Inferring interwell connectivity using multiwell productivity index (MPI)

Abstract

The multiwell productivity index (MPI) concept can be used to predict the performance of a homogenous reservoir under various operational conditions. In this paper, based on the MPI approach, we develop a semi-analytical method to evaluate interwell connectivity in waterflood projects. At first, we rearrange the MPI formula to predict waterflood performance in a homogeneous reservoir. For such cases, we can predict the liquid rates, average reservoir pressures and injectors' bottomhole pressures (BHP) accurately, in fractions of a second. For the case of an actual heterogeneous formation, we use the injection and production data to estimate the proper influence matrix by minimizing the misfit between predicted and actual reservoir performance. Defining the heterogeneity matrix as the difference between the estimated influence matrix and the one obtained as the analytical solution for the homogenous formation, we depict the normalized interwell connectivity indices. By decoupling the effect of well locations and the changes in injection rate, producer's BHP and individual skin factors from the apparent connectivity, the connectivity indices obtained by this method solely represent the reservoir heterogeneity and possible anisotropy and hence allow improved information exchange with the geologist. Comparing to the other connectivity evaluation models, the connectivity indices obtained by this method are independent of operational conditions, such as possible shutting-in existing or adding additional injectors or producers, or converting one to the other — and hence are more suitable for decision making. A further advantage of the new method is the flexibility to incorporate additional information, e.g. injectors' BHP, into the analysis process.