Exploration and Reservoir

4. Exploration and reservoir description

Reserve replacement Sedimentary basins and petroleum systems

Finding hydrocarbons Plays, leads, prospects, traps

Cost of exploration Reservoir description, characterization of reservoir rocks

Geology and formation of hydrocarbons

Prospect evaluation

Peak oil – Hubbard curve • Peak oil is the point in time

when the maximum rate of petroleum production is reached, after which the rate of production is expected to enter terminal decline.

• The total production rate from an oil region over time usually grows exponentially until the rate peaks and then declines—sometimes rapidly—until the field is depleted.

• This behavior is described by the “Hubbard curve”, and has been shown to be applicable to the sum of a nation’s domestic production rate, and is similarly applied to the global rate of petroleum production

Reserve replacement

Example 1: Oil production has peaked in non-OPEC and non-FSU countries

Example 2: Oil production from NCS fields and groups of fields

Predictions of the timing of peak oil include the possibilities that it has recently occurred, that it will occur shortly, or that a plateau of oil production will sustain supply for up to 100 years. None of these predictions dispute the peaking of oil production, but disagree only on when it will occur.

Different scenarios for world peak oil production

Portfolio production

Time

Prod

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RoCE – Return on Capital Employed Production Production costs Finding & Development costs Reserve Replacement Rate (RRR) Reserve to Production ratio (RP

ratio)

Operational indicators for a project portfolio Operational and financial indicators for measuring critical success factors are prepared in connection with the planning process in oil companies and constitute an important element of their corporate plan.

Concrete ambitions for the next year and, if possible, for the planning period, are established for each individual indicator.

R = reserves p = production rate f = supply rate maximum (technical) production rate: p = k R RRR = f/p = reserve replacement rate R/p = reserves to production ratio

Reserves, production, supply

f

p

R

Giant oil and gas fields The world's about 950 giant oil and gas fields are considered those with 500 million barrels of recoverable oil or gas equivalent. Geoscientists believe these giants account for 40 percent of the world's petroleum resources. They are clustered in 27 regions of the world, with the largest clusters in the Persian Gulf and Western Siberian Basin..

Ormen Lange

Norne Heidrun Smørbukk/Draugen

Troll/Oseberg

1453

Gullfaks

Statfjord Ekofisk/ Valhall

Large discoveries are not made very often Examples NCS

• 16/2-6 Johan Sverdrup (2010), 1761 million barrels of oil equivalents • 7220/8-1 Skrugard (2011), 241 million barrels of oil equivalents • 7122/7-1 Goliat (2000), 175 million barrels of oil equivalents • 16/1-8 on the Edvard Grieg field (2007), 161 million barrels of oil equivalents • 6406/3-8 Maria (2010), 132 million barrels of oil equivalents • 34/4-11 (2010), 125 million barrels of oil equivalents • 25/8-14 S, on the Ringehorne Øst field (2003), 87 million barrels of oil

equivalents • 16/1-9 Draupne (2008), 84 million barrels of oil equivalents • 6608/10-14 S Skuld (2010), 67 million barrels of oil equivalents • 25/4-9 S Vilje (2003), 64 million barrels of oil equivalents

The largest oil discoveries made in the period 2000 – 2011

Growth in resources on the NCS

Barents Sea Norwegian Sea

North Sea






Prospect evaluation

Finding hydrocarbons Studies of the geological and geophysical information, involving sophisticated computer modelling of geological processes, seismic interpretation, and analysis of analogue information from petroleum provinces around the world, allow the probabilities of finding reserves of a certain magnitude to be assessed. In a well-defined mature area with fully appraised discoveries, for instance, there may be a high degree of certainty about the current reserves, but little chance of finding major additions. In a speculative venture in a little known area, on the other hand, the chances of finding any hydrocarbons at all may be low, but there is also the possibility of making a very large discovery. However, as even the most sophisticated analysis can only yield a very broad indication of the chances of finding commercial quantities of oil and gas, hydrocarbon exploration remains a classic example of ‘decision-making under uncertainty’.

