This paper presents a simple approach for estimating average reservoir pressure in solution gas drive reservoirs using data of two flow tests.

The two general IPR equations developed by Couto, are the basis of this work.

With this approach, and provided that certain data is available, one can estimate flow efficiency, skin factor, and relative permeability to oil.

The present approach has many advantages over the previously published methods for estimating the above parameters.

Excellent agreement is shown between the estimated and actual results. Field example is solved to illustrate the use of the procedure presented.


The estimation of reservoir average pressure and flow efficiency is of great importance in the process of evaluating the reservoir and calculating many parameters such as the oil in-place.

Several methods have been published that can be used to estimate these two parameters. The common methods used for estimaing reservoir average pressure are:-

  1. Mathews-Brons-Hazebroek (MBH) method1.

  2. Miller-Dyes-Hutchinson (MOO) method2.

  3. Modified Muskat (MM) method3.

Although the MBH and MDH methods can be applied to a wide range of drainage area shapes, they have many disadvantages: (1) they require knowledge of drainage area size and shape and estimates of reservoir and fluid properties such as total compressibility and porosity which are not always known with great accuracy and (2) the application of these methods is limited to the availability of buildup test analysis (i.e. a buildup test must be analyzed first).

The modified Muskat method requires no estimates of reservoir properties when it is used to establish reservoir average pressure (P), except to choose the correct portion of data for analysis. This method has serious disadvantages (1) it is applicable only to a certain range of shut in time. (2) it fails when the tested well is not reasonably centered in its drainage area, and (3) it is based on trial and error calculation procedure.

The other important parameter, which is difficult to determine by the available methods, is the flow efficiency (FE).Flow efficiency is defined as the ratio of actual productivity index (PI) of a tested well to its ideal productivity index. Several mathematical forms4 are available to calculate the flow efficiency. All these forms require the analysis of a buildup test. The simplest available procedure is presented by Couto5. He stated that the flow efficiency may be determine from two flow tests. Again his method requires knowledge of reservoir average pressure which may require lengthy analysis.

Considering the disadvantages of 'the available methods for estimating P and FE a simple and general method wh1ch allows accurate estimation of both parameters without the need of knowing rock and fluid properties and analyzing buildup test, is found necessary.

In the present study an attempt: is made to make use of the two equations developed by Couto6, derived from the work of Standing7 and Fetkovich8.


Couto6 developed two general equations for predicting the inflow performance relationship of a well in a gas drive reservoir.

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