One of the unresolved issues in the evaluation of coalbed methane (CBM) reserves is whether the long-term production profile is exponential or hyperbolic. The type of decline used to forecast production can have a significant effect on the remaining reserves; hence, a thorough investigation of CBM decline is warranted.
Before investigating the decline behaviour of CBM wells, the literature on conventional gas decline analysis is summarized and the factors affecting production performance are discussed. The production performance of CBM is then investigated and compared to conventional gas performance. Simulations were conducted in order to determine the sensitivity of the hyperbolic decline exponent, to different reservoir and operating conditions. Several parameters, including the flowing pressure, were found to affect the decline behaviour of CBM wells. Finally, it was determined that, in spite of the more complex production mechanisms, the decline behaviour of CBM is similar to that of conventional gas.
The objective of this study is to review traditional decline analysis and discuss its applicability to CBM. Traditional decline analysis is based on the Arps' equation which was introduced in the 1940s. It is an empirical relation that extrapolates a production forecast based on a curve fit of historical data. The general equation is given by:
q – Equation (1) Available In Full Paper
The initial decline rate, Di, and hyperbolic decline exponent, b, are constants used to calculate rate as a function of time. The theoretical limits on the decline exponent are 0 and 1. There are three specialized forms of the equation: exponential, hyperbolic and harmonic. Exponential decline is defined by a b value of 0 and can be recognized in several ways:
The decline rate, [Eqn], is constant at all times.
A plot of gas rate versus cumulative gas production (and the logarithm of gas rate versus time) will be a straight line.
Hyperbolic decline is defined by a b value between 0 and 1 and can be recognized if:
The decline rate, D, is decreasing constantly with time.
A plot of gas rate versus cumulative gas production (and the logarithm of gas rate versus time) has a concave upward appearance.
Use of the exponential equation produces a more conservative reserve estimate than the hyperbolic equation, given the same initial decline rate, Di. It is often difficult to distinguish between exponential and hyperbolic decline without a considerable amount of data, so it is up to the judgment of the evaluator to use an appropriate b value. Analogous production history is often used as guidance. The choice of b value not only influences the estimate of reserves, but it also affects how long and at what rates the well is expected to produce, which directly affects the economic evaluations.
Though Equation (1) is widely used, it does have limitations. It is only valid during boundary-dominated flow, and when the well's flowing pressure is constant. Fetkovich(1) created type curves that combine transient solutions with the boundary-dominated stems of the Arp's equation.
