Decline curve analysis using type-curve matching has become an established engineering tool to evaluate long-tern 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 various changes that take place 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 do follow a hyperbolic decline with their decline curve exponent ("b") values 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, To the best of our knowledge, such a comprehensive study on the decline curve analysis for Canadian oil reservoirs under waterflood conditions has not been reported before.
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, There are numerous papers written on the subject However, only a handful have attempted to deal with decline curve analysis for oil reservoirs under waterflood conditions.
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 almost all of Alberta.Figure 1 shows the relative locations of the provinces in the Basin which is the most important petroleum region of Canada.
An oil reservoir's life under waterflooding conditions can generally be divided into three distinct periods as illustrated in Figure 2. The periods are:
Initial Fillup Period: This period begins with initial water injection and lasts until the first response to injection in the form of a production increase. During this period, the space occupied by gas is being filled, free gas is being dissolved, and reservoir pressure is being restored. The production rate may continue to decline or remain steady. This period, on the average, ranges from 5% to 11% of the total flood life(1), depending on the heterogeneity of the reservoir sand, the flood pattern, well spacing, and the volume of void space.
Production Incline Period: This period is from the time when oil production begins to increase to the peak of the production rate. During this period, flood fronts are moving towards producing wells, the production rate is steadily increasing, and the water cut is not increasing substantially. The length of time required in this period vanes substantially, but, on the average, it is about 20% of the total flood life(1).