Forecasting production rates from gas wells has always been of considerable importance to the natural gun industry. The conventional method of predicting the performance of San reservoirs requires data which are often unavailable. This in particularly the came when dealing with low permeability gas reservoirs. In the absence of complete reservoir data, the production decline type curves are employed to predict the future rates utilizing production history. Generally, the production decline type curves are generated based on assumption that the operating and reservoir conditions which affected gas production in the past will remain unchanged. However, there are many instances when it is necessary to forecast the production rates under conditions which are not similar to those of the past. Typical examples of such cases are:
changes encountered in well spacing due to developmental drilling programs and
backpressure changes which occur due to compressor installations.
This paper introduces a series of production decline type curves and relative methodologies which can be utilized to forecast gas production when the conditions are altered. A gas reservoir model has been utilized to simulate gas production to study the effects of changes relative to reservoir and operating conditions. The model taken into account many factors such an the non-Darcy flow effects, skin factor, and pressure dependent gas properties. The results have been utilized to develop techniques (for type curve applications) to forecast production under different reservoir and operating conditions.
The production decline type curves are employed, in the absence of complete reservoir data, to predict the future production rates solely based on past production data. The idea of using the log-log type curves for matching and interpreting production date was first proposed by Fetkovich. Several sets of production decline type curves have been published in past literature. Most of these type curves have been developed based on simplifying assumptions which limits their applications. There are several suggested modifications which have provided some improvements for gas well production decline analysis. However, the modifications involve the use of now parameters which are difficult to evaluate and an a result complicate the matching process.
A set of general production decline type curves has been introduced more recently for gas won production decline analysis which does not suffer from limiting assumptions. However, it in generally assumed that the factors affecting production in the past will been unchanged in the future when the production decline type curves are utilized to predict the future performance. Often, one or more of the reservoir parameters or the operation conditions which influence the production altered at some time during the life of the reservoir due to a variety of circumstances. Typical examples of such cases are:
Well a is altered due to developmental drilling.
Backpressure is altered due to compressor installation.
Formation permeability in altered due to stimulation.
In order to predict the future performance of the gas wells after such alterations have taken place, it in necessary to modify the type curves to account for such conditions.