Despite its many limitations, conventional decline analysis is still commonly used in gas production analysis due to its minimal data requirements and ease of application. Yet, there is strong desire and need for more sophisticated methods such as those loosely defined by the term advanced decline analysis. As a result, this paper presents an automated computer method for estimating original gas-in-place and other reservoir flow parameters. The method is semi-analytical in that it is based on a semi-empirical relationship between gas production and initial reservoir conditions, but is coupled with conventional decline analysis methods for ease adaptation into the workflow.
The procedure provides additional information comparable to flowing material balance analysis, but within the familiar Arps decline environment – a critical component that makes the procedure very amendable to traditional workflows. Another advantage is that the proposed methodology does not require a prior knowledge of net pay, formation and/or fluid compressibility, or reservoir pressure (other than initial conditions) for estimation of OGIP. Results will also show that the approach is generally accurate and robust (when used appropriately) and provide additional information such as effective permeability if a few volumetric parameters are provided. And that the procedure is extremely simple and can be implemented in desktop applications or spreadsheets with minimal computational effort.
Although there are limitations such as constant flowing pressure (as well as multiphase flow), it will be demonstrated that the procedure is generally versatile and can be applied to a number of gas systems including conventional sands and silts, thermogenic and biogenic shales, as well as (dry) coal bed methane. Results and observations are based on both simulated and field examples for a variety of operating conditions. In conclusion, a novel and rapid methodology has been introduced for estimating original gas-in-place, ultimate recovery (EUR), and other flow parameters with the simplicity of conventional decline.