A New Analytical Procedure to Estimate Interlayer Cross-Flow Rates in Layered-Reservoir Systems Using Pressure-Transient Data

Abstract

Abstract We present a new procedure to estimate analytically, from pressure-transient responses, inter-reservoir crossflow rates in multilayer systems separated by tight geological layers, streaks, or baffles. A reservoir system with two neighboring layers, reffered to throughout this study as the tested and the adjacent layers, separated by a semi-permeable barrier, is considered. As production from the tested layer causes a sustained pressure differential across the interface of these poorly-connected layers, prompting fluids to cross flow from the adjacent to the tested layer. For such flow mechanisms, an analytical method is presented to diagnose the conditions of cross flow, characterize the flow barrier, and to quantify the rate of cross flow from transient-pressure data. Pressure-derivative plots are utilized in diagnosing cross flow between the layers. By matching the test data with the model output, the barrier and the layer parameters are confirmed. The magnitude of the specific barrier permeability and the layer properties are used to compute the cross-flow rate profile as a function of time. Both the interface of the flow area and the pressure differential at that interface between the layers grow with time for a given magnitude of the specific barrier permeability (Fcb). This is also true for the cross-flow rates. At certain late times, the magnitudes of the pressure drops and cross flow rates attain and maintain their respective plateaus in infinite systems. This also implies that the cross-flow rate grows to a maximum value at some point in time. It is observed that the maximum possible cross-flow rate depends on the ratio of the reservoir flow capacity of the adjacent layer to that of the total reservoir system, comprising both layers. Pressure-derivative versus elapsed time plots are used to determine the onset of cross flow from the adjacent layer, which are impressively supplemented by the respective profiles of the calculated cross-flow rates with time. Field application of the new procedure is illustrated with actual data.