Exploration process

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Project development

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Exploration and Production work processes, phases and milestones

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General geological information, seismic interpretation, analogues and drilling

Seismic surveys

2D seismic: A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially. A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data. 3D seismic: A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data. 4D seismic: Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4C seismic: Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves.

Seismic surveys

Illustration of 2D seismic


4D SEISMIC

1985

1999


Presenter

Presentation Notes

Message: Statoil was first to apply time-lapse seismic, being a basis for IOR. In the early 90s, time-lapse or 4D seismic received a boost thanks, at least in part, to the establishment of Statoil’s Research Centre, the employment of some creative geophysicists, and the Trondheim-based BP-Statoil R&D collaboration. Designed for reservoir monitoring, time-lapse or 4D seismic involves comparing the results of 3D seismic surveys repeated at considerable time intervals (e.g. before a field starts producing versus various post-production stages): time is the fourth dimension. Robust differences seen between survey results are attributed to fluid changes and/or changes in reservoir pressures. 4D seismic technology can thus be used to monitor variations caused by production and/or the injection of fluids to improve recovery. Previously, most of Statoil’s research was focussed on qualitative aspects, as seen in this figure from the Gullfaks Fields where the ‘before and after’ diagrams show that the smaller accumulation (to the left of the fault) has been drained while much oil still to be recovered form the main accumulation (to the right of the fault). More recently, though, we have turned our attention to developing techniques for extracting quantitative information such as saturation (which tells us not only where the oil is but how much is present) and assessing uncertainties. Much of Statoil’s pioneering work was done on the Gullfaks field, which was one of the world’s first comprehensive applications; but now 4D is used on all of Statoil’s major fields if the geological conditions are suitable, and it has also been applied to partner-operated fields. In terms of value creation, a conservative figure puts the contribution of 4D seismic to the Gullfaks Field alone at around NOK 500-600 million; and the total may well exceed NOK 1 000 million by the time the field is exhausted.

Hydrophone X Y Z

Ocean bottom (4C) seismic Illustration of ocean bottom (4C) seismic

Presenter

Presentation Notes

Message: Statoil was first in developing and applying 4C seismic Within the area of exploration seismic, there have been dramatic changes over the past 10-15 years in order to reduce uncertainty related to prospects. The evolution has been from 2D via 3D to 4D and 4C seismic. 4D means that the time dimension is included, while 4C means that shear waves are measured along with pressure waves. 4C is used to drastically reduce uncertainty, and the potential is large, particularly in high cost areas. Fewer dry wells are the result. On Gullfaks, drilling cost varies from USD 3.5 mill (MNOK 30) to USD 7 mill (MNOK 60) for a new well. The cost of doing a seismic survey (shoot and process) of 30 km2 is USD 4.5 million (MNOK 40). From a technical point of view, the lack of shear-wave information is limiting because P-waves are not only influenced by rock types but also by porosity and fluids, which makes it difficult to discriminate between them. S-waves, however, are almost insensitive to a rock’s fluid content, whether hydrocarbons or water. This fact prompted one of Statoil’s former researchers – Eivind Berg – to pose the question: surely it is possible to gain shear-wave information as well if detectors are deployed on the seafloor? After all, dry land-based surveys, for which this is possible, have shown that the integration of P- and S-wave data can vastly improve our ability to distinguish between rock and fluid effects, thus making a significant, contribution to decreasing exploration risk and improving geophysical reservoir imaging and monitoring. Hence the birth of Statoil’s seafloor seismic project (SUMIC). The principle involves the deployment of hydrophones (representing one component) and three-component geophones on the seabed – hence the term 4C (four component) seismic.

