The Horner and Miller-Dyes-Hutchinson methods of pressure buildup analysis have long been the basis of such analysis. Advances in analysis theory have added to and combined these two methods to provide engineers with a thorough understanding of a reservoir. This article compiles a practical method of pressure buildup analysis for the Morrow formation in the Anadarko Basin of Central Oklahoma based on experience gained from repeated analysis of pressure data in this region. The article reviews the familiar equations for permeability and skin effect, and discusses the three plots required for proper analysis with a detailed look at afterflow, fracturing, identification of the correct straight lines on each plot, and average reservoir pressure. Buildup analyses for both pre-frac and post-frac pressure data are presented for a gas well and an oil well. The analysis pressure data are presented for a gas well and an oil well. The analysis is discussed with emphasis placed on the value of such testing in completion and workover operations.
All pressure analysis is based on the two-dimensional diffusivity equation in cylindrical coordinates written:
(1)
The diffusivity equation is assumed to describe the flow within the reservoir and the equation itself assumes constant porosity, fluid viscosity, compressibility coefficients, and formation dimensions, as well as single-phase, isothermal, radial flow toward the wellbore, a slightly compressible flowing fluid, isotropic permeability, and no gravity effects.
Horner presented a solution of this equation which resulted in the basic practical equation used in all pressure analysis. practical equation used in all pressure analysis.
