Previous work has shown that it is possible to detect the distance to a sealing fault. Recent work by several authors has been directed towards analyzing the effect of partially communicating faults on pressure transient responses. This current work concerns itself with a formation that has a fault which, rather than partially sealing the producing formation, allows influx from an adjacent formation (such as underlying aquifer). The goal is to determine the distance to this interlayer communication and the degree of communication by the use of type curve analysis.

The response caused by the producing zone alone can be filtered out using desuperposition. The remaining response is the "deviation function" caused by the "leaking" fault. Type curves were developed based on this deviation function response using dimensionless times and pressures and a dimensionless fault transmissibility as a matching parameter. The development of these type curves was based on numerical simulation.

The analysis technique is straightforward. The actual pressure response is subtracted from the extrapolated initial semi-log straight line and plotted. The resulting plot, which is the deviation function caused by the leaking fault, is compared to the type curve. The time match point provides the wel1-to-fault distance, while the matching parameter reveals the transmissibility of the interlayer communication.

A field example from a North Sea oil field illustrates the geological conditions under which the analysis technique is applicable, and how it is applied. The pressure transient data from an actual well test were first corrected for tidal effects and then analyzed using the above technique. As suspected from seismic and geologic interpretations, communication between the reservoir and an underlying aquifer could be detected. The distance to the leaking fault and its transmissibility were found. In this example case, the new analysis technique was able to provide input data essential to the reservoir description.

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