Leakoff characteristics and an equivalent leakoff coefficient in fractured tight gas reservoirs

Abstract

Leakoff prediction and control are key issues in hydraulic fracturing, particularly in fractured tight gas reservoirs. In the presence of natural fractures, leakoff behavior in these reservoirs shows characteristics of dual leakoff and pressure dependence, which is totally different from that in conventional reservoirs. Available models in the literature do not address these characteristics.In this paper, continuing from our previous studies, we further study the leakoff characteristics in fractured tight gas reservoirs in depth. A leakoff conceptual model is proposed with consideration of three leakoff media. The mathematical models in different media are developed by considering the influence of stress sensitivity. Based on this conceptual model, an equivalent leakoff coefficient is derived. Simulation results performed are consistent with field observations. The equivalent leakoff coefficient is also validated to reflect the characteristics of leakoff. The influence of multiple factors on leakoff is studied in detail, indicating that with an increase in the permeability, compressibility, density and pressure, the leakoff velocity increases, and these parameters have a linear relationship with cumulative leakoff, while with an increase in fracturing fluid viscosity, the leakoff velocity decreases, and it has a power function relationship with cumulative leakoff. After considering the stress sensitivity, both the leakoff velocity and cumulative leakoff increase, and the increasing magnitudes depend on pressure, especially in macro natural fractures. Furthermore, a pressure distribution curve also changes from concave to convex with consideration of stress sensitivity in macro natural fractures.