Traditional decline methods do not work for tight gas wells. A hyperbolic decline is often used for forecasting the production rate and estimating the expected ultimate recovery (EUR) of a gas well. This method is only valid when the well is in the Boundary-Dominated Flow (BDF) regime, and hyperbolic b-values are in the range of 0 to 1. However, most of the production data from tight gas reservoirs is in the transient flow regime and not in the BDF regime. This results in values of b bigger than 1, and a forecast which can significantly over-predict the reserves. The recently developed "power law exponential decline" method seems to be promising in predicting the production rate over both transient and BDF regimes.
In this paper, we examine the applicability of the power law exponential decline for different cases. We evaluate its behavior during both the transient and boundary-dominated flow regimes, and specifically for linear and radial transient flow. In the BDF regime, we also study the effect of reservoir size on the decline parameters.
We compare the power law exponential decline with the decline from analytical reservoir models, and we modify it accordingly. Furthermore, the sensitivity of the power law equation to any of its parameters is studied. The validity of the power law exponential decline in forecasting production in tight gas wells is tested using synthetic data.
The production of tight gas is becoming commercially significant in North America, and forecasting the future deliverability of tight gas wells is critical to the quantification of reserves. There are two methods of analyzing production data, namely the traditional methods such as exponential or hyperbolic decline, and the modern methods such as Blasingame, Agarwal and the Flowing Material Balance[–6].
The traditional methods are very popular, because they are very easy to use, and only require flow rate and time as inputs. They have served the "reserves evaluation" industry quite well for nearly 100 years. For conventional oil and gas wells, these simple methods are reasonably well behaved and their performance is well understood. However, for tight gas wells, they fail spectacularly, and typically significantly over-predict reserves.
The modern methods, on the other hand, are more rigorous, more sophisticated and more reliable. They work well for both conventional and tight gas wells, but they are more complicated to use, and require the availability of bottom-hole flowing pressure in addition to flow rate and time.
It is well known that the traditional methods should only be used with production data after Boundary-Dominated Flow (BDF) has been reached. During transient flow, these methods do not apply, and usually result in an "apparent" hyperbolic exponent, b>1. If this value is used to forecast future production, it can significantly over-predict reserves. Not withstanding this serious shortcoming, tight gas wells are commonly forecast using hyperbolic decline with b>1.