Four-dimension reservoir characterization (4DRC) has proven effective for helping manage enhanced oil recovery (EOR) and carbon capture and sequestration (CCS) operations worldwide. Whether the goal is to understand how to efficiently drain a reservoir; plan an infill drilling program; map plume injection and migration of steam, CO, or water flooding programs; or help ensure that CCS projects perform as designed, technologies are available that provide economically viable monitoring programs that enable characterizing reservoir changes over time. By monitoring surface movement over time, measurements can be inverted to interpret what is happening geomechanically at the reservoir level. The surface changes are determined through interferometric synthetic aperture radar (InSAR), global positioning system (GPS), or tiltmeter monitoring, or more comprehensively through a combination of these technologies. The geomechanical subsurface model is produced by expert analysis of the information through an inversion modeling program. The deliverable is a time-series geomechanical reservoir model and dataset that are designed for either direct interpretation or to complement other subsurface modeling programs.
Revealing reservoir-level volumetric change
Precise measurement of surface deformation form the foundation of 4DRC. Interpretation of current activities is produced, and a forward model relates possible reservoir-level deformation sources (i.e. changes of pressure, fluid volume, or temperature) to the predicted surface deformation. This model predicts the deformation (tilt and displacement) at the surface from a given deformation source in a homogeneous poroelastic or thermoelastic half-space.
The reservoir volume being evaluated is then divided into many small elements, which can possess different sizes and may be located at different depths to most closely approximate actual reservoir shapes and orientations. Each element is assumed to undergo constant changes of pressure, volume, or temperature within a defined period of time. The predicted surface deformation due to the total change in the reservoir is a summation of individual deformation caused by volumetric changes at each element used to simulate the reservoir. From this model, a linear geophysical inverse problem can be constructed and then solved to obtain the volumetric strain distribution in all of the reservoir elements.
Reservoir-level volumetric strain analysis
In fracturing, using fluid pressure to spread apart the created fracture faces causes the surrounding rock to deform in characteristic ways. The principle of tiltmeter fracture mapping is to use precise measurements of this fracture-induced rock deformation to infer hydraulic fracture orientation and geometry.
Changes in reservoir volumes, such as those produced
by fluid production and injection and thermal processes such as steam flooding, cyclic steam stimulation (CSS), and steam assisted gravity drainage (SAGD), also generate unique and measurable patterns of deformation. This pattern, or deformation field, propagates from its reservoir source and remains measurable at the earth's surface with InSAR, tilt-meters, and GPS. By solving a geophysical inverse problem, precisely measured surface deformation can be used to back-calculate reservoir-level volumetric deformation so the areal distribution of the volumetric changes at reservoir level can be identified.
Integrated technologies
InSAR is the measurement of the changes in phase from two observations of the same target. If a target has not moved in the time between the two observations, the path length from the satellite to the target is equal for both, and the phase difference is zero. Zero phase difference means no motion. If the target has subsided, the second path is longer than the first, generating a phase shift. By resolving this phase shift, the magnitude of the target motion is measured. This technique allows for the measurement of millimeter accuracy over time and multiple images.
4DRC uses space-borne SAR satellites for continuous and reliable coverage all over the world. The ability to measure surface deformation over very large areas is accomplished by the continuous coverage of repeat radar scenes. No other geodetic technique rivals InSAR's ability to measure deformation over vast areas of the Earth's surface. Integration with other measurement technologies, such as GPS for geodetic control or tilt for the measurement of microscopic motions within focused regions of interest, further enhances the precision of InSAR results.
Global positioning system
The GPS is a satellite-based system that enables precise determination of location anywhere on or above the Earth's surface. Currently, 30 NAVSTAR satellites operate as part of the GPS constellation, enough to ensure that at least eight are in view at any given time from an unobstructed location.
The ability of civilian users of GPS to process and make use of the system has come a long way since its inception and the launch of the first satellite in 1989. In the past 15 years, GPS has enjoyed an explosion in development effort, accuracy, and applications. GPS has become a primary tool for studying a wide range of geophysical phenomena, including, but not limited to, subsidence, volcanic heave, tectonic strain accumulation and release, mass wasting, glacial flow, and other phenomena requiring stable and precise measurements of small motions.
The true power of GPS comes when it is fully integrated with other measurement technologies such as InSAR, tilt, and microseismic.
How it works
Advanced tiltmeters measure their own tilt on two orthogonal axes. As the instrument tilts, a gas bubble contained within a conductive liquid-filled glass cavity moves to maintain its alignment with the local gravity vector. Precision electronics detect changes in resistivity between electrodes mounted on the sensor that are caused by repositioning of the gas bubble. The latest generation of high-resolution tiltmeters can detect tilts of less than one nanoradian.
Conclusions
Taking the surface deformation measurements down to the reservoir level to obtain the volumetric strain, pressure front, or even the thermal front in the reservoir provides a cost-effective way to monitor reservoir-level processes and improve understanding of how different recovery methods work in different types of reservoirs. The results from the surface deformation-based reservoir level monitoring can be made more useful if they are integrated with the production and injection information and possibly other imaging technologies, such as micro-seismic data or temperature measurements, to understand the significance of the induced reservoir changes. They also can be integrated into a coupled reservoir simulation model as a calibration tool to help obtain better history matching and optimize completion, stimulation, and production activities to improve recovery.
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