Well log data provide a variety of high-resolution measurements of the formation but only in close proximity to the wellbore. Surface seismic, gravity, and electromagnet-ics technologies are measurements that can image large lateral areas between wells but with limited vertical resolution compared with well log data. To close this “deep-reading gap” by providing subsurface data between the two groups of existing measurements, the oil and gas industry requires technologies that can provide higher-resolution measurements than surface data and deeper investigation than well logs Vertical seismic profiling (VSP), borehole

gravity, cross-well pressure pulses, tracers, cross-well seismic technology, cross-well electromagnetics, and microseismicity are among the technologies under consideration that may provide information to close the deep-reading gap. However, the first four ap-proaches have their own limitations: t� 741� FOUBJMT� HBUIFSJOH� 4FJTNJD� EBUB� CZ�

placing receivers in a borehole at several successive depths and directing source energy to it from a fixed point on the surface. VSP is a mature technology that can provide greater resolution than surface seismic data can and deeper formation penetration than well logs can but is still limited to the wellbore vicinity.

t� $SPTT�XFMM� QSFTTVSF� QVMTFT� JT� B� NBUVSF�technology for determining reservoir con-nectivity and compartmentalization but less applicable for fluid front monitoring between wells.

t� 5SBDFST�IBWF�CFFO�VTFE� JO� UIF�PJM� BOE�HBT�industry for decades—whether radioac-tive or chemical, gas or liquid—to deter-mine residual oil saturation. A limitation common to all conventional tracer methods is that they don’t yield direct information until detected in production wells, which could take months or even years after in-jection.

Cross-well electromagnetics, cross-well seismic and microseismic technologies are three relatively new commercial technologies that have seen marked improvement in recent years, generating increasingly robust results in imaging the subsurface.

In recent years, more oil and gas reserves have been added to the world’s proven hydro-carbon endowment through reserves growth than through discovery. Opportunities for significant discoveries in simple geological settings or hospitable locales have greatly di-minished. With the consequent greater emphasis on improving oil and gas recovery rates in existing fields, it is more critical than ever to obtain a clear picture of the subsurface to optimize oil and gas reservoir management. !e key parameters in imaging the subsurface are the quality of resolution and the diversity of measurement methods.

Illuminating the reservoir. Deep-reading measurements bridge the gap between log- and surface-scale measurements.

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- Deep-reading technologies image the interwell space at higher resolution and provide a new scale of characterization that was previously not possible.

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Cross-well seismic data acquisition. The source is moved at regular intervals and then re-leased at each position. The signal is recorded by an array of receivers placed in another well.

!is article will outline the technologies, data acquisition methods, and application of these three promising deep-reading technologies in respective case studies. !e integration of the technologies and how they complement each other for enhanced reservoir description in the interwell space also will be described.

- Gathering, processing and analyzing surface seismic data is a proven, key technology for imaging the subsurface in hydrocarbon explo-ration and production. !e vertical subsurface resolution is, however, limited. VSP acquisi-tion improves vertical seismic resolution by placing the receivers in the wellbore. Short-ening the ray path and limiting the signal at-tenuation greatly improves resolution, but the

subsurface coverage is limited to the wellbore vicinity. However, placing both receivers and source energy in multiple individual wells to image the interwell space can significantly enhance both vertical resolution and horizontal sub-surface coverage. Cross-well seismic data provide vertical resolution imaging of the reservoir layers of up to an order of magnitude better resolution than surface seismic data. Both direct arrival and reflected information can be processed to provide a detailed subsurface image of the reservoir or zone of interest. !e cross-well seismic data improve the understanding of the reservoir geometry and rock properties from the reflection seismogram and details of fluid migration, including steam chambers,

from both the velocity tomography and re-flection seismogram. By repeating acquisition of cross-well seismic data, it is possible to map in detail fluid migration over time.

