38 Oilfield Review

Looking Deep into the Reservoir

Crosswell monitoring technology is moving formation measurements away from the

near-borehole region and farther into the reservoir. Tomographic images derived from

these measurements are helping asset managers monitor the movement of fluids deep

inside formations.

Zaki A. Al-AliMuhammad H. Al-BualiSaleh AlRuwailiShouxiang Mark MaAlberto F. MarsalaSaudi AramcoDhahran, Saudi Arabia

David AlumbaughLuis DePaviaCyrille LevesqueAjay NalonnilPing ZhangRichmond, California, USA

Colin HulmeClamart, France

Michael WiltAbu Dhabi, United Arab Emirates

Oilfield Review Summer 2009: 21, no. 2.Copyright © 2009 Schlumberger.For help in preparation of this article, thanks to Martin Isaacs, Sugar Land, Texas, USA.DeepLook-EM, Petrel, RST and ToolPlanner are marks of Schlumberger.

An elusive yet essential element of reservoir management is the ability to understand and pre-dict the movement of fluids in a producing field. A key strategy for optimizing production involves continuous or repeated monitoring of reservoir properties. This practice helps operators identify and respond to changes in reservoir conditions requiring intervention. Reservoir monitoring options vary—from daily surveys carried out by intelligent completions to repeated measure-ments of pressure transients across multiple wells and time-lapse seismic data acquired throughout the producing life of a field. Each option has its strengths, weaknesses and tradeoffs.

Interwell pressure transients enable an operator to monitor fluid movements, but these pressure measurements provide information pri-marily from the near-wellbore vicinity. Seismic surveys illuminate reservoir volumes but tend to exhibit poor vertical resolution, and they are more sensitive to rock matrix than to fluid type

and distribution within a reservoir.1 To better manage hydrocarbons lying between wells, operators need a fluid-saturation survey that investigates farther into a reservoir than electri-cal logs but at better resolution than is attained with seismic surveys.

In their quest to monitor fluid movements at a reservoir scale, some asset managers are turning to crosswell electromagnetic induction technol-ogy. This approach capitalizes on the advantages of electromagnetic induction and tomographic imaging to produce a cross-sectional display of the resistivity distribution between wells. Electromagnetic (EM) induction offers a greater depth of investigation than conventional logging tools and is sensitive to fluid content. Crosswell EM induction surveys investigate the region between wells, filling an intermediate role between high-resolution well logs and lower- resolution surface measurements (below).

> Bridging the resolution gap. Most oilfield measurements fall at the ends of a spectrum; they are obtained by probing either the near-wellbore vicinity at high resolution or a basinwide area at lower resolution. Crosswell measurements sample at an intermediate depth of investigation and resolution to help geoscientists characterize and image the reservoir within the interwell region.

Figure—01

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1. Luling MG, Babour K, Habashy TM, Nichols E, Wilt MJ and Zhang P: “Crosswell Electromagnetic Surveys for Monitoring Large-Scale Reservoir Saturation Changes,” paper SPE SA-55, presented at the SPE Technical Symposium of Saudi Arabia Section, Dhahran, May 15–17, 2004.

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40 Oilfield Review

Induction measurements respond primarily to electrical conductivity; in the downhole envi-ronment, high conductivity values generally signify the presence of salt water. The reciprocal of electrical conductivity is resistivity. High resistivity values, which correspond to low

conductivity, may indicate the presence of hydro-carbons; however, the measurement is influenced by temperature, porosity, pore-fluid resistivity and fluid saturations. Crosswell EM resistivity is employed to distinguish the contrast in electrical properties between resistive oil and conductive

saline water.2 Interwell resistivity data are also useful for mapping variations in reservoir proper-ties and for ascertaining heterogeneity and reservoir connectivity or compartmentalization between wells.3 Crosswell EM logging is not a new concept. It has proved to be a viable technology and has evolved into a system that permits increased logging speeds and provides advances in measurement accuracy, resolution, acquisi-tion, modeling and processing.4

The DeepLook-EM crosswell reservoir moni-toring system operates similarly to conventional logging tools. But, instead of lowering a transmit-ter and receiver into the same wellbore, this crosswell technique uses a transmitter placed in one well and an array of receivers in another well to investigate the area between them. By trans-mitting from one well to another at frequencies in the hertz to kilohertz range, it is possible to propagate signals over a distance of nearly 1 km [0.6 mi] from the source, depending on wellbore and formation characteristics (left).

