Production Rate Decline In Pressure-sensitive Reservoirs

Abstract

This paper investigates the influence of pressure-dependent fluid and rock properties on well production decline in con-stant wellbore pressure tests. The rock properties considered variable are permeability, porosity, pore compressibility and formation thickness, and thefluid properties are density, com-pressibility and viscosity. Singlephase flow through the porous medium is considered. Various geometries, ratios of in-itial to wellbore pressure, and data sets of rock and fluid pro-perties are studied. For allpractical ratios of initial to wellbore pressure and for transient flow conditions, production rate decline expressed in terms of a dimensionless rate qdis essen-tially the same as the production rate decline qo] for constant property liquid flow. The only exceptions occur for bounded reservoirs after the flow is affected by the outer boundary. These deviations are such that in pressure-sensitive systems production rate declines faster than in constant-property systems. It is shown that variable property decline solutions, when compared to constant property decline solutions, do not follow any of the three common types of production decline curves-exponential, hyperbolic or harmonic. Type curve matching of variable property production rate decline to con-stant property flow solutions gives the correct size of the reser-voir, but other reservoir parameters can be in error. When variable property type curves are used for matching, all reser-voir parameters are correctly estimated. The method of Jacob and Lohman(19) and of van Poolien(21) areproperly modified to accountfor the pressure-dependency of rock andfluid proper-ties. Introduction It is well accepted that porous media are not always rigid and non-deformable. This should influence the transient well behaviour. A frequent assumption is to use average values for both pressure-dependent rock and fluid properties. This helps to reduce the errors involved, but does not totally eliminate them. When rock and fluid property changes are important over *Now with Petroleos Mexicanos, Mexico City. pressure range of interest, then these changes cannot be neglected and a variable property solution should be obtained. A flow equation considering the pressure dependency of all rock and fluid properties has been presented in the literature(l). This equation, when expressed as a function of a pseudo-pressure m(p), resembles the diffusivity equation. Samaniego et al.(2.3) studied this variable property problem for a greater variety of flow conditions. These authors only investigated constant rate cases.There are two basic radial flow cases in the flow of fluids through porous media: (a) constant well rate; and (b) constant well pressure. It is the purpose of this study to investigate the influence of pressure-dependent rock and fluid properties on the constant pressure case. One of the objectives is to find out how production rate decline would vary in these pressure-dependent systems, and if methods of analysis for constant pressure flow testing can be properly modified...

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