Decline Curve Analysis For Canadian Oil Reservoirs Under Waterflood Conditions
Authors: Ambastha A.K. , Wong K.H. 
Published: March 2010

Decline curve analysis using type-curve matching has become an established engineering tool to evaluate long-term production characteristics of oil and gas reservoirs producing under various drive mechanisms. Decline curve analysis is often used to satisfy various regulatory requirements, to establish reserves, and to evaluate the success or failure of operational changes made during a reservoir's producing life.

In this study, a detailed investigation of production decline data from 78 Western Canadian Sedimentary Basin oil pools under waterflood conditions is presented. This study demonstrates that the majority of the waterflooded oil pools follow a hyperbolic decline with decline curve exponents ("b") less than 0.5. The decline exponent "b" does not exhibit any obvious functional relationship to the type of rock, oil density, geographical area, and/or geological formation. Nonetheless, such a study provides a basis for "b" values to be expected for Canadian oil reservoirs under waterflood conditions.

Introduction

Decline curve analysis is one of the oldest and most extensively used methods of predicting the future performance of a producing well, a lease, or a depleting reservoir. The purpose of this study is to present an analysis of the production declines of a group of waterflooded oil pools in Western Canada to establish any obvious decline trend. All of the pools selected are located in the Western Canadian Sedimentary Basin, which covers northeastern British Columbia, the southern half of Saskatchewan, and most of Alberta.

This study uses a type-curve matching technique(1) based on the general hyperbolic decline equation:

Equation 1 (available in full paper)

Previous Studies

Fetkovich(2) reported that in a predominantly solution-gas-drive reservoir in Texas, both the primary decline and the waterflood decline appeared to have the same decline exponent "b" of 0.3. Reasons for this phenomenon were uncertain to him. Schuldt et a l .(3), indicated that waterflooded oil reservoirs are generally expected to follow hyperbolic decline behaviour, but they neither explained why this should be so nor indicated their source of information. Mead(4) wrote that pools under different producing mechanisms, waterflood or water influx, should follow exponential or hyperbolic decline with decline exponents in the range of 0 to 0.2. Lijek(5) showed that if a plot of log (WOR) versus cumulative oil produced is linear, the decline curve of the pool is either hyperbolic or harmonic, depending on whether the gross rate is constant or variable. From his experience, he concluded that the decline exponent is very close to one (that is, harmonic decline) at a constant gross rate.

Currier and Sindelar's work(6) was similar to that of Lijek(5). They used log (WOR) versus cumulative oil recovery and a frontal advance cross-plot to aid them in establishing waterflood production decline trends from early data of a waterflood field in Alaska. Their study also showed that waterflood declines are harmonic or hyperbolic with a decline exponent much higher than those presented by Mead(4). In their study, cases with a straight line log (WOR) versus cumulative oil produced yielded decline exponents between 0.6 and 0.7.