Using models of the field, along with wellbore trajectory data, analysts processed the survey data to generate tomographs—vertical slices of the reservoir bounded by each well in the survey. These tomographic slices are color-coded to show differences in formation resistivity.

Asset managers utilize DeepLook-EM surveys for a variety of applications, such as monitoring sweep efficiency, identifying bypassed pay, plan-ning infill drilling locations and improving effectiveness of reservoir simulations.5 This article describes how DeepLook-EM surveys are conducted and how the data are used. It begins with an overview of the survey equipment and measurement process, then discusses applica-tions through case studies from the Middle East and the USA.

Acquisition SystemIn broad terms, the DeepLook-EM inductive crosswell system consists of a transmitter tool-string placed in one well, a receiver array placed in another well, surface acquisition consoles at each well and a wireless telemetry system between the two. The tools can be run in each well using a standard wireline system and a mast (left).

The transmitter toolstring is 9.88 m [32.4 ft] long and 8.6 cm [3.375 in.] in diameter and contains an induction-coil transmitter with a 3.7-m [12-ft] antenna (next page, top right). The transmitting antenna broadcasts a continuous sinusoidal signal at programmable frequencies. A gamma ray detector in the transmitter cartridge

> Surveying between distant wells in the Middle East. A receiver array is poised above a wellhead (center) before being lowered into a well. On the horizon, a transmitter toolstring hangs suspended in the wellbore beneath a slanted hoisting boom.

> Crosswell EM acquisition system. A magnetic dipole transmitter, five orders of magnitude more powerful than conventional induction logging transmitters, is lowered into the first well (left ). In the second well (right), an array of four receivers is deployed. Magnetometers in the receiver array can measure magnetic fields 10 orders of magnitude smaller than the Earth’s static magnetic field. The transmitter and receiver components are linked wirelessly between wells spaced up to 1,000 m apart.

Figure—02

Receiver 1

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Well separation distance up to 1,000 m

Receiver well

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Summer 2009 41

provides depth correlation with previously obtained logs.6

The EM receiver toolstring is 22.5 m [73.8 ft] long and 5.4 cm [2.125 in.] in diameter. The tool comprises an array of four induction-coil magnetic-field receivers, spaced 4.5 m [14.75 ft] apart. These detection coils—extremely sensitive magnetometers—accurately measure signals generated by the transmitter. The receiver section also includes a gamma ray detector for depth correlation.

Like any induction tool, the DeepLook-EM system requires accurate measurements of phase and amplitude. To obtain synchronous phase measurements, timing at the surface transmitter console and receiver acquisition console is refer-enced against a GPS satellite. A wireless telemetry system links these consoles, enabling the field engineer to remotely monitor and con-trol the acquisition from a single location. The downhole transmitter current, voltage and field measurements are sent by wireless link to the receiver acquisition console to normalize the received signals.

Survey PrinciplesReservoir monitoring surveys are typically carried out during secondary- or tertiary-recovery pro-grams, and the results are used to update reservoir models or to revise field-development plans. By measuring formation resistivity between wells, a DeepLook-EM survey can help engineers and geo-scientists map the resistivity distribution within a reservoir. Because resistivity measurements are sensitive to porosity, fluid saturations and temper-ature, variations in resistivity distribution may reflect changes in these properties.

The detection of these changes can be a valu-able diagnostic tool for asset managers. For instance, changes in porosity may be symptomatic of subsidence within a reservoir. Changes in satu-ration often signal the advance of a waterflood, whereas static saturation measurements may indi-cate bypassed pay. Temperature variations can help asset teams evaluate the effectiveness of a steamflood. During fluid injection, differences in resistivity distribution over time provide informa-tion about fluid movement, which in turn may provide clues to permeability distribution.

