Abstract

The author has encountered a number of situations where drastically different interpretations are possible from oil material balances. The paper focusses on two of these field situations. In the first case, a gas cap could be interpreted which subsequent analysis disproved. In the second case, regression techniques converged to a total of three different combinations of gas cap and oil leg sizes. The existence of multiple solutions was not readily apparent.

The discussion is approached from three different perspectives, which are referenced to two different field examples:

  1. How production, geological, PVT (oil, gas and water) and historical information may be used to screen results and to apply the mathematical technique.

  2. A new mathematical approach to error analysis for the Havlena and Odeh material balance has been developed which the author has not seen elsewhere in the literature. Additionally, graphical interpretation can identify situations where multiple interpretations are likely.

  3. A spreadsheet has been used to implement the Havlena and Odeh material balance. Using some simple macros it is possible to quickly generate all of the diagnostic plots. This is a cost effective alternative to purchase of a specialty program. Aquifers are also included as well as statistical regression. Convergence is demonstrated as a criteria for confidence in material balance calculations.

The new interpretations were significant. In the first case, it was an important factor in deciding to proceed with a string of successful workovers designed to recover attic oil. In the second case, the new interpretation cut short a field extension drilling, program which was resulting in unexpected dry holes.

Introduction

This paper is based on half a dozen material balance studies that the author has prepared on a consulting basis. Four were for carbonate reservoirs and two were for sandstone reservoirs. The contents are based on this experience and thus represent a mixture of field case, theoretical analysis (developed during projects) and an evolving approach to material balance calculations. Two examples are given, however, they represent the combined experience on all of the above reservoirs. Scales have, for the most part, been removed; since the data is from actual studies. The paper has been structured as follows:

  1. A quick summary is made of the material balance equations. These are used for the derivation of the error analysis. No derivation of the equations was made since this is comprehensively covered in many reservoir engineering textbooks. A summary of the various Havlena and Odeh plots is also made, since this material is discussed in the paper.

  2. An analysis of the effects of errors in pressure measurement is presented. The author uses this for weighting data points.

  3. Some practical observations are made on statistical minimization.

  4. The general approach used by the author is outlined.

  5. Two typical material balance situations are described, which highlight error analysis and data screening.

The paper differs slightly from the norm for a technical paper.

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