Production history analysis in predicting “dry” gas-well performance

Abstract

The determination of future production rates and recoverable gas reserves are the primary requirements for making an evaluation of natural gas reservoirs. Often production history is the only available performance information on gas wells. The conventional method of using bottomhole pressure and gas composition to predict gas reservoir performance cannot be utilized inasmuch as these data are not available. At the same time, the use of finite-difference numerical reservoir simulators to analyze the production history is impractical when important reservoir data are lacking. Production decline type curves are employed, in the absence of complete reservoir data, to analyze production history and predict the future production performance of the reservoir. Generally, production decline type curves are considered to be long-term constant pressure solutions. These solutions are expressed as a plot of dimensionless flow rate versus dimensionless time on a log-log paper. The published gas well production decline type curves, however, have been derived from empirical and/or simplified models which do not represent actual gas reservoir performance. The utilization of production history to determine future rates implies that the factors influencing gas production remain constant for the entire life of the reservoir. For the most part, this assumption is not valid under normal operating conditions. A set of type curves are presented based on a comprehensive model which simulates a “dry” gas well production decline. This model accounts for a number of factors which are usually ignored in the derivation of other type curves. These include: the pressure dependency of the gas properties; pressure loss owing to non-Darcy flow; skin factor; and shape factor. In addition, methodologies are introduced for predicting the future rates when the factors influencing gas production are not constant with respect to time. Applications of the type curves have been illustrated by utilizing data from several gas wells in the northeastern United States.