Illustration of other geophysical methods Resistivity-based seafloor logging

Presenter

Presentation Notes

Message: Statoil was first in developing and applying 4C seismic Within the area of exploration seismic, there have been dramatic changes over the past 10-15 years in order to reduce uncertainty related to prospects. The evolution has been from 2D via 3D to 4D and 4C seismic. 4D means that the time dimension is included, while 4C means that shear waves are measured along with pressure waves. 4C is used to drastically reduce uncertainty, and the potential is large, particularly in high cost areas. Fewer dry wells are the result. On Gullfaks, drilling cost varies from USD 3.5 mill (MNOK 30) to USD 7 mill (MNOK 60) for a new well. The cost of doing a seismic survey (shoot and process) of 30 km2 is USD 4.5 million (MNOK 40). From a technical point of view, the lack of shear-wave information is limiting because P-waves are not only influenced by rock types but also by porosity and fluids, which makes it difficult to discriminate between them. S-waves, however, are almost insensitive to a rock’s fluid content, whether hydrocarbons or water. This fact prompted one of Statoil’s former researchers – Eivind Berg – to pose the question: surely it is possible to gain shear-wave information as well if detectors are deployed on the seafloor? After all, dry land-based surveys, for which this is possible, have shown that the integration of P- and S-wave data can vastly improve our ability to distinguish between rock and fluid effects, thus making a significant, contribution to decreasing exploration risk and improving geophysical reservoir imaging and monitoring. Hence the birth of Statoil’s seafloor seismic project (SUMIC). The principle involves the deployment of hydrophones (representing one component) and three-component geophones on the seabed – hence the term 4C (four component) seismic.

Example: Mapping of Aldous Major/Avaldsnes

In August 2011 a large oil discovery was made on the Aldous Major South prospect in the Norwegian sector of the North Sea. A minimum 65-metre oil column was confirmed. The exploration well also established a common oil/water contact between the Aldous and Avaldsnes structures. An additional well will be drilled in Aldous Major North to clarify the further potential and any communication with Aldous/Avaldsnes. Aldous/Avaldsnes has been described as the largest offshore discovery in the world in 2011.

Exploration drilling

• Exploration drilling may be undertaken to determine whether subsea geological structures, identified by seismic surveying, contain oil or gas.

• Dependent on the results of the technical work carried out in the initial period, exploration drilling will be considered.

• Exploration drilling is undertaken to determine whether subsea geological structures, identified by seismic surveying, contain oil or gas.

• A well is drilled using on board equipment for hoisting pipes, pumps for circulating fluids, motors to rotate the pipe and generators to provide electrical power.

• After completion of exploration drilling operations, wells are sealed off with downhole cement plugs. No metal is left on the sea floor.

Drilling and logging The only way to confirm whether a structure does contain oil or gas is to drill a well. This confirms the presence of hydrocarbons, but also provides additional information on which to base further exploration and a future field development plan. Rock cuttings, core samples and geophysical data from well surveys are used to gain information from wells. Rock cuttings brought to the surface by drilling mud and specially taken core samples enable geologists to understand the geological history and, if hydrocarbons are discovered, the nature of the reservoir. Key physical properties of the rocks drilled are obtained from wireline logs. Drilling is halted and a recording device known as a ‘sonde’ is passed down the bore hole on an electric cable. Alternatively, formation data can be measured during drilling with special downhole tools in the drill-string. By measuring the electrical, acoustic and radioactive properties of the rocks, the presence of hydrocarbons can be detected and information collected on the different formations.

If a well finds oil or gas, additional insight into reservoir properties and well performance under operating conditions can be obtained from a flow test. Depending on such issues as the value of information gained and environmental constraints, this could be a short ‘drill-stem’ test or a longer test using temporary production facilities (Extended Well Test -EWT).

Well testing

Exploration drilling – some definitions The definitions used for wildcat well , segment, segment well and appraisal well are as follows: Wildcat well The first well to test a new, clearly defined geological unit (prospect). "Find new oil" Segment A segment is that part of a prospect, defined by geological and engineering data, whose petroleum volumes may confidently be explored for by the drilling of a single exploration well. The petroleum volume distributions of un-drilled segments carry a risk, even though discovered resources may already have been booked in other drilled segments. Segment well The first well to test an un-drilled segment. "Find new oil" Appraisal well A well drilled to establish the extent and the size of a discovery. "Delineate old oil"

Exploration wells spudded on the NCS 1966–2010

(Source: Norwegian Petroleum Directorate)

Resource growth per wildcat well on the NCS – five year rolling average

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101520253035404550

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Growth in resources and the number of wildcat wells on the NCS






Prospect evaluation

Average exploration cost per well NCS 1985-2010

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Drilling costs General investigations Field evaluation Administration

Cost of exploration

Average exploration cost per well NCS 1985-2010

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Exploration costs as a percentage of investment costs

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Number of wildcat wells on the Norwegian continental shelf per year and the trend in the

nominal price of oil in 1966-2011.

