Abstract

This work presents procedures on how to apply a newly developed method to simultaneously determine the average reservoir pressure and initial fluid in place using 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. Reservoir simulation was used to generate production history and flowing bottom hole pressure via hydraulic tables, which were used to calculate average reservoir pressure and initial fluid in place in order to validate the accuracy of this new method. Five examples were investigated for production with vertical and horizontal wells in oil and gas reservoirs at constant rate or constant flowing bottom hole pressure. Calculated initial-fluid-in-place and average reservoir pressures agree very well with that derived from reservoir simulation. This new method is useful in analyzing surface production data in the following conditions:

  1. reservoirs significantly lack data,

  2. buildup tests are inconclusive in determining average pressure,

  3. buildup tests are expensive or difficult to run such as wells in tight and over-pressured reservoirs or with subsea completion.

Introduction

A frequently encountered difficulty in material balance calculation is lack of shut-in reservoir pressure. 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 simultaneously. This new method does not require shutting-in of production, nor does it require a prior knowledge of drainage area, shape factor, and permeability under pseudo-steady state condition.

Objectives

The objectives of this study are

  1. to present procedures on how to simultaneously determine average reservoir pressure and initial fluid in place from production data and flowing bottom hole pressure for oil and gas reservoirs, and

  2. to test the validity of this new method with five examples employing vertical and horizontal wells.

Mathematical Formulation

This section gives a brief summary of a generalized material balance formulation of hydrocarbon fluid at reservoir condition. Detailed derivation of all the equations presented in this work can be found in reference

  1. and definitions of symbols are given in the nomenclature section.

Oil Reservoirs
(I) Material Balance Equations for Cumulative Fluid Production

For oil reservoirs, the material balance equation with constant Cf is represented by equation (1).

  • Equation (1)

(II) Material Balance of Oil Production Rate

Assuming no water influx and negligible water production, the material balance equation of oil production rate at reservoir condition is represented by equation (2).

  • Equation (2)

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