Traditional ("Arps") decline analysis is the most common reservoir engineering tool used for production performance forecasting. It has several advantages over other techniques in that it is simple to use, requires minimal data and is well understood by the industry. Currently, however, these methods are being misused in unconventional applications, such as tight gas.
Production performance from tight gas reservoirs is characterized by steep initial decline rates and long periods of transient flow. If decline analysis is performed using this transient production data, the main assumption of boundary dominated flow (BDF) is violated and inaccurate forecasts may result. The goal of this work is to understand the behaviour of tight gas reservoirs during transient flow so that the familiar Arps method may be applied. The effects of different tight gas production responses (bilinear, linear, pseudo-radial, boundary dominated) are investigated. Finally, a methodology for applying traditional decline curve analysis to tight gas, with reference to long term transient flow, is presented.
The goal of this work is to outline a method of production forecasting for tight gas reservoirs without the use of complicated tools. The most accepted and well understood forecasting tool is traditional decline curve analysis and for this reason is the focus of this paper. The general, hyperbolic form of the Arps decline equation is shown as Equation 1.
This equation is used to predict the gas flowrate (q) as a function of time (t). The hyperbolic decline exponent (b) can be determined by matching past production performance to Equation 1. For gas wells, hyperbolic decline exponents (0<b<0.5) are expected during BDF [Fetkovich (1) (1996), Okuszko (2007)]. However, decline exponents much greater than one ("superbolic") have been used to forecast tight gas production with limited success.
Many authors [Maley (1985), Cox (2002), Cheng (2007), Rushing (2007)] have investigated the use of traditional decline curve analysis to forecast production from tight gas reservoirs. Some authors [Maley (1985), Robertson (1988)] suggest limiting a hyperbolic or superbolic decline curve to an exponential decline curve at a certain time or at a specific decline rate. Other authors suggest that decline curve analysis be avoided altogether during transient flow and instead recommend using modern decline analysis methods [Fetkovich (2) (1980), Palacio (1993), Agarwal (1998), Anderson (2005)] as they are applicable to both transient and BDF flow behaviour. However, these methods are complicated, dependent on a complete (pressure/rate) dataset and not always available to the practising engineer.
Figure 1 shows the production history of a typical tight gas well in transient flow and outlines the common problems associated with tight gas decline analysis. The blue line is an exponential (b = 0) fit of the final portion of the production history. This extrapolation is likely conservative and underestimates recovery. Conversely, the red line is a superbolic best fit of the production history with a decline exponent of 2.0 and likely overestimates recovery from this well.