Superpressured Gas Reservoirs: Case Studies and a Generalized Tank Model

Abstract

SPE Members Abstract Pressure decline data from 21 superpressured gas reservoirs in the Gulf Coast were reviewed to determine the characteristic slopes of the P/Z (pressure divided by the gas non-ideality factor) vs. cumulative production plots. Twelve of the reservoirs were substantially depleted. The data were fit with one or two straight lines which would indicate a change in the reservoir pressure support mechanism. The slopes of the plots were reviewed in particular to determine whether the apparent compressibility at reservoir pressures above the hydrostatic level was different than the apparent compressibility below the hydrostatic level, as proposed by Hammerlindl and others. Only four of the plots could reasonably be fit by two straight lines which indicated that the rock compressibility correction factor proposed by Hammerlindl and others, that changes at the hydrostatic pressure point is not general and the method cannot be used indiscriminately. To better identify the factors that cause downward curving P/Z plots the gas law equation for a gas/aquifer reservoir was rewritten in a generalized form with additional coefficients included the fluid and rock compressibility changes with pressure and gas exsolving from solution. The P/Z plot calculated by the generalized model departs from a straight line, and curves downwards depending on the compressibilities and aquifer size. The generalized equation, with appropriate expressions for the non-ideal factors, can be solved analytically or with numerical computer methods to fit both the shape and position of the decline data. An example fit, using both the two straight line method and the generalized tank model, to a reservoir with a curving P/Z plot is given. Introduction Gas reservoirs with abnormally high pressure have been encountered in all continents of the world. The pressure of the gas or other fluids in the formation can exceed hydrostatic pressure when the fluids are trapped and some of the pressure is supported by the weight of the overlying rock in addition to the weight of the fluid. These abnormally pressured reservoirs are called superpressured, or geopressured, if the pressure exceeds the normal pressure that would occur from a column of water at the same depth (about 0.45 psi per foot of depth). They can occur at any depth in the ground below a few hundred feet. The superpressured zones are generally found in sand/shale sequences or evaporite/carbonate sequences where the porous sand or carbonate becomes sealed in place with surrounding impermeable shale or evaporites. In the United States, superpressured zones are found in all of the hydrocarbon bearing provinces, but are concentrated in the Gulf Coast, Anadarko Basin, Delaware Basin, and the Rocky Mountain area. When gas is removed from a sealed container, the pressure inside the container will decline proportionate to the amount removed (corrected for non-ideal gas behavior). This simple principle is the basis for plotting the graph of the P/Z vs. cumulative production for gas wells. The reservoir is visualized as a sealed system such that a plot of P/Z vs. cumulative production will give a straight line that begins at the initial reservoir pressure for the start of production and declines linearly to zero pressure when the reservoir is depleted. If the reservoir is a depletion-type reservoir, the P/Z plot is a valid way to predict the gas reserve in the reservoir. In general, however, gas reservoirs are not simple closed containers and the P/Z plots deviate from straight lines. Previously published papers have pointed out that, in many cases, straight line extrapolation of the early production history P/Z data project a reserve that is too high. As the reservoir is produced, a downwards correction is needed to obtain a more correct value for the original gas-in-place, as shown in Figure 1. P. 269^

A Semianalytical p/z Technique for the Analysis of Reservoir Performance from Abnormally Pressured Gas Reservoirs

“…To demonstrate (and validate) the application of our method to typical field data, we analyzed production data taken from twenty (20) abnormally pressured gas reservoirs. The data from eighteen (18) of these cases are taken from published literature sources, 4,6,13,16,17,18,20 while the other two data cases were obtained from unpublished sources.…”

A Systematic Dynamic Characterization Method for Abnormally High-Pressured Gas Reservoirs

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