DeepLook-EM crosswell data are acquired by lowering the receiver array into a wellbore to a specified depth, or station. While this receiver array remains stationary, the EM transmitter in the other well continuously broadcasts as it is moved between selected depths. Once the profile—signals from a complete transmitter

traverse—is obtained at a station, the receiver tool is repositioned to another station, typically uphole, and the process is repeated. Thus, as the transmitter and receivers are moved across the logging interval, the target zone between wells is

investigated from multiple angles. For better control of tool movement during logging, the operation is usually directed from the transmit-ter well, with commands entered through a laptop computer (below).

> Tool specifications. The DeepLook-EM system, rated to 150°C, can be run in any wellbore fluid.

Figure—03

Specifications DeepLook-EM Transmitter DeepLook-EM Receiver

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> Inside the logging unit. The logging job is typically controlled from a laptop computer at the transmitter well, where commands and data are exchanged wirelessly between the transmitter and receiver wells.

Figure—04

2. Levesque C: “Crosswell Electromagnetic Resistivity Imaging: Illuminating the Reservoir,” Middle East and Asia Reservoir Review no. 7 (2006): 23–33. Available online at http://www.slb.com/media/services/resources/mewr/num7/illuminating_reservoir.pdf (accessed April 2, 2009).

3. Wilt M, Morrison HF, Becker A, Tseng H-W, Lee K, Torres-Verdin C and Alumbaugh D: “Crosshole Electromagnetic Tomography: A New Technology for Oil Field Characterization,” The Leading Edge 14, no. 3 (March 1995): 173–177.

4. DePavia L, Zhang P, Alumbaugh D, Levesque C, Zhang H and Rosthal R: “Next-Generation Crosswell EM Imaging Tool,” paper SPE 116344, presented at the SPE Annual

Technical Conference and Exhibition, Denver, September 21–24, 2008.

For an earlier perspective on crosswell EM techniques: Kretzschmar JL, Kibbe KL and Witterholt EJ: “Tomographic Reconstruction Techniques for Reservoir Monitoring,” paper SPE 10990, presented at the 57th Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, New Orleans, September 26–29, 1982.

5. Sanni ML, Yeh N, Afaleg NI, Kaabi AO, Ma SM, Levesque C and Donadille JM: “Crosswell Electromagnetic Resistivity Tomography: Pushing the Limits,” paper SPE 105353, presented at the 15th SPE Middle East Oil and Gas Show and Conference, Bahrain, March 11–14, 2007.

6. DePavia et al, reference 4.

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The DeepLook-EM transmitter generates a magnetic field that is sent into the formation at user-defined frequencies between 5 Hz and 1 kHz. This magnetic field, known as the primary field, attenuates with increasing distance from the transmitter. The primary field induces a current in conductive formations (above). This current gen-erates an opposing secondary electromagnetic field, such that the total field decreases in ampli-tude with decreasing formation resistivity.7 An array of four sensitive induction-coil receivers is stationed in the receiver borehole to detect the primary electromagnetic field generated by the transmitter and the secondary electromagnetic field generated by the induced currents.8

A dataset may span 30 to 60 receiver stations and frequently constitutes several thousand mea-surements. The strength of the secondary signal is quite small, so to reduce the signal/noise ratio, the incoming signals are stacked and averaged over several hundred cycles per station. Depending on the amount of averaging and the transmission frequency, transmitter logging speeds may range from 600 to 1,520 m/h [2,000 to 5,000 ft/h], and a typical deployment requires roughly 12 to 30 hours of field recording for a vertical section of 300 m [1,000 ft]. The distance covered by the four evenly spaced coils in the receiver array helps reduce logging times by spanning a wide interval for each station.