Source!e source for acquiring cross-well seismic data can be piezoelectric or magnetically clamped. Piezoelectric source technology has been in use for more than a decade and con-tinues to be a very good source for generating high frequencies (100 Hz–2,500 Hz). !is approach uses multiple piezoceramic radiator elements coupled into several radiating arrays that are swept by surface-programmable source controllers. !ese arrays have radia-tion patterns optimized for cross-well seismic usage. Its high duty-cycle rates and the fact that the piezoelectric source does not require clamping in the wellbore mean that data can be acquired rapidly without having to stop the source at each acquisition level. A more recent design for a magnetically clamped source is intended specifically for greater-attenuating formations and longer well offsets. Operating at a lower frequency range (30 Hz–800 Hz), this source produces about 20 dB more amplitude than the piezo-electric source. It also produces direct com-pressional and shear energy waves, allowing for advanced wavefield analysis. Both sources are borehole friendly and distribute the energy produced over an area in the wellbore that is large enough not to cause damage to the casing or cement. !e cross-well seismic data acquisition is conducted by deploying an array of receiv-ers in one well and a source in another well. !e source is moved up at regular intervals, emitting a controlled pulse measured by the receiver array in the other well and then moved to the next location. !e goal is to il-luminate as much of the subsurface as possible through the aperture of the source locations and range of receiver locations. In a typical ac-quisition both source and receivers are placed at the base of the reservoir or zone of interest. !e source is raised and pulsed until the entire zone is imaged at that receiver station. !e receivers are raised to the next station, the source is lowered to the base of the reservoir, and the pulsing process is repeated. Once complete, several fans of raw data are acquired and processed. !e base of the reservoir image is filled in using reflection tomography.

Case StudyIn a case study involving the Cretaceous McMurray formation in the Western Canadian Sedimentary Basin, efforts were

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made to identify thin shale laminations that were potential permeability barriers in the reservoir and to provide time-lapse evalua-tion of steam movement in a steam-assisted gravity drainage project.Six cross-well seismic sections were acquired to provide a high-resolution characterization

of the McMurray reservoir. !e processed data provided reflection images at a resolu-tion of 2 meters or better in the McMurray formation, detected lateral and vertical facies changes and shale stringers, and indicated severe reservoir heterogeneities, all of which helped to more accurately characterize the

reservoir and better predict reservoir perfor-mance under thermal operations. A compari-son with conventional seismic data shows the substantial improvement of the cross-well seismic data resolution that makes it possible to interpret previously unknown faults.

- Until now, logging has measured only within the borehole and near-well environment, imaging only the rock surface in terms of pro-viding detailed reservoir information. Seismic imaging provides guidance for interpolating information between and beyond wells, but it delivers relatively low vertical resolution, and there are often uncertainties in the tie between surface seismic and well data. Cross-well electromagnetics expands the scale inves-tigated by resistivity logging to the reservoir level for monitoring fluid distribution and movement, which are critical factors for suc-cessful reservoir management. !e cross-well electromagnetics acquisition system consists of a transmitter tool deployed in one well and a receiver tool deployed in a second well. With such a system, the two wells can be located up to 1,000 meters apart, depend-ing on how the wells were completed and the formation and resistivity contrasts. A global positioning system is used for synchro-nized communication of the tools, which are conveyed on standard wireline equipment. !e 9.88-meter transmitter antenna used in the source borehole is a vertical-axis, magnetically permeable core wrapped with several hundred turns of wire and driven to broadcast a continuous sinusoidal signal at user-defined frequencies that depend on the particular borehole environment. !e trans-mitter signal induces electrical currents to flow in the formation between the wells. !e currents, in turn, induce a secondary magnetic field related to the electrical resistivity of the rock and fluids where they are flowing. At the receiver borehole, induction coil re-ceivers detect the electromagnetic field gen-erated by the transmitter (primary field) as well as the magnetic field from the induced currents (secondary field). !e receiver tool comprises an array of four receiver coils, which reduce logging time by simultaneously recording the signals. For each receiver station, the transmitter in the other well is moved between the depths of interest while continuously broadcasting. To reduce noise, the incoming signals are averaged several hundred times per station. Depending on the amount of averaging

Comparison of cross-well seismic data and surface seismic data. The comparison illus-trates the sub-stantially increased cross-well seismic resolution.

Cross-well electromagnetics data acquisition. The transmitter traverses the logging inter-val while continuously propagating the primary electromagnetic field. The receiver col-lects the primary and secondary (formation) fields.

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and frequency of operation, the transmit-ter logging speed ranges from 600 to 1,520 meters per hour. Once a complete transmitter traverse, or profile, is collected for a receiver position, the receiver tool is repositioned and the process repeated. Logging is conducted at the locations of the moving transmitter. !e operation is controlled via laptop computer at a logging surface station and requires a wireline field unit and mast or crane at each of the two wells. Inversion of cross-well electromagnetic data to generate interwell resistivity images follows a workflow that integrates existing res-ervoir information. !e field data is compiled in a seismic-to-simulation workflow to create a field model of possible fluid movement scenarios. Simulation based on the scenarios then ensures that the appropriate measure-ment sensitivity is applied.

