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

Injection/fall-off testing is one of the unconventional well test methodologies used to eliminate hydrocarbon flaring and thus gas emissions into the atmosphere. Except for fluid sampling, all of the main well testing targets can be achieved, while complying with the environmental regulations. 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 an additional bi-phase skin component. Furthermore, natural or induced fractures can be intercepted by the well, reducing the total skin but adding complexity to the test interpretation. Typically, the application of traditional analytical models to interpret injection tests 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 well productivity. In this paper, an effective correlation to determine the mechanical, the fracture and the bi-phase components of the skin in the case of injection tests is presented; this correlation was empirically derived with the aid of a numerical simulator. The equation expresses the total skin as a linear composition of the three skin 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, 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.