The amount of money that oil companies are willing to spend on exploration is closely related to the oil price!

Typical costs for seismic data • Onshore: 2D: 8000 – 12000 USD/km 3D: 20000 - 40000 USD/km2

• Offshore: 2D: 500 USD/km 3D: 10000 USD/km2

Seismic data from Lofoten (Nordland VI and VII and Troms II) have been obtained by NPD over three years at a total cost of 420 mill.NOK. The data will be sold (made available) to oil companies at a cost of 15% of the 420 mill.NOK (about 60 mill.NOK)

Acquisition of seismic data in Nordland VII and Troms II

Accumulated costs during exploration and appraisal

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Prospect evaluation

Geology • Geology is the science comprising the study of solid Earth

and the processes by which it evolves. • Geology provides primary evidence for plate tectonics, the

history of life and evolution, and past climates.

• Petroleum geology refers to the specific set of geological disciplines that are applied to the search for hydrocarbons

Geology

Subsurface disciplines

Seismic acquisition & processing Seismic data

acquisition Seismic processing

Play & prospect-evaluation Prospect

analysis

Reservoir modelling

Geological reservoir modelling &

uncertainty analysis Production geology

Petrophysics Petrophysics and fluid-rock

analysis Core analysis

Basin analysis & geochemistry Basin modelling and geochemistry Petrology

Geo operations Operations

geology and data acquisition

Seismic interpretation

analysis Seismic

interpretation Seismic LFP

Structural geology Structural geology & tectonics

Reservoir simulation Reservoir simulation

Drainage strategy Drainage

strategy Reservoir technology

Seismic & well sequence stratigraphy

& sedimentology Sedimentology

&Sequence stratigraphy

Biostratigraphy

http://www.discoveringfossils.co.uk/Earth.htm






Prospect evaluation

Basins are large scale areas of the Earth’s crust with a long history of subsidence and within which a thick sequence of sediments has accumulated.

Basin analysis involves making an interpretation of the formation, evolution, architecture and fill of a sedimentary basin by examining geological variables associated with the basin.

Basin analysis provides a foundation for extrapolating known information into unknown regions in order to predict the nature of the basin where evidence is not available.

Basin analysis Basin

Geo-seismic section through the North Sea Basin – Viking Graben

Basin

Sedimentary basins and petroleum systems

At Svalbard you will see rocks protruding through the ice that you normally just find under the seabed, which is very special.






Prospect evaluation

Basin, petroleum system, play, lead, prospect

• A sedimentary basin is a depression filled with sedimentary rocks. The presence of sedimentary rocks proves that a basin existed.

• Presence of petroleum proves that a petroleum system exists within the basin. Presence of a petroleum system implies that the basin comprise mature source rock, migration pathway, reservoir rock, trap and seal.

• Plays are developed in order to define areas where leads and prospects may be identified. The critical elements are: source rock, reservoir rock and top seal rock.

• Lead is a structure which may contain hydrocarbons (rough indication of a prospect).

• Prospect is a lead which has been fully evaluated/mapped and may be ready for drilling.

The relation between basin, play, prospect and discovery

Basin Play Prospect Discovery/field

Petroleum system • A petroleum system is defined by geologic components and

processes necessary to generate and store hydrocarbons, including a mature source rock, migration pathway, reservoir rock, trap and seal.

• Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved.

• The components and critical timing relationships of a petroleum

system can be displayed in a chart that shows geologic time along the horizontal axis and the petroleum system elements along the vertical axis.

• Exploration plays and prospects are typically developed in basins or

regions in which a complete petroleum system exists or has some likelihood of existing.

