a triaxial compression test suite at pressures 3, 6, 10, 15 MPa, can have a 2 MPa confining pressure unload-reload cycle at 3 MPa during the isotropic compression stage of all the tests. The bulk modulus determined from these cycles provides a directly comparable measurement made under the same conditions on each sample, which can highlight anomalous behavior/samples. The same test stage can be added to other test types (isotropic compression, constant-mean-stress, uniaxial-strain) that at least begin with an isotropic compression to at least 3 MPa, and so extend the comparison across all tests used for developing or calibrating a constitutive material model.

Use of synthetic rocks for quantification of coring induced rock alteration - REPRINTED FROM THE 1ST WORKSHOP, 2012

Rune M Holt; SINTEF Petroleum research & NTNU, Trondheim, Norway

Abstract:

Synthetic rocks formed under stress have been used to demonstrate how compaction behavior, porosity and wave velocities are altered as a result of stress release, mimicking the situation during core drilling. This technique enables a better understanding of core damage mechanisms, and how they may be corrected for or mitigated. Two sources of such core alteration have been identified, i.e. formation of microcracks in competent sandstone and collapse of pore structure in poorly consolidated sand and sandstone. Discrete particle modeling has been used as an additional tool, permitting simulation of stress paths representative of coring, and extending the studies of stress release effects as seen in the synthetic rock experiments. This includes studies of stress memory (Kaiser effect) in granular rock.

If time permits, the presentation will also contain some results obtained with synthetic analogues to study fractured rock, comparison between compacted clay and shale behaviour, and ideas around manufacturing of artificial shale.

The main message is that use of synthetic rock permits controlled experiments that may contribute to an improved understanding of physical and mechanical processes in rocks. Such understanding can be used to improve models for rock behavior, but one needs to be careful in direct use of results for field applications.

Wave Velocities in Stressed Shales and Sands: How can we obtain representative measurements in the laboratory? - REPRINTED FROM THE 2nd WORKSHOP, 2013

Authors: Rune M Holt1, Erling Fjær2, Andreas Bauer2 and Jørn F Stenebråten3 1 NTNU & SINTEF, Trondheim, Norway, Houston. 2 SINTEF & NTNU, Trondheim, Norway 3 SINTEF, Trondheim, Norway

With the continued development of 4D seismic, quantification of subsurface stress evolution caused by pore fluid withdrawal or injection is becoming feasible. Sensitivity of sonic well log data to the near-well stress field provides a potential to map in situ stress anisotropy, which has an important control on hydraulic fracturing as well as borehole failures. From these perspectives, it is important to be able to assess wave velocities and their stress dependence under in situ conditions from core measurements.

In this presentation, laboratory experiments on shale / clay and sandstone / sand will be shown, in order to see how differences between in situ and laboratory conditions may affect the observations. The issue of core alteration as a result of stress relief has been studied extensively by experiments on synthetic rocks formed under stress. Furthermore, the influence of stresses and pore pressure and in particular of the stress path during laboratory experiments will be focussed. Brief discussions will also be given to issues of frequency dependence (dispersion) and temperature dependence, in particular for shales.

THE SCRATCH TEST: A HIGH RESOLUTION LOG OF ROCK STRENGTH WITH APPLICATION TO GEOMECHANIC AND PETROPHYSIC. C. Germay, T. Lhomme, T. Richard EPSLOG SA The scratch test is a transportable, non-destructive technique born at the end of the 90’s at the University of Minnesota to measure the uniaxial compressive strength of rock material (see Figure 1). It is a simple, non-destructive, fast (average of 6 feet of core tested an analysed per hour) and robust test that bears the unique advantage of producing a log of mechanical strength with a high spatial resolution (centimetric) along the core sample (Figure 1) providing a unique insight on the scales and distribution of heterogeneities along the core samples, thereby offering new perspectives for routine and special core analysis.

In this presentation, we first show how the mismatch between the resolution scale of core plug samples and well log measurements combined with the discrete nature of plugging can lead to an erroneous interpretation of the apparent correlation between petrophysical properties. The knowledge of small heterogeneity length scales brought by the scratch test enables a better selection of representative plug samples (Figure 2) reducing the uncertainty in the integration of core and wireline data for the characterization of reservoir rock units.

We then illustrate with some examples how rock strength profiles averaged to the relevant length scale can be correlated with other petro-physical properties measured both on core plugs or inferred from well logs (Figure 3).

With these case studies we also highlight that relationships between strength and petro-physical properties are not universal and can vary drastically from one formation to another, meaning that such empirical laws must be considered with caution before being generalized to an entire core section, or a fortiori exported towards other non-cored wells. For this matter, the extensive data set provided by the scratch test is a major asset to derive statistically more robust correlations.

Finally, we discuss how standard and special core analysis routines could benefit most from the advantages and practical features of the scratch test (fast, full cores, flat-clean groove left by the blade ideal for micro-perm and UV photography) when introduced at a very early stage of the global core analysis workflows (Figure 4).

Figure 1: (Left) Views of the Wombat and log of strength superimposed to high resolution picture of the rock. (Right) one to one correlation between scratch strength and unconfined compressive strength.

