No abstract

No abstract

Summary Rate-transient analysis (RTA) has been widely studied to evaluate the shale-gas-reservoir parameters. However, few attempts have been made on composition-transient analysis (CTA) to evaluate formation properties about shale-gas reservoirs. In this work, we proposed a new CTA method and provided a physical interpretation of CTA. We first established a compositional simulator incorporating slip flow and extended Langmuir (EL) isotherm with multiple components. Then, a fully implicit iterative numerical solution of the nonlinear model was developed on the basis of perpendicular-bisection (PEBI) gridding. Finally, we defined the composition change and derivative as functions of pressure change and derivative. One important finding is that the free-gas composition is obviously different from the desorped-gas composition because of the different desorption capacity of different gas components. This contributes to the changes of composition of the produced gas over time. Another finding is that ultimate adsorption capacity determined the value of the composition change and composition derivative, which made adsorption content be easily determined by CTA. If the flow regimes are defined by composition, several flow regimes could be identified from the composition change and the composition derivative, which may not exist on the curves of pressure change and pressure derivative. The findings in this study could provide a more-effective and practicable way to identify reservoir parameters than the traditional well-test method. Furthermore, gas compositions could be measured at the wellhead, which makes the CTA feasible in practice.

No abstract

With the widespread drilling of gas wells in Marcellus shale, there are high potentials for wellbore instability problems when wells are located in longwall mining areas, which in many areas such as southwest Pennsylvania, West Virginia, and eastern Ohio are being used for extraction of the coal seam overlaying the gas reserves. The ground deformation, caused by coal mining, could generate large horizontal displacement and complex stress change in subsurface rock. This in turn triggers ground movement which can cause casing failure, and thus interruption in the operation of the well that raises safety and environmental concerns. This could result in shutting down the well for repair, or permanent abandonment. Thus, it is critical to characterize the parameters related to the longwall mining process and to propose a general casing design guideline in such areas. In this paper, numerical modeling was utilized to simulate the complex ground conditions and resulting stresses and strains in longwall mining areas. A casing design spreadsheet was then constructed for design of appropriate selection of casings, based on the results of the numerical modeling. Our results were validated with field practices of wellbore design in southwest Pennsylvania. This paper also provides a methodology for investigating potential ground deformations, resulting stress/strain changes, and wellbore stability issues for oil and gas wells drilled in longwall mining areas in Marcellus shale or similar formations worldwide with active coal mining activities.