Assessing Critical Survey ParametersNo two crosswell EM surveys are the same—each is influenced by local conditions such as casing characteristics, distance between transmitter and receiver wells, formation conductivity and wellbore deviation—all of which can affect the quality of signals detected at the receiver. The interval to be logged, logging speed and transmit-ter frequency can also affect the signal and its response to various formation parameters. For this reason, every survey is designed to account for the unique combination of constraints imposed by any given well pair.

In all but the most competent of formations, one very common survey constraint is caused by steel casing. Electromagnetic signals experience severe attenuation as they diffuse across steel casing.9 Attenuation increases with casing conductivity, casing magnetic permeability and transmitter frequency. Steel casing therefore limits the range of frequencies available for a crosswell EM survey. Such casing, which may be eight orders of magnitude more conductive than the surrounding formation, hinders the transmis-sion and reception of high-frequency signals; this constrains the ability to resolve the conductivity distribution in the interwell region.10

Other components of the casing string can also degrade crosswell EM data. Additional metal present in collars and centralizers further increases the casing effect and complicates the measurement. When the positions of these com-ponents can be determined prior to or during the survey, their effects can be nearly eliminated by careful sensor positioning and monitoring. Fortunately, DeepLook-EM logging tools can detect the locations of collars and centralizers to help logging crews recognize and avoid those areas where the casing effects are greatest.

Because casing effects also limit the interwell distance across which meaningful survey data can be obtained, these effects must be modeled in advance of the survey. To this end, different casing materials have been tested (left). Nonconductive and nonmagnetic fiberglass casing is essentially transparent to the transmitter and receivers. Acquiring data through this casing is equivalent to logging in open hole. On the other hand, carbon steel casing is both highly conductive and highly magnetic, so it poses the worst-case option. When fiberglass casing is not feasible, chromium steel casing may be recommended over the logging interval because it is nonmagnetic and less con-ductive than carbon steel.

> Primary and secondary fields. An alternating current excites the magnetic dipole transmitter coil to send an electromagnetic field into the formation. This primary field induces eddy currents that, in turn, generate a secondary alternating electromagnetic field whose strength is inversely proportional to formation resistivity. The secondary electromagnetic field is detected at the receiver array along with the primary field.

Figure—05

Transmitter well Receiver well

Secondary field

Target

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> Distance versus casing. DeepLook-EM surveys are constrained by casing type and formation conditions.

Figure—06

Transmitter Well Receiver Well Maximum Well Spacing

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Open hole

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Tool positioning must also be considered when designing a crosswell EM survey. Ideally, the logging interval should extend above and below the reservoir by a distance that equals at least half the thickness of the reservoir. In gen-eral, the best response will be obtained when •the anomaly is significantly more conductive

than the surrounding formation (because the tool is inductive, it is more sensitive to conduc-tors than to resistors)

•the reservoir thickness is equivalent to about5% or more of the well-separation distance

•the casing or completions are nonmetallic orthey are metallic but not magnetic

•thespanoftheintervaltobeloggedisatleasthalf as long as the spacing between wells.

To determine which crosswell logging param-eters will best characterize the client’s reservoir, DeepLook-EM survey experts model both the res-ervoir and tool response before committing the equipment to a job.

7. For additional information on the physics of EM acquisition: Brady J, Campbell T, Fenwick A, Ganz M, Sandberg SK, Pereira Buonora MP, Rodrigues LF, Campbell C, Combee L, Ferster A, Umbach KE, Labruzzo T, Zerilli A, Nichols EA, Patmore S and Stilling J: “Electromagnetic Sounding for Hydrocarbons,” Oilfield Review 21, no. 1 (Spring 2009): 4–19.

8. Levesque, reference 2. 9. Kim HJ and Lee KH: “Electromagnetic Fields in a

Non-Uniform Steel-Cased Borehole,” Geophysical Prospecting 54, no. 4 (July 2006): 433–439.

10. Nekut AG: “Crosswell Electromagnetic Tomography in Steel-Cased Wells,” Geophysics 60, no. 3 (May–June 1995): 912–920.

