Utilizing Log-derived Permeability to Predict Rates of Gas Production
Authors: Denoo S.A. , Hammar R.F. , Maxwell J.B. 
Published: March 2010

Introduction

Log data have been utilized to identify hydrocarbon-bearing reservoirs and identify hydrocarbon types. To date, limited use has been made of this data to evaluate how a well will actually perform in terms of flow rates.

Log-derived permeability and standard reservoir fluid flow equations can be used to calculate absolute open flow for a natural flowing well. From this, various rates can be established for different draw-down pressures. The various rates can then be used to monitor completion efficiency.

The technique has been tested in the Morrow Springer, a gas sand in Oklahoma and the Texas Panhandle. Test results indicate this to be a useful method for predicting the rate at which a well will produce.

Introduction

Reservoir modelling and computer processing of well logs can be used to predict the absolute flow potential of natural gas reservoirs. The standard induction, density and neutron suite and computer processing can give flow rate potentials and volumetric reserves in addition to the usual porosity, water saturation and correlation. This is an important step in totally utilizing the information gained from well logs. The technique presented here is to calculate permeability from open-hole log data and combine it with physical reservoir parameters and Darcy's radial flow equation to predict a well's performance. This is done in much the same way that a reservoir engineer would take data from a four-point test and back calculate the average permeability. This paper will suggest a more useful sequence, beginning with permeability and working to a synthetic four-point test.

The ability to predict a well's potential has several advantages. A standard is set to judge completion efficiencies an economic evaluation of the well can be done before or during the pipe-setting operation, and the date of payout can be more closely estimated. In addition, knowing the magnitude of the well potential, an operator can order equipment of the proper size, thus saving waiting time and getting the well on line as soon as possible. Calculation of flow potential allows an operator to monitor his well during the critical completion stages and to detect abnormalities before further work is done. This ensures the best possible completion and the most gas the well can deliver.

Computer integration makes it possible to determine accurately the downhole hydrocarbon type and volume. Again, by using accepted engineering practices, the actual recoverable gas reserves can be obtained with very little error, save for the true drainage radius. Reserve estimates done by a computer take the human bias out of the calculation and give the reserves credibility and repeatability.

Any interpretation is very dependent upon the model or models used. By providing a direct link between log data and well performance, an erroneous interpretation model can be detected and corrected rapidly, thus making the information more useful to everyone concerned.

Theory

Darcy's equation for radial. steady-state flow of a compressible fluid in the region of laminar flow is used to describe the well's behaviour in the text of this paper.