Determination of Gas In-Place From Analytical Analysis of Shut-in Or Flowing Pressure Survey Data

There is evidence that the current need for natural gas reserves has made certain low permeability reservoirs, heretofore considered marginal or uneconomical, candidates for development. Accurate estimates of the gas in-place in these reservoirs will not only be important for pipeline contracting purposes, but will become fundamental requirements for establishing the most economic well spacing and the best facilities and operations design. Considerable effort has been directed to the problem of estimating the gas initially in place and in predicting future performance of natural gas wells. Basically, the techniques described in various reviews of the methods used fall into the volumetric or the performance categories. The volumetric technique, based on geological considerations, is useful in the very early- stages of a reservoir's life. The pressure production methods, based on data recorded during the producing life of a well, are generally considered more accurate and can sidestep the error in volumetric estimates caused by unknown reservoir properties. Most theories for estimating the gas in-place have been based on one or more idealizations. Most performance calculations assume Semi-steady-state flow, small and constant compressibility, gas viscosity calculated at an average pressure, radial flow and all other ideal reservoir considerations. Most calculation methods require that the pressures on which the solution is based be weighted average representations of reservoir pressure. With the acceptance of real gas potential, and the development of equations defining the average pressure in bounded reservoirs of different configurations, it becomes feasible to develop a process for estimating the initial gas in-place from shut-in or flowing pressure data that is not stabilized. The method can take into consideration the geometry of the drainage area and the fact that the gas compressibility may vary widely because of large pressure drops in a low permeability system. The equations expressing the semi-steady state pressure drawdown or build-up in terms of the real gas potential at the wellbore are presented as Eqs. (2) and (3) below. In order to prove the validity of the concept, three separate executions of a numerical simulation model were performed to develop pressure production data suitable to check out the new procedure. Two different geometries and two different well flowing conditions were imposed on the numeric models so as to develop data under a range of assumptions. In all instances, a known amount of gas was assigned to the models and a pressure history corresponding to the assigned withdrawal schedule was developed. This pressure history, if correctly evaluated by the proposed procedure, should generate the known gas in-place to a high degree of resolution.