The potentially severe impact of reservoir compaction on production can be assessed with a geomechanical model calibrated to field data. Yet field data acquisition programs can be quite costly, and optimizing the design of reservoir monitoring equires prior understanding of reservoir behavior. Forward geomechanical models can help here to define the best strategy for field data acquisition, maximizing value for model calibration and other purposes, whilst minimizing costs. This paper illustrates the use of geomechanical models to optimize measurements of vertical displacement of the seafloor (subsidence) for two cases of depletion-induced compaction in stacked unconsolidated sandstone. The first model is for a field with reservoirs buried 3 to 5 km below the seafloor (BSF). In this case, quantitative analysis of subsidence to detect lateral variation in compaction (and thus depletion) of the reservoirs is not feasible, mainly due to the large depth of the reservoirs. However, difference maps of subsidence can reveal areas of relatively high and low cumulative reservoir compaction. Prospects for monitoring and quantitative analysis look much better in the second model, which was for a field with shallow reservoirs buried only 0.5 to 1.2 km BSF. In this case, the geomechanical model shows good agreement between the pattern of lateral distribution of depletion, compaction and associated compaction-induced subsidence. Having demonstrated the feasibility of reservoir monitoring with subsidence data for the second model, its forward-model results were used to define the resolution, design, and timing of seafloor subsidence surveys.
1.1 Reservoir compaction and surveillance
Production-induced depletion in water and hydrocarbon reservoirs leads to deformation, compaction, displacement, and stress change both inside and around the reservoir [1-3]. Potentially serious consequences for production include well damage, permeability reduction, and vertical displacement at the Earth surface (subsidence). Well damage can occur inside the reservoir by e.g. buckling of the casing due to compactioninduced along-well shortening. Wells can also be damaged by compaction-induced fault slip, which is reported to occur mainly above the reservoir [4, 5]. Permeability reduction caused by compaction is typically a few percent of the pre-production value in consolidated rocks like sandstone, but can be tens of percent in poorly-consolidated granular rocks . Subsidence on land is a problem when operating in lowlying areas [7, 8], certainly given observations and recent predictions of sea level rise due to global warming . Subsidence at the seafloor reduces the safety-gap between the average sea level and the base of the platform structure. Large compaction in shallow-buried reservoirs is likely to cause inhomogeneous subsidence, controlled by lateral and vertical distribution of reservoir compaction and by overburden geology. For instance, stepped-subsidence patterns (terraces) could form in case of compaction-induced near-seabed normal faulting. These could destabilize the near-surface sediments and damage production installations at the seafloor. Other effects of reservoir compaction include additional reservoir energy provided by the compaction-induced porosity reduction (compaction drive), earthquakes, and bulk density and acoustic velocity changes in the reservoir and overburden, leading to acoustic impedance changes and timeshifts in reflection seismic [10-12].