Estimation of interwell connectivity in the case of unmeasured fluctuating bottom hole pressures

Injection and production rates are often the most readily available data in a waterflood. Yousef and coworkers have shown that these data can be analyzed using a method, called the capacitance model (CM), to determine interwell connectivity. The connectivities can, in turn, provide a tool for reservoir characterization and management. The CM gives best results when the number of producers is constant and when the producer pressures (BHPs) are known. If pressures are unavailable (as is often true with legacy assets), they are assumed to be constant over the time period being analyzed. In many cases, however, these conditions are not satisfied. This paper describes two enhancements of the CM to relax these requirements: the segmented CM and the compensated CM. Taken together, these become the segmented/compensated CM and provide a means to increase the CM tolerance to common field conditions. The segmented CM can be used where unknown BHPs change during the analysis interval. The compensated CM overcomes the requirement to rerun the model after adding a new producer or after shutting-in an existing producer. If both BHP changes and shut-in periods occur, both modified CMs can be used simultaneously to construct a single model for a period of data. In several simulated cases with unmeasured fluctuating BHPs, the segmented and compensated CM successfully determined the true interwell connectivities. Ignoring the BHP data typically gave 2-10 times the root mean squared error of the segmented/compensated CMs. In an application to North Buck Draw field, which both lacks BHP data and has several producer shut-ins, the CM connectivity parameters were compared to the tracer test results and an earlier geological study. We observed good agreement between the modified CM and the amounts of higher-quality fluvial facies in the system. Also, the CM results correlated better with the results from tracer tests run in the field than when the unmodified CM was applied. (C) 2012 Elsevier B.V. All rights reserved.