oa Undepleted Pockets of Gas Detected Using Coupled Modeling of Subsidence and Reservoir Depletion Processes

An important cause of subsidence is the withdrawal of large volumes of natural gas, oil and water from<br>hydrocarbon reservoirs. Every point of depletion in the subsurface causes a subsidence bowl with a<br>radius corresponding to the depth of the depletion point. The depletion of a large volume in the<br>subsurface leads to a subsidence bowl of which the shape and the size are directly related to the<br>subsurface properties and processes. Besides the amount of hydrocarbons produced and the geometry<br>of the reservoir, relevant features include the subsurface elasticity profile, the compaction coefficient<br>and the strength of the aquifer.<br>Surface subsidence measurements (e.g. Interferometric Synthetic Aperture Radar (InSAR), leveling<br>measurements, or GPS) can be used to better quantify the subsurface parameters and processes.<br>Simple inversion approaches do not usually provide a sensible solution due to non-uniqueness of the<br>solution and the sensitivity of the inverse problem to small fluctuations in the data. We have developed<br>a time-integrated inversion scheme for resolving the spatial and temporal reservoir pressure drop from<br>surface subsidence observations. This inversion procedure is unique because it utilizes all the available<br>prior knowledge including the uncertainty and the correlations within it. Elements of this prior<br>knowledge are the geological model, the reservoir model, the sealing properties of faults, and the acquifer activity.<br>We have applied our inversion method to a highly compartmentalized reservoir in the Netherlands<br>where we used the observed surface subsidence (leveling), the geological model, the reservoir model,<br>and the prior knowledge about possibly sealing faults, to obtain insight into undepleted pockets of<br>natural gas in the reservoir. We used prior models which complied with the measured flux and pressure<br>measurements of the production profile. With these data, we were able to better quantify the amount<br>of compaction in the different compartments in the reservoir and the pressure depletion causing it.<br>The approach followed is essentially a two-step approach: the history match is the first step, and the<br>inversion of subsidence measurement the second. The risk of this approach is that not the complete<br>solution space is being searched. We are currently also developing an approach in which the inversion<br>of production data and subsidence measurements are integrated.