11. DePavia et al, reference 4.

Designing the SurveyWith the outcome of a crosswell survey influenced by so many factors, planning is essential for opti-mizing logging parameters such as frequency, logging speed and traverse depths. More impor-tantly, to determine whether survey objectives can be met, this exercise is required for quantifying the sensitivity of a measurement to various reservoir conditions such as formation layering, movement of fluid-injection fronts and effects of fractures and faults.11 Only after modeling and simulationshaveconfirmedthatasurveyisfeasi-ble will the data be acquired and processed.

A quicklook evaluation to determine whether crosswell EM measurements will be sensitive to a given client objective is performed using ToolPlanner software (above). Parameters for building a basic layer-cake structure of the reser-voir and adjacent formations, along with borehole deviation and completion information for the transmitter and receiver wells, are entered into the ToolPlanner program. The transmitter and receiver wellbores are placed in the reservoir model by specifying the depth and deviation at

which they intersect the top of the reservoir. The program then builds a basic 2D model of the reservoir area surrounding the wells.

A display of the simplified reservoir modellets users test various scenarios to simulate con-ditions the client wishes to characterize. The program calculates the sensitivity of the survey to each scenario and recommends survey parame-ters and logging sequences or further simulation requirements. Sensitivity to each scenario is ranked on a scale from 1 to 10, and a sensitivity map is generated to show how tool response var-ies depending on where the resistivity changes

> The ToolPlanner interface. From this simplified reservoir model, DeepLook-EM survey designers can model basic scenarios to simulate a wide range of reservoir conditions.

Figure—07

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44 Oilfield Review

occur between the wells. The quicklook model allows survey designers to rapidly determine whether they should undertake more-detailed modeling. If this initial evaluation shows prom-ise, then in-depth modeling and simulations will

be conducted to more closely approximate and assess the outcome of each scenario.

In-depth modeling begins by building a geo-logical model of the area. The more structurally correct the model, the more accurate the sensi-

tivity response will be. Relevant field data are captured, including seismic lines, wellbore- deviation surveys and resistivity and gamma ray logs from offset wells, along with geological infor-mation such as stratigraphic features, horizon tops, and fault location and displacement data. Ideally, survey designers seek to model the geology within a radius of twice the distance between wells.

This information is incorporated into a struc-tural model using Petrel simulation software. Having established the geometry of the horizons of interest, modeling experts populate the structural model with well log resistivity measurements. Geostatistical interpolation is then used to help produce a baseline 3D resistivity volume of the proposed survey region (left).

The baseline model is imported into a DeepLook-EM simulation package and sensitivity and image reconstruction studies are performed (next page, top). The sensitivity study evaluates the survey’s capacity to detect formation resistiv-ity changes in response to different reservoir scenarios, such as moving fluid-injection fronts, formation layering, fractures or faults—plus the unique parameters associated with each scenario. During this step the electromagnetic field is computed for all combinations of trans-mitter-receiver pair locations. Here, 2D forward modeling is performed for the baseline and again for each of the various scenarios.12

The scenario response is compared with the baseline, and survey designers look for resistivity differences in each case to determine whether a given scenario generates changes that can be measured using the survey parameters specified (left). If the sensitivity is sufficient to recognize differences in electromagnetic fields between a scenario and base model, an inversion is per-formed on the scenario data to estimate the ability of the imaging process to reconstruct changes in resistivity distribution between the two wells.13

These response models serve as a framework for the final inversion and interpretation that take place after the crosswell survey is run. Once the actual field data are acquired, an image of the resistivity distribution between wells is gener-ated through a nonlinear data-inversion process. This process employs an iterative procedure to update and minimize the differences between the measured data and the modeled responses in a least-squares sense. The resulting resistivity distributions are used to track fluid movements between the wells.

> Resistivity volume for prejob planning. The baseline resistivity model of the formation serves as a basis for geometry correction and high-frequency data inversion. The baseline model is constructed using conventional resistivity logs that have been acquired in offset wells prior to conducting the DeepLook-EM survey. These log data, along with structural information derived from seismic surveys, are integrated and smoothed using a geostatistical program to create a 3D resistivity volume that covers the depths of interest.

