Double-porosity model for tracer transport in reservoirs having open conductive geological faults: Determination of the fault orientation

Conductive geological faults in oil reservoirs have a major impact in the efficiency of fluid-flooding and enhanced oil recovery schemes. Inter-well tracer tests are a traditional mean to determine underground communication channel properties, and therefore have the potential to estimate fault features. In this work an analytical model is developed to describe inter-well tracer tests in reservoirs with open conductive faults. Specifically, a fault that intersects the flow path between the injection and production wells without reaching them is treated. In open faults fluid can be exchanged along the fault to external regions. In addition, the present model captures two new relevant aspects of the process: (i) the fault region is a fractured media, and (ii) a Danckwertʹs discharge condition is established at production well. To treat open faults a new source-coupling scheme is designed for linking the three independent path regions in which the system is discomposed. The first region comprehends the path from the injector to the fault, the second represents the zone along the fault, and the third one the path from the fault to the production well. The involved coupled advection–dispersion equations are analytically solved by the Laplace technique and numerically inverted. There are twelve parameters in the model; from them five concern the fault properties. By exploring their effect on the tracer breakthrough curve it is found that the curve is mainly sensitive to the tracer fault path length and the fault fluid velocity. The two factors describing the fracture–matrix coupling become important only when the fluid velocity in the fault is relatively low. No relevant sensitivity is found on the fault dispersivity. An outstanding attractive feature of the new model is its capacity to determine the fault orientation, which is not possible by the standard characterization means. The model has been successfully applied to data from a real reservoir tracer test performed in Loma Alta Sur Field. Here, the orientation of two faults have been determined.