Abstract

One frequently encountered difficulty in material balance calculation is lack of shut-in reservoir pressure. This work presents a new method that can be applied under pseudo steady-state conditions to simultaneously determine the average reservoir pressure and initial fluid in place using only surface production data and flowing bottom hole pressure. This new method is derived from a combination of a generalized material balance formulation and pseudo steady state theory and is applicable to both oil and gas reservoirs for single-phase flow with no water influx. This method does not rely on pressure build-up tests, knowledge of drainage area or permeability, but it requires flowing bottom hole pressure. For constant flowing bottom-hole pressure or variable production rate, this work presents an algorithm to convert fluctuating or constant flowing bottom hole pressure into corrected flowing bottom-hole pressure with a decline rate essentially identical to that of average reservoir pressure. Methods for analyzing transient flow are also discussed in this work. This new method is useful in analyzing surface production data and flowing bottom hole pressures for reservoirs significantly lacking data.

Introduction

A frequently encountered difficulty in material balance calculation is lack of measured reservoir pressures, which are usually obtained from shut-in build-up tests. Without shut-in reservoir pressure, it is very difficult to perform material balance calculation. However, production data seems to be available most of the time. If we can make use of sand-face production rate and flowing bottom hole pressure, then it is possible to develop a new method which can be used to determine average reservoir pressure and initial fluid in place (IFIP) simultaneously.

This new method is derived from a combination of generalized material balance method and pseudo-steady state theory to determine reservoir pressure and initial fluid-in-place simultaneously without relying on pressure build-up tests. Furthermore, this new method does not require shutting-in of production, nor does it require a prior knowledge of permeability, drainage area, and shape factor under pseudo-steady state condition.

Objectives

The objectives of this study are

  1. to present detailed mathematical derivation of a new method, which can be used to simultaneously determine average reservoir pressure and IFIP for oil and gas reservoirs without relying on pressure build-up tests and

  2. to provide solution algorithms of this new method.

Mathematical Derivation

The material balance of hydrocarbon fluid at reservoir condition states that the initial hydrocarbon pore volume is equal to the current hydrocarbon pore volume plus the hydrocarbon pore volume reduction due to

  1. pore volume reduction,

  2. expansion of initial water saturation,

  3. water influx,

  4. water injection, and

  5. water production.

This concept is expressed in equation (1). Detailed derivation of equation (1) can be found in references (1, 2, 3, and 4). Definition of each symbol is given in nomenclature.

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