Figure—08

123456789101112

Deep resistivity,ohm.m

> Scenario model. Tool responses to various scenarios are modeled to determine the survey’s capability for sensing and ultimately resolving features in the subsurface. A resistivity image models the response to a waterflood scenario (left). If the response differs significantly from the baseline model, an inversion is performed on the scenario data to estimate the ability of the imaging process to reconstruct the resistivity changes between the two wells (right).

Figure—10

Inversion-howwell are thescenariosreconstructedwith certainparameters

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12. Forward modeling is used to determine the general response of many electromagnetic logging measurements. This technique was developed to determine the value that a given sensor would measure in a specific formation environment by applying a set of theoretical equations for the sensor response. Forward modeling is also used for interpretation, particularly in horizontal wells and complex environments. The set of theoretical equations constituting forward models can be 1D, 2D or 3D. The more complex the geometry, the greater the number of factors that must be modeled, which results in longer computing times.

13. For more on the DeepLook-EM inversion process: Gao G, Alumbaugh D, Zhang P, Zhang H, Levesque C, Rosthal R, Liu J, Abubakar A, and Habashy T: “Practical Implications of Nonlinear Inversion for Cross-Well Electromagnetic Data Collected in Cased-Wells,” SEG Expanded Abstracts 27 (2008): 299–303.

For a different perspective on inversion in the oil field: Barclay F, Bruun A, Rasmussen KB, Camara Alfaro J, Cooke A, Cooke D, Salter D, Godfrey R, Lowden D, McHugo S, Özdemir H, Pickering S, Gonzalez Pineda F, Herwanger J, Volterrrani S, Murineddu A, Rasmussen A and Roberts R: “Seismic Inversion: Reading Between the Lines,” Oilfield Review 20, no. 1 (Spring 2008): 42–63.

Imaging Fluid Flow on a Reservoir ScaleUneven sweep, or water override, was occurring in a prolific UAE field operated by Abu Dhabi Company for Onshore Oil Operations (ADCO). To understand how water injection was affecting production, the operator needed to monitor sub-surface waterflood movements between wells. However, lateral coverage using conventional well logging suites was not sufficient for monitor-ing interwell fluid flow, and neither was the vertical resolution provided by seismic surveys. These survey limitations impeded stochastic flow simulations and overall reservoir modeling that ADCO used to predict water-front dimensions.

DeepLook-EM profiles of well pairs were obtained between a set of three dedicated observer wells having chromium steel comple-tions. A baseline survey conducted before injection established a basis for comparison to enable tracking of changes in the interwell space. Survey images obtained six months after the baseline indicated successful waterflooding from the horizontal injector well in the direction of observer Wells 1 and 3, with a large volume of water swept along a preferential east–west flow direction (right). This flow preference was fur-ther confirmed by logs from an RST reservoir saturation tool.

At the time of the six-month survey, water had not migrated to the pair of observer Wells 2 and 3. A subsequent survey has recently been com-pleted, providing still more detail on the progress of the waterflood. The results will be used to develop a more-accurate flow-simulation model for reservoir engineering. This information will guide ADCO engineers and geologists in their strategy for placement of new injectors and

> Feasibility workflow. Tool responses to various reservoir scenarios must be tested and verified before conducting a DeepLook-EM survey.

AcquisitionPlanning

Figure—09

ModelPreparation

Petrel Model DeepLook-EM Simulation and Processing Software

Prepare initialbase resistivitymodel fromwell log data

Load model toDeepLook-EMsoftware tocreate base model

Create scenarioof expectedfield conditionor feature usingbase model

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Determine howwell thescenarios arereconstructedusing specifiedparameters

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> Time-lapse survey images. DeepLook-EM surveys (right) conducted six months after a baseline survey (left) show a significant water sweep (purple zone) running preferentially from east to west between Wells 1 and 3.

Figure—11

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infill development wells while helping them fine-tune performance of existing injectors for optimal recovery.

