Estimation of skin in the interpretation of injection tests in fractured reservoirs

Injection/fall-off testing is one of the unconventional well test methodologies used to eliminate gas emissions into the atmosphere. Except for fluid sampling, all of the main well testing targets can be achieved, while complying with the environmental constraints. However, the interpretation of injection tests in oil reservoirs is complicated by the presence of two immiscible mobile phases in the reservoir: the hydrocarbon originally in place and the injected fluid. As a result, the total fluid mobility is reduced and an additional pressure increment occurs, which affects the total skin with a supplementary bi-phase skin component. Furthermore, natural or induced fractures can intercept the well, reducing the total skin but adding complexity to the interpretation. Typically, the application of traditional analytical models only provides the total well skin while its mechanical component, due to permeability damage in the near wellbore zone, cannot be isolated. However, the mechanical skin is a fundamental well testing target because it is essential to estimate the well productivity. An effective relationship to determine the mechanical, the fracture and the bi-phase components of the skin in the case of injection tests was empirically derived with the aid of a numerical simulator. The equation expresses the total skin as a linear composition of these three components and is of general applicability; in mono-phase flow conditions or in the absence of fractures it reduces to well-known formulas available in the technical literature. By means of this equation the true permeability damage can be assessed and, in turn, the well productivity calculated. Additionally, the total skin factor and thus the expected pressure increase during injection can be estimated when designing a well test. A real field case where the formula was successfully applied is presented in the paper.