Play definitions • A play (or a group of interrelated plays) generally occurs in a single

petroleum system (Norwegian: play = “letemodell”)

• Play: A geographically and stratigraphically delimited area where a specific set of geological factors is present so that petroleum should be able to be proven in producible volumes. Such geological factors are a reservoir rock, seal, mature source rock, migration routes, and that the seal was formed before the migration of petroleum ceased.

• Play: A family of prospects, leads, undeveloped and developed pools and drilled unsuccessful features that are known or conceived to share the same gross reservoir, hydrocarbon charge system and regional top seal

• Play: A group of prospects within a given geographical area in which a set of common geological factors, such as reservoir rock, trap and the formation of hydrocarbons from a mature source rock, must be present simultaneously in order that petroleum accumulations can occur

• Play: A conceptual model for a style of hydrocarbon accumulation used by explorationists to develop prospects in a basin, region or trend and used by development personnel to continue exploiting a given trend

Confirmed and unconfirmed Plays

• All Prospects and Discoveries within a Play share the same set of necessary attributes (reservoir, source, seal) and are hence distinguishable from Prospects and Discoveries belonging to other Plays

• Plays are classified as confirmed when at least one accumulation of producible quantities of hydrocarbons is discovered. All discoveries and prospects in the same play are characterised by the play's specific set of geological factors.

• Unconfirmed plays are defined as those yet to yield a discovery. Plays are subject to varying degrees of uncertainty, depending upon the statistical probability that the geological factors which define them are present.

Prospect • A possible petroleum

trap with a mappable, delimited volume of rock.

• or…..an area of

exploration in which hydrocarbons have been predicted to exist in economic quantity.

• or……an anomaly, such as a geologic structure or a seismic amplitude anomaly, that is recommended by explorationists for drilling a well.

• a lead is an indication of a prospect

Basin

Petroleum system

Play

Lead/Prospect

Basin Screening

Basin – petroleum system – play – lead/prospect

The presence of hydrocarbons (1)

The presence of large quantities of hydrocarbons in a sedimentary basin, indicates that six independent requirements have been met. The first three relate to the charge (source/migration), the formation of hydrocarbons within the basin: I) there must have been a source rock, rich in organic carbon to be converted to hydrocarbons II) there must have been sufficient heat over long periods of time to convert the organic carbon into hydrocarbons (such temperatures can only be achieved at depths of between two and four kilometres, depending on the age and geological setting, and the basin will need to be deep enough to ensure that the source rock reaches the required depth) III) there must have been migration pathways to enable the hydrocarbons to migrate upwards from the source rock, and, perhaps, reach a trap

The presence of hydrocarbons (2) The other three conditions concern the reservoir and the trap, which prevents migrating hydrocarbons from escaping to the surface, and which therefore must pre-date the charge: IV) there must be a suitable reservoir rock, such as limestone or sandstone, which must have sufficient porosity to store the hydrocarbons and be permeable enough to allow them to be produced at economic rates V) there must be an effective seal of impermeable rock, such as clay, shale or salt, above and against the reservoir VI) there must be a closed structure, a geometric disposition of the reservoir and a seal to arrest the upward migration of the hydrocarbons

The presence of hydrocarbons (3) • In some literature this description is simplified, and only three

conditions instead of six are listed:

• Three conditions must be present for oil reservoirs to form: – a source rock rich in hydrocarbon material buried deep enough for

subterranean heat to cook it into oil (ref. I, II, III) – a porous and permeable reservoir rock for it to accumulate in (ref. IV) – and a cap rock (seal) or other mechanism that prevents it from

escaping to the surface (ref. V, VI) • Within these reservoirs, fluids will typically organize themselves

like a three-layer cake with a layer of water below the oil layer and a layer of gas above it, although the different layers vary in size between reservoirs.

• Because most hydrocarbons are lighter than rock or water, they often migrate upward through adjacent rock layers until either reaching the surface or becoming trapped within porous rocks (known as reservoirs) by impermeable rocks above.

