American Institute of Mining, Metallurgical and Petroleum Engineers, Inc.

Abstract

A study of the Coyanosa, Ellenburger and Devonian reservoirs was made using a two-dimensional, single phase, unsteady-state computer program. The model was made to fit the general physical conditions thought to exist in the reservoir under study and then adjusted by a history-matching technique.

The matching technique is discussed with an example of how the reservoir parameters were manipulated to achieve a match. An example is given of the use of the adjusted reservoir model as a tool for investigating the effects of various allowable rates on availability of gas over periods of several years.

The authors believe the approach outlined in this paper to be of considerable value in availability projections and reserve determinations. Use of the model allows projections to be made on the basis of individual well performance that includes interaction and performance that includes interaction and changing drainage patterns of all wells in the reservoir.

Introduction

This paper presents a discussion of a history-matching method using a mathematical reservoir model for projection of gas reserves and availability, and the result of a study to apply this method to the Coyanosa field of West Texas. This field was chosen for the study because it is in a newly developing area, and because it is an important new source of supply in the Permian Basin.

Pressure data resulting from the short production history of the two deepest Coyanosa production history of the two deepest Coyanosa reservoirs indicated that actual reserves were less than the originally accepted reserves, which had been based on volumetric estimates.

Therefore, an attempt was made to simulate the Devonian and Ellenburger reservoirs using a two-dimensional, unsteady-state, single phase model. The procedure followed was to try to force the computer model to match the appropriate cumulative production information by wells and the stabilized pressures obtained from a fieldwide shut-in period in May, 1967, and then to use the model for forecasting reservoir performance to depletion. This forecast estimate performance to depletion. This forecast estimate is thought to be more reliable than estimates arrived at by conventional methods for approximating availability of gas and individual well recoverable reserves.

DISCUSSION OF METHOD

To approximate the physical reservoir as a mathematical model, certain steps must be followed.

  1. The reservoir isopach map must be overlaid with a grid base and a grid size and location must be selected that will provide the well interference and drainage information necessary, keep computer time to a minimum, and minimize distances between well locations and the nearest grid intersection.

  2. The reservoir boundary must be approximated with grid lines knowing that, in this model, the mathematical boundary is one half grid size outside the drawn boundary [Fig. 1].

  3. Each well must be located with respect to the grid system and a boundary thickness estimated.

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