Application of Linear Flow Analysis to Shale Gas Wells—Field Cases

Abstract

Abstract It has often been observed that tight gas wells behave as though they are controlled by transient linear flow. This behavior can be seen as a "half-slope" on a type curve and certain reservoir parameters can be evaluated. There can be many causes of linear flow, so the engineer must choose the most likely model for a particular well. Linear flow behavior has also been observed in shale gas wells but these wells tend to exhibit a significant "skin effect" that is not so common in tight gas wells. This "skin effect" can mask early linear behavior but may be accounted for with a modified equation form which is presented in this paper. Two mathematical models are presented which represent plausible relationships between fractures and matrix rock. Each model can be fit by linear dual porosity solutions. But in either case observed production rates can be represented by transient linear flow out of the matrix since the transient flow in the fracture system dies out in just hours or a few days. Calculation procedures are given which can be used to calculate size of the drainage volume, and the area of the interfaces between the hydraulic fractures and the rock matrix, Acm, and the matrix permeability in some cases. The numerical value of Acm may provide useful information in evaluating a well's completion and stimulation effectiveness. Both the "skin effect" and "drawdown dependence" are accounted for in the proposed method. Several field examples are presented which show ideal applications of the calculation procedures as well as other cases for which the method does not seem to apply, or at least does not work so well. Some factors that may cause difficulties in analyzing flow behavior are liquid loading, interference with adjacent wells, and fracture patterns that cannot be represented by our idealized model. The proposed method should be thought of as idealized models which may be found to be useful in analyzing production data from shale gas wells in their early life. Other methods may be more useful and more accurate in a well's later life.