Pressure Buildup Analysis of Prudhoe Bay Wells
Authors: Brown Michael E. , Mao Ming-Lung 
Published: December 2007

Recognition of a distinct characteristic common to pressure buildup curves obtained from the Prudhoe Bay field has led to a better understanding of reservoir properties. Our method of analysis identifies two straight-line segments on the conventional Horner plot and postulates the correspondence of these segments to two distinct regions in a well's drainage area. The first segment corresponds to a region close to the wellbore and appears to reflect reservoir properties over the intervals contributing to production. The second segment yields information on reservoir properties farther away from the wellbore and over the entire net pay. Consistent application of the method to pressure buildup surveys at Prudhoe Bay has proved a valuable toot in the selection of stimulation candidates, the understanding of high GOR mechanisms, and the development of an overall reservoir description.

Introduction

Frequent and regular reservoir pressure surveys have been conducted in the Prudhoe Bay field. In 4 years of production, more than 900 pressure buildup surveys have been run in more than 300 wells. Extremely sensitive Hewlett-Packard bottomhole pressure gauges have been used for the surveys. A conventional method, commonly called the Horner method, was selected to analyze the pressure data. The procedure here is an extension of the Horner method. The line source solution to the diffusivity equation for fluid flow through a porous medium provides the basis for the widely accepted Horner method of pressure buildup analysis. Important assumptions made in the derivation of the differential equation are single phase flow, a small constant compressibility, constant viscosity, and a homogeneous porous medium. Since these assumptions and the boundary conditions for the line source solution are not strictly satisfied by actual reservoir flow systems, an engineer must exercise considerable judgment when analyzing buildup test data by the Horner method. The method is, however, straightforward and economical to apply. With care and attention to relevant well data other than from the buildup test itself, the Horner method should yield sound estimates of formation permeability, well damage, and average pressure within the drainage area, At Prudhoe Bay, the presence of a free gas cap and near saturated crude oil are two of the factors that complicate the interpretation of buildup tests. It was recognized early that as reservoir depletion progressed multiphase flow would become increasingly prevalent and complicate the interpretation process. Therefore, an intensive effort was committed to pressure buildup analysis during the early production period before depletion became advanced. This paper summarizes that effort and the results obtained. One requisite of conventional theory that is rarely satisfied at Prudhoe Bay is that radial flow into a well occur from the entire thickness of the formation. Many of the wells have large standoffs between the perforations and the gas cap and between the perforations and the aquifer. Secondly, effective formation permeability to oil will not be constant throughout the drainage area. Bottomhole flowing pressures have been 500 to 1,500 psi below the bubble point. Fig. 1 is a schematic of fluid flow to the wellbore for a partially penetrating well. For the analysis discussed in this paper, two distinct regions around the wellbore are postulated (Region 1 and Region 2 in the diagram) corresponding to two distinct straight-line segments observable on the Horner semilog plot. The semilog slopes and partial penetration relationship formed the basis of Nisle's work in 1958. Later, Brons and Marting presented completion pseudo-skin factors resulting from this geometrical flow contraction. Recently, Bilhartz and Ramey presented simulated pressure buildup data for partially penetrating wells.

JPT

P. 387^