Decline curve analysis for real gas wells with non-Darcy flow

Abstract

Isochronal testing of gas wells suggests that high velocity flow in wells containing hydraulic fractures results in an additional pressure drop down the fracture that is flow rate dependent. In addition, wellhead pressures are also flow rate dependent due to the turbulent flow and liquid loading in the tubing. This high velocity flow effect in the tubing is referred to in the petroleum literature as turbulent flow. In the formation or hydraulic fracture, it is called non-Darcy flow. This work proposes new 'equivalent liquid variables' for rate, time and pressure that account for non-Darcy flow and real gas properties. This allows the engineer to predict and to history match gas well production using analytical solutions for a slightly compressible liquid. These methods were verified using a numerical model. For the normal hydraulic fracture stimulation of lower permeability formations, non-Darcy flow in the fracture dominates the total rate-dependent pressure drop in the reservoir. This study concentrates on creating a method to account for non-Darcy flow in a finite conductivity fracture. When compared to simulated data, the error analysis was within the accepted engineering accuracy. The major advantages of this method are that it extends the applicability of analytical solutions and that it improves the accuracy of the interpretation of early rate-time data. The interpretation of the this early data is very important since it is used to make decisions on infill drilling, sizing production tubing and facilities, and determining drainage area size and shape. One practical problem encountered in field is the very low frequency of measured rate and pressure data. The well production is reported monthly and the well shut-in pressure is usually reported biannually. A flowing sales line pressure is usually reported monthly with days the well was on line. The engineer usually has to present reserves and a 5 year production rate versus time forecast for a given sales line pressure each year for each gas well. The purpose of this research is to give the engineer a fast, medium-accuracy method that accounts for changing gas properties, stimulation treatments and major reservoir properties to analyze lower quality data. In summary, this work presents a method to analyze and to predict a variable rate and surface pressure production history of a stimulated gas well...