Following the cross-well electromagnetic survey, the interwell resistivity distributions are exported back to the reservoir model for data integration and interpretation to provide critical insight for tracking fluids such as water and steam, detecting bypassed pay and characterizing the reservoir.

Pilot studyPeripheral waterflooding at a large onshore oil field in the United Arab Emirates has typi-cally bypassed layers with lower permeability. To address this issue and develop a more opti-mized reservoir management strategy, a pilot was established in 2007 to design and test strategies for recovering any remaining oil. !e pilot features three chrome-cased ob-servation wells surrounding a horizontal water injector, which was completed in a number of reservoir units. Five thousand barrels of high-salinity brine are injected into this formation

daily with the expectation that breakthrough will occur in the producers within seven years. !e pilot’s objectives are to determine sweep and conformance in the reservoir to estimate recovery and improve waterflood design. !e benefits of cross-well electromagnetics moni-toring are that the fluid movement can be tracked between wells and the flow observed within months, or long before it would be observed in any wells. !is allows adjusting parameters to optimize performance during the pilot operation. Baseline field data were collected in 2007, and time-lapse data sets were collected in June and December 2008, corresponding to six months and one year, respectively, after the initiation of water injection. !e pilot’s injected brine as of time lapse 2 has not broken out of the section as peripheral flooding seems to have done in the past and continues to do so even now. !e results are being used to develop a static and dynamic simulation model of the pilot area. !e dynamic models will be used to predict the pilot behavior and to help develop designs for other pattern waterflood pilots within the field. Microseismicity provides continuous real-time information about the stress changes in the reservoir away from and between wells that are caused by injection and production operations. Induced fractures can be mapped, and these changes in both space and time provide a unique insight into the effect of pressure change on the lithologic framework and associated movement of reservoir fluids. In connection with the injection of prop-pants for hydraulic fracture monitoring, mi-croseismicity is used to monitor the induced fractures in real time. To monitor the res-ervoir when injecting H2S or CO2, induced microseismicity can potentially indicate if the cap rock is being breached or faults are being reactivated that would create a leakage path to the surface. When hydrocarbons are being produced from a reservoir, induced mi-croseismicity may indicate which part of the reservoir has pressure communication and is producing and thus may highlight potential areas of bypassed hydrocarbons. Microseismic events generally have low magnitude (Mw), from -4 to 1. Record-ing microseismicity requires a microseismic sensing system that can consistently record high-fidelity, low-noise data and maximize the located event population. Surface acquisi-tion using sensors on the surface and buried in shallow holes is possible, especially onshore.

Time-lapse pilot results. The time-lapse cross-well electromagnetic surveys clearly image a low-resistivi-ty volume associated with the water injection. The cross-well electromag-netic images indicate that the injected water has spread approximately halfway between the wells but has largely remained within the injected reservoir units.

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The final processed microseismic events in relation to the well trajectory. The red dots are the first phase of the well stimulation, the green dots are the second phase, and the yellow dots are the third and final phase.

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However, issues with a noisy surface envi-ronment and with too great a distance to the reservoir to record the low-magnitude mi-croseismic events may limit the application of these two deployment methods. To bring the sensors closer to the reser-voir to better record the low-magnitude mi-croseismic events, the sensors can be posi-tioned in a well. But this requires placement of a microseimic sensing system that does not interfere with field operations and that mitigates any risk to well integrity. When monitor wells are available for installing microseismic sensing systems, this is not an issue. !e microseismic sensing system can either be cemented in place for abandoned wells or left for short- or long-term moni-toring and then retrieved. It is also possible to locate the sensors behind casing, which will influence the well completion because the casing diameter increases and the casing size has to be constant down to the sensor deployment depth. !e limited availability of monitoring wells, especially in the oil field environ-ment and particularly offshore, has meant that without an active monitoring well solution, microseismic monitoring most likely will remain a niche technology for reservoir management. However, a tubing-deployed device, released downhole to clamp against the casing for the sensors to be de-coupled from the flow noise, makes record-ing microseismicity during well operation possible. Furthermore, a new design of geo-phones with a tetrahedral arrangement of four sensors at a 109° angle provides both improved signal fidelity and long-term re-liability if one sensor fails. Deployment methods are therefore now available for all types of microseismic acquisitions. To design the microseismic sensing system, the array design is modeled to ensure that the induced microseismicity is recorded.