Waterflood MonitoringFor several years, seawater was pumped into peripheral water injectors to maintain reservoir pressure in an oil field in Saudi Arabia. There, injected water replaces produced oil through a bottom-up process. Over time, this water has moved upward to break through in some of the producer wells. The distribution of injected water is variable, owing to heterogeneities in this car-bonate reservoir.14

Water-cut measurements at the producer wells have been used to track the progress of the water front over time; however, this method does not define the shape of the front between the wells. In one part of this giant field, resistivity measure-ments obtained by conventional wireline logs indicated two producing wells were nearly watered out. Because the resistivity distribution between the wells was unknown, the fluid saturation in the interwell region could not be determined.

The injected water is typically obtained from seawater or recycled produced water; in either case, the salinity of the injected water hovers around 60,000 ppm. Fluid-swept zones can some-times be characterized by a drastic change in formation resistivity, typically from 30 to 50 ohm.m to less than 2 ohm.m, depending on the saturation and porosity. There are also corresponding changes in density, but only minor changes in seis-mic velocity. A high resistivity contrast between injected water and oil in the reservoir would likely be apparent in deep-reading electromagnetic pro-files. Crosswell survey results would also enable the association of formation resistivity with fluid saturation.

After assessing feasibility, Schlumberger con-ducted a DeepLook-EM crosswell survey between producing wells 845 m [2,772 ft] apart on the west flank of the reservoir. The survey obtained several thousand data points at a frequency of 100 Hz. The processed survey indicated that the resistivity dis-tribution differs significantly from the interpolated baseline cross section based solely on presurvey well logs (above left).

Although conventional resistivity logs indi-cated the near-wellbore section was essentially watered out, the DeepLook-EM survey revealed areas of variable sweep efficiency between the wells.

> Resistivity profile between wells. Low resistivity (blue shading) is a typical characteristic of shale layers and steam-swept zones; at the other end of the spectrum, higher resistivities (orange to red) are a characteristic of unswept oil sands. Intermediate resistivities (green) represent transition zones between steam and oil. The DeepLook-EM survey imaged an abrupt boundary midway between wells—the resistivity changes from 2 ohm.m in the depleted zone to more than 50 ohm.m in the unswept zone. The low resistivity most probably results from replacement of oil by formation water and steam condensate injected from a well to the left of this survey.

Figure—12

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> A big difference. The baseline model (top panel ) inferred from conventional resistivity logs seems to indicate the reservoir interval between the wells is watered out, and the conductive injected fluid (blue area, top of the section) is sweeping the reservoir. By contrast, the inverted DeepLook-EM resistivity image (middle panel ) shows a higher-resistivity range (green to yellow) in the same section, indicative of oil. The resistivity differences between the baseline and inverted data (bottom panel ) indicate the efficiency of the waterflood is variable in an area known for higher porosity.

Figure—13

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> Time-lapse crosswell seismic monitoring. Crosswell seismic tomography was used for mapping miscible CO2 between an injector and producer in an enhanced oil recovery program. The display shows high-resolution crosswell seismic reflection data (black traces) overlying color-coded velocity-difference data. This color scale helps geoscientists track changes in seismic velocity between a baseline preinjection survey and a later survey conducted nine months after CO2 injection commenced. The zone in the center (yellow and red) experienced a greater velocity decrease than surrounding layers (blue and green), indicating a potential CO2 accumulation. In this case, the operator concluded the CO2 sweep was not uniform between the injector and producer wells.

Figure—14

Distance between wells, ft2000 600400

Injector

Reservoirinterval

Producer

50 ft

0

5

10

Velocitydecrease, %

Imaging Steam Fronts The operator of a heavy-oil field in the San Joaquin Valley of California, USA, uses steam- drive to enhance production in several reservoirs. Reservoir engineers are applying a cyclic steam-injection technique, in which a well is taken off production and used to inject steam into the res-ervoir. The well is shut in for several days during what is known as a “soak period,” which gives the steam time to disperse and thin the oil. The well is then opened and the thinned oil is produced.