Figure 2: Case study: (Left): correlation between well log porosity and (i) strength derived from UCS on plugs (3 red data points) and (ii) scratch strength (much larger data sets). (Right) With one of the plug sitting as an outlier the strength log derived from the porosity well log and the UCS strongly overestimate the strength overall.

Figure 3: Example of correlation between scratch strength (averaged to the scale of plugs, i.e. 5 cm) the porosity and permeability measured on plugs (clastic reservoirs) and correlation between well logs (total porosity, sonic) with the scratch

strength (averaged to the scale of logging tool resolution, 50 cm) for clastic, carbonate and shale formation.

Figure 4: (Left) Practical advantages of the scratch test that makes it fit to slot prior to slabbing. (Right) Core analysis workflow with the scratch test carried out early on full cores following Gamma Ray and prior to slabbing.

• FAST • CAN BE CARRIED

OUT ON FULL CORES

• FLAT CLEAN GROOVE IDEAL FOR MICRO_PERM

• EXPOSED FRESH ROCK FIT FOR UV PHOTOGRAPHY

Rapid Heterogeneity Assessment Using the Impulse Hammer

Rathbun, A.P., Carlson, S.R., Ewy, R.T., Hagin, P.N. and Bovberg, C.A. Chevron Energy Technology Company, San Ramon, California, USA G. Boitnott, New England Research, White River Junction, Vermont, USA

Typical reservoir rocks can have natural heterogeneity from the submicroscopic to more than 10’s of m with most standard well logs measuring properties that are averaged over 0.1-1m. Measuring the core and well heterogeneity is both time consuming and costly. This has led to several indexing methods for measuring core heterogeneity. Here, we present a rapid method of determining a coupled Young’s Modulus (E) and Poisson’s Ratio (ν) called E* at cm or smaller scale for meters of core in one day. In addition to heterogeneity, shale cores are often naturally fractured and it can be difficult to acquire plugs for testing. Elastic properties such E and ν are key controlling factors in the design of fracture stimulation in unconventional reservoirs such as gas/oil shales and tight gas sands. The Hertz modulus, E*, is a key parameter in controlling hydraulic fracture width.

We use a rapid, non-destructive, index method based on impulse for the measurement of a constrained Young’s modulus. The determination of E* is based on the combination of Hertzian Contact Theory and impulse. A small mass with a hemispherical tip is dropped on a flat core surface from 12-19 mm height. The mass rebounds from the core; the resulting force-time curve is measured during contact of the mass and rock sample. This curve is inverted to measure the coupled E and ν of the sample. Measurements are conducted on either slabbed core or whole-round with a 20mm-wide flat section ground using a surface grinder. Samples must be thicker than ~2.5 cm for quantitative measurement. Fractured core that may not be suitable for traditional rock mechanics testing is easily handled in most situations. Measurements can be made at any desired spacing down to ~2 mm; currently our standard for heterogeneity measurement is 1 cm. At this spacing, it is possible to test 4 m of core in one day, with complete data analysis in a second day. This provides a rapid, in-house, determination of the mechanical properties with no permanent damage to the core. Data are reported as E* against depth and integrated into a built-in core photography system. The measured values of E* can be used directly for completion design and other applications, as well as for general core characterization. For complete technical details of the method please refer to Rathbun et al. [2014] presented at the U.S. Rock Mechanics /Geomechanics Symposium.

We find that E* varies on the cm scale with changes on the order of 106 psi (6.8 GPa) possible between measurement locations. In some cases transition is gradual and likely reflects subtle changes in mineralogy or fabric, while in other cases the change is abrupt. Measurements of E* are used to match ‘twin’ rock mechanics plugs and forecast properties to other core intervals. Impulse measurement at the cm-scale has the potential to be combined with traditional rock mechanics and up-scaled to entire mechanical layers of core and matching to well logs. The local nature of the measurement makes impulse testing a practical method for

rapid determination of mechanical heterogeneity due to mineralogical or fabric variations in core.

Figure 1. Example of impulse measurement of heterogeneity for three different cores. The core length is 1 ft in each case. Note the axis is different on the first core due to the high modulus of the carbonate.

Rathbun, A.P., S.R. Carlson, R.T. Ewy, P.N. Hagin, C.A. Bovberg, and G.N. Boitnott, Non-destructive impulse based index testing or rock core, American Rock Mechanics Association, USRMS, 2014.

Enhanced Rock Strength Profiling, Combining Triaxial Compressive Strength, Non-Destructive Core Scratching and Index Tests - REPRINTED FROM THE 1ST WORKSHOP, 2012 Abbas Khaksar and Feng Gui: Baker Hughes Inc. Australia Christophe Germay and Thomas Richard: EPSLOG SA, Belgium and Australia Knowledge of rock mechanical properties including rock strength is essential for accurate in situ stress analysis and geomechanical evaluations. Reliable quantitative data on rock strength parameters can only be derived at specific depths from laboratory tests on core samples typically through destructive tests on cylindrical samples. However, laboratory data are limited, discontinuous and often biased toward stronger intervals. In practice, many geomechanical problems are often addressed in the absence of core samples for laboratory testing. Consequently, rock strength evaluation is primarily based on log strength indicators, calibrated where possible against limited measurements on cores.

