Abstract

Procedures for computing turbulent flow of gas in steady state near the well bore and a graphical method for predicting unsteady state laminar flow at distances from the well have been combined to compute pressure gradients in gas reservoirs.

Methods are discussed for predicting single and multiple transients at constant flow rate in an infinite reservoir, predicting constant flow rates in a finite reservoir, reproducing and interpreting back pressure test data, and prediction of the behavior of a closed-in gas well. An example of the graphical method is given for a single transient and the results are compared to a published analytical solution. A typical calculation of the pressure gradient in a reservoir and the production of gas is made starting with data from a back pressure test.

Introduction

The calculation of pressure gradients throughout a gas reservoir at any point in its production history is a complex problem involving turbulent flow near the well bore and unsteady state flow of the gas from the reservoir. The problem is complicated further by the variety of boundary conditions that may be imposed upon the flow. Such boundary conditions include finite or infinite reservoirs, constant or variable rates of production, constant or varying bottom hole pressures, complex initial pressure distributions throughout the reservoir, and pressure maintenance through cycling.

In addition to these problems there are the practical difficulties associated with natural gas reservoir analysis. These might include:

  1. variations in the permeability and porosity throughout the formation,

  2. variations in the thickness of the formation,

  3. communication of the producing formation with other producing zones,

  4. radial variations in permeability due to influx of drilling fluid, acidizing, or the buildup of liquid in the formation around the well,

  5. partial penetration of the producing zone,

  6. water flood, and many others.

These problems will be considered eventually. In the meantime, it is necessary to present methods of handling the case of radial flow through a homogeneous, regular, producing stratum to a single well uncomplicated by other factors.

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