• However, the process is influenced by underground water flows, causing oil to migrate hundreds of kilometres horizontally or even short distances downward before becoming trapped in a reservoir.

Play Probabilities P(Reservoir): The probability of occurrence of reservoir facies on a regional scale. P(Seal): The probability of occurrence of a regional top seal, capable of preventing hydrocarbons from upward migration. P(Source): The probability of occurrence of a rock unit that can generate and expel oil or gas in sufficient quantity to form one or more accumulations within the play.

Prospect Probabilities P(Reservoir): The probability of occurrence of reservoir facies with effective porosity/permeability, capable of holding hydrocarbons above a specified minimum volume in the prospect (IV). P(Trap): The probability of occurrence of a structural or stratigraphic configuration that provides a trap for migrating hydrocarbons (V and VI). P(Charge/Migration): The probability of presence, quality and maturity of source rocks in the drainage area of the prospect, sufficient migration of hydrocarbons into the trap and of in-reservoir biodegradation (I, II and III).

Illustration of types of traps

Illustration of types of traps

Faults - earth quake is natural

A normal fault; one of the dominant structures of sedimentary basins

Illustration based on seismic surveys

In reflection seismology, a bright spot is a local high amplitude seismic attribute anomaly that can indicate the presence of hydrocarbons and is therefore known as a direct hydrocarbon indicator

Traps, leakage, spill-point

Source Rocks and migration

Short summary • Prospect: a defined (mapped) trap with reservoir rock and associated

source rock

• Lead: an indication of a not yet mapped prospect

• Play: a geographically and stratigraphically delimited area that may contain discoveries, prospects and leads - all based on the same source-, reservoir- and seal rocks

• Petroleum system: a geographically and stratigraphically delimited area that may contain several plays – all based on the same source rock

• Basin: basins are large scale areas of the Earth’s crust with a long history of subsidence and within which a thick sequence of sediments has accumulated. A sedimentary basin may contain petroleum systems






Prospect evaluation

Reservoir • A reservoir is a subsurface body of rock having sufficient porosity

and permeability to store and transmit fluids.

• Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and form under temperature conditions at which hydrocarbons can be preserved.

• A reservoir is a critical component of a complete petroleum system.

An accumulation of hydrocarbons

Oil

Water

Impermeable rock (seal)

Permeable reservoir rock

Gas

Source rock Migration

Sea bottom

In geology, rock or stone is a naturally occurring solid aggregate of minerals. The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary and metamorphic.

Igneous rocks Igneous rocks are the first great class. "Igneous" comes from the Latin for fire, and all igneous rocks began as hot, fluid material. This material may have been lava erupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic. The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.

basalt granite

Sedimentary rocks

sandstone limestone shale

Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types, Clastic, Organic and Chemical Sedimentary Rocks. Clastic: The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud and sand and gravel and clay. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocks. Sand and mud is carried down rivers to the sea, mostly. Sand is made of quartz, and mud is made of clay minerals. As these sediments are steadily buried over geologic time, they get packed together under pressure and low heat, not much more than 100°C. In these conditions the sediment is cemented into rock: sand becomes sandstone and clay becomes shale. Organic: Another type of sediment actually forms in the sea as microscopic organisms — plankton — build shells out of dissolved calcium carbonate or silica. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica).

Metamorphic rocks Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain.

Gneiss Marble Quartzite

For a rock to form a reservoir: a) It must have a certain storage capacity (it must have many tiny spaces or pores): This property is characterized by the porosity. b) The fluids must be able to flow in the rock (the pores must be connected) : This property is characterized by the permeability. c) It must contain a sufficient quantity of hydrocarbons, with a sufficient concentration: The impregnated volume is a factor here, as well as the saturations. The methods used to characterize reservoir rocks are essentially core analysis and well logging.

Petrophysics the study of the physical properties of rocks.