The operator sought to better understand how heat is distributed through the reservoir after steam is injected. However, the field, which encompasses approximately 20 mi2 [52 km2], is known for steeply dipping and complex geology. This environment made it difficult for the operator to determine the vertical and radial distributions of steam around each injector.

The approach proposed by Schlumberger involved imaging the reservoir’s resistivity response to steam injection. Induction logging measurements show that resistivity values typically decrease by 35% to 80% after steam is injected into a reservoir. This response is influenced by two factors: As tem-perature increases, rocks become more conductive, or less resistive; just as important, the response will be seen when high-resistivity oil is replaced by lower-resistivity salt water.15 The sensitivity of DeepLook-EM surveys to resistivity changes made this crosswell technique an obvious choice for tracking the distribution of injected steam and iden-tifying swept zones in reservoirs. Using crosswell EM measurements, the operator would be able to infer structure, temperature distribution and resid-ual saturation of affected reservoir volumes.

A baseline model for data inversion was con-structed during the planning stage. This model incorporated resistivity logs obtained by conven-tional wireline tools when the wells were first drilled and before they were cased or subjected to steam injection. Once the feasibility of the survey had been determined for mapping the steam-affected zone between wells, the DeepLook-EM transmitter and receiver were deployed in two wells. A 750-ft [229-m] interval was logged at a rate of 20 ft/min [6 m/min], then

the transmitter was lowered to the base of the traverse and the receiver array was repositioned.This process was repeated until all transmitter and receiver stations had been logged. The opera-tion was completed in about 12 hours.

The resulting survey clearly profiled depleted and unswept zones within the reservoir (previous page, bottom). Data from this crosswell survey enabled the operator to map the effects of steam saturation, guided field development plans and improved reserves estimates.

Expanding Deep-Reading CapabilitiesOnce a well is drilled and completed, operators must concern themselves with the region beyond the wellbore—particularly that area of the reser-voir between producing wells. Beyond a few meters from the wellbore, however, there is a dearth of detailed information, owing to a trade-off between depth of investigation and vertical resolution: Most surveys can obtain one, but not both. Operators have adapted to this short coming in characterizing the reservoir. Until recently, they built most reservoir-scale models upon a broad foundation of surface seismic data, using geostatistics to extrapolate log-derived formation properties beyond the individual wellbore and into the interwell region.

Deep-reading measurements obtained through DeepLook-EM crosswell surveys provide operators with greater petrophysical control in their reser-voir models. Crosswell specialists are now working to expand the scope of services beyond resistivity measurements. With the recent acqui-sition of Z-Seis Corporation, Schlumberger is adding seismic measurements between wells to its range of deep-reading capabilities. Obtained with an acoustic source in one wellbore and a receiver array in another, borehole-seismic crosswell surveys image the interwell region in greater detail than conventional surface-seismic surveys (above). In time-lapse applications, these surveys can distinguish changes in velocity of just a few percent to help reservoir engineers identify and monitor the movement of steam, gas and CO2.

The capability to identify and monitor fluid fronts using crosswell EM measurements is thus complemented by this seismic technique, which can delineate structure and monitor gas movement in the reservoir. The detailed reservoir-level information obtained from deep-reading measure-ments gives geoscientists and reservoir engineers greater insight into how fluids travel and interact, which will ultimately determine the course of field management and production. —MV

14. Wilt M, Donadille JM, AlRuwaili S, Ma SM and Marsala A: “Crosswell Electromagnetic Tomography in Saudi Arabia: From Field Surveys to Resistivity Mapping,” presented at the 70th EAGE Conference and Exhibition, Rome, June 9–12, 2008.

Marsala AF, Ruwaili S, Ma SM, Ali Z, Buali M, Donadille JM, Crary S and Wilt M: “Crosswell Electromagnetic Tomography: From Resistivity Mapping to Interwell Fluid Distribution,” paper IPTC 12229, presented at the International Petroleum Technology Conference, Kuala Lumpur, December 3–5, 2008.

15. Wilt et al, reference 3.

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