Modelling and Analysis of Shale Gas Production With a Skin Effect

Abstract

Summary Many hydraulically fractured horizontal wells in shale gas reservoirs have been observed to exhibit transient linear behaviour. This transient linear behaviour is characterized by a one-half slope on a log-log plot of rate against time. This transient linear flow regime is expected to be caused by transient drainage of low-permeability matrix blocks into adjoining fractures. This transient flow regime is the only flow regime available for analysis in many wells. In this paper, a linear dual-porosity model is used to approximate the shale gas completions with a horizontal well and multiple hydraulic fractures. Five flow regions are identified with this model. It seems that field production data exhibit Region 4 behaviour, which is primarily transient drainage from the matrix. However, a skin effect for Region 4 is observed in many wells. This modifies the shape of the log-log rate plot and the square root of time plot. A new analytical equation is presented to model the effect of skin on linear flow behaviour. The resulting curve shapes are somewhat surprising and tend to explain the early curve shapes of actual wells. A procedure is presented in this paper to analyze field data.