Maximizing the Potential of Decline Curve Analysis

Abstract

Abstract The data most collected within the oil industry is the rate-time data. This data is analyzed with decline curve to primarily determine well/reservoir remaining reserves. However, rate-time data, which is a form of extended well testing, can also be used to estimate other reservoir parameters such as permeability and/or relative permeability curve. In this paper, we utilized Masoner's decline derivation to show that estimates of the relative permeability curve can be obtained from a well under voidage rate control in a solution gas drive system. For a well under pressure control in a solution gas drive reservoir, however, we show that the decline is exponential and obtain an expression for the permeability. The results were applied to data from solution gas drive simulation models and are presented. Application to field data is also presented. Introduction Decline curve analysis has been in use for several years within the oil industry but limited to reserves estimation and future well/reservoir performance. However, according to Horne, decline curve is a form of extended well testing and can be used to diagnose other reservoir properties, particularly long term effects. This work presents the use of decline curve to obtain the oil relative permeability curve and/or the absolute permeability. Arps pioneered the work on decline curve analysis. He empirically derived expressions for three forms of decline - exponential, harmonic and hyperbolic. His work was given theoretical basis by Fetkovich using material balance principles. Specifically, for single phase systems, Fetkovich showed that the decline function is exponential. Other authors also arrived at this same conclusion using the single phase flow diffusivity equation. Fetkovich applied his work to a number of single-phase-flow field cases and obtained estimates of the permeability, which compared well with those from transient testing. For solution gas drive systems, however, Fetkovich introduced a pressure term to account for relative permeability effects. He later derived a hyperbolic expression with an Arps exponent of 0.33 for depletion drive systems. Frederick and Kelkar combined the single-phase work of Fetkovich and the work of Palacio et al. to iteratively compute the ultimate recovery for depletion drive systems.