The Material Balance as an Equation of a Straight Line

Abstract

Abstract The material balance equation used by reservoir engineers is arranged algebraically, resulting in an equation of a straight line. The straight line method of analysis imposes an additional necessary condition that a successful solution of the material balance equation should meet. In addition, this algebraic arrangement attaches a dynamic meaning to the otherwise static material balance equation. The straight line method requires the plotting of one variable group vs another variable group. The sequence of the plotted points as well as the general shape of the resulting plot is of utmost importance. Therefore, one cannot program the method entirely on a digital computer as is usually done in the routine solution of the material balance equation. If this method is applied, then plotting and analysis are essential. Only the appropriate equations and the method of analysis and interpretation with comments and discussion are presented in this paper. Illustrative field examples for the various cases treated are deferred to a subsequent writing. Introduction One of the fundamental principles utilized in engineering work is the law of conservation of matter. The application of this principle to hydrocarbon reservoirs for the purpose of quantitative deductions and prediction is termed "the material balance method of reservoir analysis". While the construction of the material balance equation (MBE) and the computations that go with its application are not difficult tasks, the criteria that a successful solution of the MBE should fulfill have always been a problem facing the reservoir engineer. True and complete criteria should embody necessary and sufficient conditions. The criteria which the reservoir engineer uses possess a few necessary but no sufficient conditions. Because of this, the answers obtained from the MBE are always open to question. However, the degree of their acceptability should increase with the increase in the number of the necessary conditions that they should satisfy.