Surface deformation measurements have been used for years in oilfields to monitor production, waterflooding, waste injection, steam flooding, and Cyclic Steam Stimulation (CSS). They have been proven to be a very effective way to monitor the field operations and save money for operators wishing to avoid unwanted surface breeches, casing failures and excessive subsidence due to production. This paper demonstrates that more information can be extracted from surface deformation measurements by inverting the surface deformation for the volumetric deformation at the reservoir level, so the aerial distribution of volumetric deformation can be identified. First, a poroelastic model is presented to calculate the deformation due to the volumetric change in the reservoir. Then, a linear geophysical model is formulated to invert for the reservoir volumetric deformation from the measured surface deformation (or tilt). Constraints are added into the procedure as necessary to better resolve the inversion problem. After each inversion, the theoretical surface deformation (displacement, tilt, reservoir compaction and volumetric strain) can be calculated from the inverted volumetric deformation distribution which best fits the measured deformation data (or tilt) at the surface. The technique of mapping fluid flow using surface deformation was applied to real data from a cyclic steam injection project. Introduction Through the decades, many oil companies and individual researchers have studied reservoir compaction and its associated surface subsidence. Two techniques are used: forward modeling for prediction and direct measurements (or monitoring). The forward modeling includes numerical analysis using Finite Element Method and analytical or semi-analytical analysis. The most common monitoring techniques used in oil and gas fields are:Optical instrument leveling surveys or Global Positioning System (GPS) surveys[1]. These are conducted continuously or periodically to determine changes in position of monuments across the field.Interferometric Synthetic Aperture Radar (InSAR) [1]. This enables mapping of surface displacement along the satellite line of sight over large areas.Tiltmeter-based surface deformation monitoring[2,3]. High precision tiltmeters are placed near the earth's surface to measure the displacement gradient (tilt) induced by field operations such as fluid injection and production. Each technique has advantages and disadvantages, and in some cases two or even all three can be used in combination to get the necessary combination of precision, spatial coverage and temporal resolution. In the case history shown here, only tiltmeter data is used and the inversion process calculates and compares measured and theoretical tilt, but only minor changes are needed to perform the same calculations with displacement.

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Unlocking shale gas has been extremely successful during the last decade. Nevertheless, new challenges will continuously arise. One of the most pressing current issues is to know the stimulated reservoir volume (SRV), the part of the reservoir that actually received fracturing fluid. The most widely used technology for estimating SRV is downhole microseismic mapping. Under many conditions, it yields reasonably accurate/reliable information about SRV (Mayerhofer et al. 2008). Unfortunately, it requires an observation well in close proximity to the treatment well in which to place the geophone string used to sense the small seismic signals. This requirement often makes it impossible to provide an SRV estimate for many of the hydraulic-fracture treatments that could benefit from this information. This paper presents an alternative to the downhole-microseismic SRV mapping. A new method, stimulated reservoir characterization (SRC), is based on surface microdeformation measurements obtained with a precision surface tiltmeter array. Tiltmeter-based hydraulic-fracture diagnostics have been successfully used for more than two decades on more than 10,000 hydraulic-fracture treatments. SRV is typically calculated for highly jointed shale reservoirs or coal seams in which injected fluids can inflate a myriad of interconnected hydraulic fractures in two or more dominant orientations. The measured surface deformation is the superposition of all the deformation fields resulting from the inflation of each individual fracture. This makes the deformation-based approach for SRV estimation a convoluted process and requires much more complex tilt-data analysis than does a simple planar hydraulic fracture. This paper describes a new technique for tilt-based SRV estimation, which is capable of resolving spatial distribution, orientations, and volume percentages of the major components of a fracture network. Hence, the new technique allows not only an insight into the areal penetration of treatment fluids into the reservoir, but also an understanding of how multiple joint sets, each with unique orientations, are actually accepting the injected fluids and proppant. This paper includes synthetic data examples and SRV results derived by applying the new technique to a hydraulic-fracture-stimulation project in the lower Eagle Ford shale formation.

Surface deformation measurements have been used for years in oilfields to monitor production, waterflooding, waste injection, steam flooding, and Cyclic Steam Stimulation (CSS). They have been proven to be a very effective way to monitor the field operations and save money for operators wishing to avoid unwanted surface breeches, casing failures and excessive subsidence due to production. This paper demonstrates that more information can be extracted from surface deformation measurements by inverting the surface deformation for the volumetric deformation at the reservoir level, so the areal distribution of volumetric deformation can be identified. First, a poroelastic model is presented to calculate the deformation due to the volumetric change in the reservoir. Then, a linear geophysical model is formulated to invert for the reservoir volumetric deformation from the measured surface deformation (or tilt).Constraints are added into the procedure as necessary to better resolve the inversion problem.After each inversion, the theoretical surface deformation (displacement, tilt, reservoir compaction and volumetric strain) can be calculated from the inverted volumetric deformation distribution which best fits the measured deformation data (or tilt) at the surface. The technique of mapping fluid flow using surface deformation was applied to real data from a cyclic steam injection project. Introduction Through the decades, many oil companies and individual researchers have studied reservoir compaction and its associated surface subsidence. Two techniques are used: forward modeling for prediction and direct measurements (or monitoring). The forward modeling includes numerical analysis using Finite Element Method and analytical or semi-analytical analysis. The most common monitoring techniques used in oil and gas fields are:Optical instrument leveling surveys or Global Positioning System (GPS) surveys[1]. These are conducted continuously or periodically to determine changes in position of monuments across the field.Interferometric Synthetic Aperture Radar (InSAR) [1]. This enables mapping of surface displacement along the satellite line of sight over large areas.Tiltmeter-based surface deformation monitoring[2,3]. High precision tiltmeters are placed near the earth's surface to measure the displacement gradient (tilt) induced by field operations such as fluid injection and production. Each technique has advantages and disadvantages, and in some cases two or even all three can be used in combination to get the necessary combination of precision, spatial coverage and temporal resolution.In the case history shown here, only tiltmeter data is used and the inversion process calculates and compares measured and theoretical tilt, but only minor changes are needed to perform the same calculations with displacement.

Surface-deformation measurements have been used for years in oil fields to monitor production, waterflooding, waste injection, steam flooding, and cyclic steam stimulation (CSS). They have been proved to be a very effective way to monitor field operations and save money for operators wishing to avoid unwanted surface breaches, casing failures, and excessive subsidence because of production. This paper demonstrates that more information can be extracted from surface-deformation measurements by inverting the surface deformation for the volumetric deformation at the reservoir level using the inversion techniques from the literature, so that the areal distribution of volumetric deformation can be identified. This leads to a better understanding of reservoir behavior and also provides additional data for integration into coupled reservoir simulation modeling. This paper shows the results of mapped reservoir volume changes from two cyclic steam injection projects using tiltmeter-based surface deformation measurements.