Reservoir Rocks The main reservoir rocks are made up of:

•sandstones •carbonates

These are sedimentary rocks, in other words rocks made up of sediments formed at the earth's surface by debris (mineral, animal and vegetable) or chemical precipitations. They are stratified in successive beds.

http://www.earth.lsa.umich.edu/earth118/gate.jpg

Porosity

A rock sample is considered. Its apparent volume or total volume VT consists of a solid volume VS and a pore volume VP. The porosity is: φ = VP/VT It is often stated that the porosity is: Low: if φ < 5% Mediocre: if 5% < φ < 10 % Average: if 10% < φ < 20 % Good: if 20% < φ< 30 %

Excellent: if φ > 30%

Nearly all rocks and sediments contain openings called pores or voids, which come in all shapes and sizes. The fraction of total volume occupied by pores or voids is called porosity. Materials containing a relatively large proportion of void space are described as porous or said to possess "high porosity."

Permeability During production, the fluids flow in the rock pores with greater or lesser difficulty, depending on the characteristics of the porous medium. Darcy's Law The specific or absolute permeability of a rock is the ability of the rock to allow a fluid with which it is saturated to flow through its pores. Permeability can be determined by Darcy's Law, an experimental law. <1 mD: Very low 1 to 10 mD: Low 10 to 50 mD: Mediocre 50 to 200 mD: Average 200 to 500 mD: Good > 500 mD: Excellent NB.: in a porous medium, the permeability generally varies with the flow direction.

Connected pores give rock permeability

Permeability – Darcy’s law

Porosity (%)

Perm

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lity

(mD

)

Permeability versus Porosity

Saturations In the pore volume Vp there may be found a volume Vw of water, a volume Vo of oil, and a volume Vg of gas (Vw + Vo + Vg = Vp). The oil, water and gas saturations are: expressed in percent, with Sw + So + Sg = 100 %. Knowing the volumes of oil and gas in place in a reservoir requires knowing the saturations at every point, or at least a satisfactory approximation.

Viscosity • A property of fluids and slurries that indicates their resistance to

flow, defined as the ratio of shear stress to shear rate.

• Poise is the unit for viscosity, equivalent to dyne-sec/cm2.

• Because one poise represents a high viscosity, 1/100 poise, or one centipoise (cp), is used for measurements.

• One centipoise equals one millipascal-second.

• Viscosity must have a stated or an understood shear rate in order

to be meaningful. Measurement temperature also must be stated or understood.






Prospect evaluation

Prospect evaluation • Prospect evaluation is a technical/economical calculation of the

expected value of a prospect. • The calculated expected value is used as basis for decision making

(drill or not drill, buy or sell etc.)

• Prospect evaluation is a complex multidisciplinary task.

Geology Reservoir Commercial Economy Production profile

Cost&schedules

Tax, price, tariff Wells

Facilities

E(NPV)

Prospect evaluation the main steps

Basin evaluation Play evaluation Prospect evaluation

Probabilities Resources initially in place Recoverable

Thickness

Area

Quality

Maturity

Migration Avai

labl

e ch

arge

, vol

ume

and

HC

type

Trap

cap

acity

(HC

PV

) R

ock

volu

me

Calibration

Thickness

Sealing, closure

Net/Gross ratio

Porosity

Saturations

HC volume in place

Recoverable resources

Recovery factor

Gas/Oil ratio

Shrinkage Expansion

P(reservoir)

P(seal)

P(source)

P(p

lay)

P(reservoir)

P(trap)

P(source) P(p

rosp

ect)

P(discovery)

Cash-flow Income Costs

Production

Development

Technical / Economical evaluation

Net Present Value (NPV) E(NPV)

Technical Economical E(NPV) = Expected Net Present Value of prospect

Trap and reservoir Source rock

10 Questions

1. Define the reserves replacement rate 2. What is a basin? 3. What is a petroleum system? 4. What is a play and what is a prospect? 5. What is a trap? Name 4 types. 6. What is meant by migration? 7. List 6 requirements that must be satisfied if significant

volumes of hydrocarbons are to be found in a basin. 8. What is a well-testing and what is the purpose of it? 9. What is meant by 2D, 3D and 4D seismic? 10. What is the unit for quantifying permeability?

Documents

Exploration and Reservoir