A number of techniques have been developed to replace or supplement plug based destructive tests to measure the strength properties of rocks. Scratch and Schmitt hammer tests test are examples of such techniques that have demonstrated the ability to provide continuous or semi-continuous, fine-scale measurements of rock mechanical properties. In contrast with conventional triaxial tests, both of these index tests are non-destructive and do not cause significant damage to the core and no special core preparation is required. These tests can be conducted either in the lab, core store or, in principle, on the rig, almost immediately after recovery of core material.

The Schmidt hammer, originally designed for concrete testing and is being widely used in civil engineering and more recently in geotechnical works and mining and applications. The hammer contains a spring-loaded mass that is automatically released against an impact plunger when the hammer is pressed against the test surface. Elastic recovery of the rock is dependent upon its surface hardness. Since hardness is related to mechanical strength, the rebound distance travelled by the returning hammer mass is a relative measure of the surface hardness, and therefore the strength. Taylor and Appelby in SPE 101968 and Khaksar et al in SPE 121972 showed the use of Schmitt hammer to characterise rock strength on cores from North Sea wells with application in sanding evaluations. Impact (rebound number) measurements were made and compared between matched depth sections of whole core, half cut core and slab sawn, resinated core, and all were found to correlate well with each other and many other petrophysical properties; e.g. GR, RHOB, NPHI, Dtc, Dts, Helium porosity, air permeability and grain density. Four intervals of high rock strength can be clearly seen and correspond to zones of intense quartz cementation. Direct measurement, semi-continuous impact logs such as this provide an excellent pre-filtering tool for plug selection for rock strength laboratory testing and qualitative assessments of rock strength. Scratch testing- Experimental results with sharp cutters at small depth of cut show linearity between the energy required provided to the cutter and the volume of rock removed by scratching. The intrinsic specific energy or rock strength is well correlated with the uniaxial compressive strength (UCS) of the tested material. The variation of the internal friction angle of the rock can also be assessed by using a blunt cutter. Experimental data on various sedimentary rock types are used to derive generalized correlations between the intrinsic specific energy and UCS and friction angle. Strong correlations are also found between intrinsic specific energy and core porosity and permeability. We present experimental results from laboratory testing using conventional destructive triaxial testing on core plugs and continuous non-destructive scratch testing along on a series of silisiclastics rocks. The rocks types represent a range of strength from very weak and unconsolidated with UCS ~ 2-6 MPa to moderately strong with UCS ~ 20-40 MPa and strong and competent rocks with UCS > 60 MPa and measured friction angle ranging from 17 to 55 deg. The experimental results show that albeit the use of generalized (global) correlations of scratch testing may provide relatively good estimates of rock strength comparable with triaxial tests, a local calibration based on triaxial tests would be required for more accurate and robust rock strength profiling.

It is shown that besides the direct application to geomechanical modeling the scratch test can also be used as a screening tool to optimize the plug location to characterize the range of heterogeneity of cored interval. Once calibrated to the conventional plug tests, the continuous scratch and Schmitt hammer test results can be correlated with core sedimentological and petrophysical properties to generate more reliable rock strength profiling coupled with rock facies analysis and diagenetic classification for improved strength profiling.

Using Continuous Profiles of Core Properties to Map Heterogeneity and Improve Geologic Core Descriptions. - REPRINTED FROM THE 2nd WORKSHOP, 2013

Author: Roberto Suarez-Rivera1 1 Schumberger, Terratek, Salt Lake City, Utah, U.S.A.

Understanding scale-dependent heterogeneity in tight shales and other unconventional reservoirs is important for hydrocarbon production and recovery. It is also important for characterization, modeling, and for extending our observations, experience and understanding from one scale (e.g., core-scale) to another (e.g., log- or seismic-scale). The presence of scale-dependent heterogeneity also poses additional important questions regarding sampling for characterization, including the number of samples needed, the adequate scale for sampling and others. Addressing and solving these questions will lead to significant progress on tight shale exploration and efficient production. Here, we describe continuous measurements along the length of the core that result in significant improvements to geologic core descriptions and heterogeneous rock characterization. Using multiple high-resolution measurements (e.g., of strength, thermal conductivity, CT atomic number, and XRF mineralogy) we define the principal rock classes, with similar characteristic properties, that define the heterogeneous system. The thickness and cyclic stacking patterns of these units provide quantitative information of the depositional system and its sequences. The method also differentiates transitional contacts from abrupt contacts, and provides additional information for developing a geologic model. Although the cyclic nature of shale and silt sequences is often visually apparent, the variability in properties within these sequences is only accessible by the continuous measurements. We analyze these via multivariate statistical analysis, to define rock classes with tightly constraint measured properties and visualize their variability with colors (similar colors represent similar rock classes). Results allow us to define the geologic system more quantitatively. They also allow us to understand the variability represented by larger samples used for core-based reservoir quality and completion quality evaluations.