Analytic Technique to Forecast Production for Non-Declining Oil Wells in Bounded Solution-Gas-Drive Reservoirs

Abstract

Abstract A methodology has been developed to forecast the performance of an oil well which has not gone into the decline mode as yet i.e., which is exhibiting (a) increasing, (b) constant or (c) fluctuating production rates. This procedure has been adapted from the solution of the familiar "Diffusivity Equation" under conditions which give rise to the well-known "exponential decline" behavior. The technique has been illustrated with the help of both a synthetic case and an actual field example from a Middle Eastern reservoir. The analytically-derived predictions have been compared with the results from single well numerical simulation. There is good agreement between the two. The analytic technique gives conservative predictions. Introduction Historically, one of the most common tasks that a petroleum engineer is confronted with, while analyzing the performance of a single well, a reservoir or a field is the determination of :Future production rates;Producing life and,Remaining reserves before reaching its economic limit. One may require the knowledge of these quantities both for a mature producing field as well as a newly-discovered one. These parameters have a large impact on important financial decisions. Numerical simulation provides a means to calculate them through modeling the well/reservoir/field. However, the process of modeling can be quite time-consuming and expensive. It is, therefore desirable to develop analytic means to furnish quick yet reliable answers. The technique most often employed to derive the above quantities is the so-called "Decline Curve Analysis" which is applicable during the declining production phase. The procedure essentially consists of (i) plotting oil production rates versus time, (ii) establishing a trend through the past performance by fitting a suitable curve and (iii) extrapolating the curve to make forecasts. It has been recognized for a long time that the decline curves can be of three types :Exponential,Hyperbolic andHarmonic. Originally it was believed that the curves were purely empirical in nature with the hyperbolic curve representing the most general type whereas exponential and harmonic curves were special cases when the "decline exponent" was equal to zero or one, respectively. In recent times, it has been demonstrated that the exponential type of decline does, indeed, have a sound theoretical basis. Constant-pressure solution of the radial Diffusivity Equation for a well in a bounded reservoir under the well-known standard or ideal conditions e.g., single phase fluid, laminar flow etc. proved to be an exponential decline during the pseudo-steady state regime. Discussion The topic of decline curves has been vigorously debated in the petroleum literature since J.J. Arps proposed his famous equations. P. 61^