Determination of Reservoir Properties From Backpressure Tests With Applications to Reservoir Simulation
Authors: Chain R.Y.L. , Mountford C.J. , Raghavan R. , Thomas G.W. 
Published: December 2007

The Odeh-Jones technique for determining values of formation flow capacity and skin factor from backpressure-test data is investigated using a three-dimensional reservoir simulator. An application of the technique to the simulation of a gas storage reservoir is also presented.

Introduction

A number of situations exist in gas reservoirs where a fast, cheap, reliable method for determining reservoir properties - especially formation capacity - is properties - especially formation capacity - is desirable. In some cases a standard buildup test may be out of the question because of the long buildup times required. Yet, formation capacities are necessary to predict gas deliverability. One may desire to simulate predict gas deliverability. One may desire to simulate the behavior of a gas reservoir where good pressure decline data are available for each well but where the permeability information required for the simulator is permeability information required for the simulator is nonexistent. The question then arises: Is it possible to quickly and cheaply obtain such data without appealing to core analyses or well logs?

This paper focuses attention on a technique for analyzing backpressure-test data to determine formation capacities and skin factors. The method is advantageous since field measurements can be made rapidly, easily, and at low cost; and well shutdown is not required. It is based on the Odeh-Jones procedure for analyzing multiple-rate flow tests. Their work was first adapted by Fraser to the analysis of backpressure tests assuming the existence of small pressure gradients and ideal gas. Subsequently, Essis B. and Thomas extended the Odeh-Jones procedure to real gas flow and analyzed backpressure-test data from 16 Oklahoma and Texas gas wells. Flow capacities calculated by the Odeh-Jones technique were compared with those obtained from pressure buildup tests. However, in applying this pressure buildup tests. However, in applying this method Essis B. and Thomas observed various anomalies. These took the form of abrupt changes in slope, breaks in plots, etc. Accordingly, they implied that the utility of this technique was limited. Their attempts to interpret these changes in terms of formation characteristics or wellbore effects were largely based on conjecture since the formation characteristics were, for the most part, inferred.

The main objective of this investigation was to determine reasons for the anomalies observed by Essis B. and Thomas. We shall determine whether the various effects (turbulence, skin effect, fractures, etc.) attributed by Essis B. and Thomas to be the cause of anomalous behavior do really cause unusual bends or breaks when flow/afterflow data are plotted in the manner suggested by Odeh and Jones. In addition, it was our purpose-to delineate those reservoir conditions where one could expect reasonably reliable results from the Odeh-Jones method. Finally, we demonstrate how the technique can be useful in determining permeabilities for subsequent use in a reservoir simulation study.

Procedure Procedure To achieve the first two goals we used a reservoir simulator to generate the necessary backpressure-test data under a wide range of well and reservoir conditions. This insured better system control; that is, formation and well characteristics were those actually used as input data in the simulator.

JPT

P. 603