Abstract

As an ongoing function the National Energy Board monitors the performance of producing gas reservoirs throughout Canada. Recently computerized plotting of reservoir performance data has allowed this monitoring program to be made more comprehensive and currently more than three hundredproducing gas reservoirs are included in the program. This paper describes the form of the gas reservoir performance plots being used in the NEB monitoring program. Typical examples are presented. The interpretation and use of the plots in identifying performance problems are discussed.

Introduction

Government agencies, such as the National Energy Board, in serving the public interest, have the task of maintaining an accurate estimate of domestic energy reserves. In Canada, gas reserves contained in a multitude of individual pools are amajor component in the total energy package. New technical information concerning these gas reserves is continually being generated by exploration and development drilling and operating experience in producing reservoirs. The task of assessing the impact of this new information on reserves estimates is not a small one. Monitoring programs which enable resources to be channeled for maximum effectiveness play an important role.

The computerized production data files of the provincial agencies of the oil and gas producing provinces in Canada contain a large amount of information concerning producing gas wells. This information can be rapidly retrieved and presented in anyone of a variety of formats using computer graphics, a process which is well-known and widely used.

Recently the NEB's gas reservoir performance monitoring program has been expanded by the use of computer graphics. The computer generated performanceplots of this program are similar, except perhaps in a few minor details, to those used by most oil and gas companies in their reservoir evaluation work. They merely present raw data required for pressure and production rate decline analyses along with the water/gas ratio history. To date more than three hundred gas reservoir performance plots have been prepared, which represent a large portion of Canada's initial reserves of conventional gas. The sheer volume of information presented in graphical form has permitted some interesting observations and conclusions of an empirica1 nature.

RESERVOIR PRESSURE DECLINE

The material balance equation expresses the relationship between the stabilized shut-in reservoir pressure (P) and the cumulative production (q). For a volumetric gas reservoir the relationship is:

Equation (1) (Available in full paper)

It is usual to present the material balance graphically as a plot of P/Z versus Q. For a volumetric reservoir the plot can be extrapolated linearly to obtain an estimate of the initial gas-in-place (Qi) as the x-axis intercept and of the recoverable serves (Qi) as the value of Q at the abandonment P/Z (Ref. 1).

The practical application of the material balance relationship is not without its difficulties, In low-permeability reservoirs prohibitively long shut-in times may be required in order to directly measure stabilized shut-in pressures (Ref. 12). In such cases a value must be estimated by build-up analysis (Ref. 2, 3).

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