Case studyTo calculate the location of the recorded mi-croseismic events, source and magnitude, a velocity model also was developed for both compressional (Vp) and shear wave (Vs) parameters using available velocity infor-mation from well logs, surface seismic, and other data sources. Microseismicity was recorded during a three-step acid stimulation job, and the data were processed. A single well was used for monitoring during the acid stimulation, which at the reservoir depth of about 15,000 ft was highly deviated. A two-level, tubing-deployed system that clamped the sensors to

Deployment methods for microseismic sensing systems.

A model of the microseismic array design.

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Measurement integration overview. Results from the three deep-reading technologies complement each other and provide important input to improving reservoir manage-ment.

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Fluid front monitoring within detailed seismic defined reservoir

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Microseismicevents for

verificationof seismicfaults andfractures

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the casing recorded microseismicity during flow in the tubing. Good-quality microseis-mic events were detected and located with effective sensor coupling to the casing and a low noise floor providing reliable results and interpretable microseismic events. 1e development over time of the mi-croseismic events associated with the well stimulation indicates a pathway for the acid fluids away from the reservoir section, reducing the impact of the well stimulation. After 2 months the microseismic sensing system was successfully retrieved, and a new stimulation strategy is being considered. 1e microseismicity highlighted a previously unknown permeable zone, which has been included in the fracture network model and static model.

Because the acquisition equipment is deployed in the well and is more focused on the reservoir than other techniques, deep-reading technologies image the interwell space at higher resolution and provide a new scale of characterization that previously was not possible. Each of the deep-reading tech-nologies can be used to populate the static reservoir model by adding reservoir proper-ties understood at a higher resolution. Cross-well seismic sections provide cali-bration points for the reservoir geometry, highlighting smaller faults and internal het-erogeneities that control the flow of reservoir fluids. Both the fracture network and static

model become more accurate. With the ac-quisition of time-lapse cross-well seismic data, the tomographic velocity model can in many cases be related to fluid changes and used to monitor the fluid front movement and to identify bypassed hydrocarbons. 1e time-lapse data can be used to update the dynamic reservoir model. Cross-well electromagnetics sections provide an apparent saturation measure-ment, which, with the acquisition of time-lapse data, can be used to directly monitor the fluid front. 1e electromagnetics satu-ration measurement is more direct than what is derived from the cross-well seismic images, but it requires sufficient resistivity differences between the hydrocarbons and formation water. Microseismic data provide insight into the geomechanical behavior of the reservoir due to production and injection. 1e mi-croseismic events can provide information about the fractures and be used to update the fracture network model. Pore pressure changes from moving reservoir fluids may also provide insight into the moving fluid front. 1e deep-reading results are complemen-tary. A velocity model is required to accu-rately locate microseismic events. Velocity tomography from the cross-well seismic may provide additional velocity information in the interwell space that is not available from other sources. Microseismic events could indicate fracturing or the activation of smaller faults that may be visible only on

high-resolution cross-well seismic sections. 1e interpretation of the fluid front from any of the three deep-reading technologies becomes more accurate with the detailed reservoir geometry provided by the cross-well seismic section. Fluid front monitoring is important to optimize reservoir production and locate bypassed oil. Cross-well electromagnet-ics provides a direct means of calculating apparent fluid saturation changes, which can be better understood in terms of structurally guided flow and verified by the tomographic velocity changes from the cross-well seismic method and potentially the induced micro-seismicity through changes in pore pressure. If time-lapse cross-well seismic and cross-well electromagnetics data are acquired, both direct and indirect monitoring of the fluid front is possible, allowing an update of a dynamic reservoir model. 1e technologies provide important information individu-ally, but the technologies can also reinforce each other through integration both during data processing and interpretation. 1e final results become more accurate and reliable and can be better verified.

1e three deep-reading technologies covered in this article—cross-well seismic, cross-well electromagnetics, and microseismicity—are varied in their maturity and commercial use. 1rough recent developments, all three technologies may now provide both new measurements and higher subsurface reso-lution. Even though the measurements are different, they can be related to the same res-ervoir model, complement each other, and improve subsurface understanding. With a better understanding of how to interpret and apply these measurements, improved reser-voir management is possible.

AcknowledgmentsENI SpA has kindly allowed the publication of the microseismic data.