Abstract

In this work the performance of wells in solution gas-drive reservoirs during the boundary-dominated flow period is examined. Both constant wellbore pressure and constant oil rate production modes in closed systems are considered.

For the constant wellbore pressure production mode, the conditions under which procedures in the literature can be used to analyze data are discussed. Specifically, Arps's equations for performance prediction are examined and it is shown that predictions performance prediction are examined and it is shown that predictions of future performance are strong functions of well spacing, well condition, and fluid properties. The parameters, b (the decline exponent) and d (the initial decline rate), in the Arps equations are expressed in terms of physical properties. The conditions under which these equations can be used are specified. An empirical procedure to protect production rates is also presented. procedure to protect production rates is also presented. In the case of constant oil rate production, an expression to correlate the pressure distribution in the reservoir is presented. The correlating function permits us to extend the definition of pseudo-steady-state flow to solution gas-drive systems. Its use pseudo-steady-state flow to solution gas-drive systems. Its use also allows the simultaneous computation of average properties (pressure and saturation) during boundary-dominated flow from wellbore information.

Introduction

The intention of this work is to document some theoretical results that are useful for predicting well performance from production data in solution gas-drive reservoirs during the boundary-dominated flow period. In the process of documenting these results it is also period. In the process of documenting these results it is also intended to furnish a theoretical support for empirical observations that exist in this subject.

Both constant wellbore pressure and constant oil rate production modes in circular closed systems are considered. The outcomes presented in this communication take as a basis the theoretical results presented in this communication take as a basis the theoretical results presented in Refs. 1 and 2. Specifically, the results given here follow presented in Refs. 1 and 2. Specifically, the results given here follow from our ability to correlate responses of solution gas-drive systems with the response of a slightly compressible liquid flow during the boundary-dominated flow period for both constant oil rate and constant wellbore pressure production modes.

To accomplish our goals, this work is divided into three parts. In the first part, the theoretical results related to boundary-dominated flow given in Refs. 1 and 2 are outlined to establish a framework for the findings presented in this communication.

In part II, the case of a well flowing at a constant pressure during the boundary-dominated flow period is analyzed. For this case, Fetkovich showed that the empirical family of curves of Arps can be combined with the slightly compressible liquid flow solution (exponential decline response) to obtain a family of curves that can be used to predict future performance and estimate the reservoir pore volume. Refs. 1 and 6 report that during the boundary-dominated pore volume. Refs. 1 and 6 report that during the boundary-dominated period the rate response plotted versus time does not match a fixed period the rate response plotted versus time does not match a fixed value of the decline exponent, b, in the type curves of Ref. 3. An explanation for this observation is presented. Refs. 3 and 7 emphasize that the decline exponent, b, must be less than or equal to unity. They also note that if transient data are used, then the value of the decline exponent, b, of the Arps's solution can be greater than unity. Using the development given in Ref. 8 for transient flow, a theoretical justification for this observation is provided. An empirical procedure to predict production rates of wells produced at a constant pressure, over a short time spans, is also presented.

Part III is devoted to the situation when the production is held at a constant oil rate. For this case it is known from Refs. 1, 2, and 9 that the reservoir does not achieve the condition of pseudosteady-state, i.e., the derivative of pressure with respect to pseudosteady-state, i.e., the derivative of pressure with respect to time is not constant and is also not independent of position in the reservoir. In this work, a correlation for the pressure distribution in the reservoir during the boundary dominated flow period is developed. This correlating function permits us to obtain an extension of the pseudosteady-state concept to solution gas-drive reservoirs. Furthermore, pseudosteady-state concept to solution gas-drive reservoirs. Furthermore, this function also allows us to compute simultaneously the values of average pressure,, and average saturation,, having wellbore information.

The numerical results presented in this paper were obtained with a finite difference model described in Ref 1. Procedures followed to ensure the accuracy of the solutions are given in Refs. 1 and 2.

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