Shale Water As A Pressure Support Mechanism in Superpressure Reservoirs
Abstract: 4 --G. 'more welle penetrete tha deeper hori--ABSTRACT One problem confronting! reeervoir engiaeers tcday la an analyeie of the L.-.--. reeervoir mocnanammm iii~~l?=d iil :h= depletion of abnormally hish preeeure reeervolre.Two mechanleme that have baen propoeed ara (1) hitth rock compreeaibility and (2) shale water influx. This paper la a study of the qhale water i"nflux theory. A litqrature search waa conducted to qetablieh what ie known qbout shala . ...--. .L. ,.--permeaox~azy, c0xIpres8aoLAALY, poroeity, … Show more
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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^
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^
Graphical application of the gas Material Balance Equation (MBE), P/Z plots, have typically been used to predict the gas in place (GIP) of volumetric reservoirs. Few reservoirs however, are truly volumetric and several workers have developed procedures to correct such P/Z plots, particularly in over pressured regimes, for the pressure maintaining effects of rock collapse and/or shale water influx. The paper examines and discusses the applicability of eight (8) such procedures to the mildly overpressured, relatively low yield, gas condensate Kiskadee Field in which the original GIP is known from late life P/Z data and in which pressure maintaining effects on the early life P/Z data were not previously recognised. Production, bottom hole pressure, PVT and pore volume compressibility data are used to analyse the early and late life P/Z data of each identified fault block and to determine the extent and causes of apparent early life pressure maintenance as well as observed variations in the degree of pressure maintenance in individual blocks. A modification is proposed to the Bourgoyne procedure that accurately corrects the early life P/Z data of mildly overpressured reservoirs. A further modification is proposed to correct late life P/Z data for the effects of decreasing shale water influx and increasing water saturation. This further modification is successfully applied and results in GIP estimates that are within 3 % of the known volume. Introduction An estimate of the Original Gas In Place (OGIP or Gi) for a volumetric gas reservoir can be obtained from volumetric gas material balance considerations and yields:P/Z=Pi/Zi-(P i/ZiGi)Gp . . . (1) This linear relationship is the expression of a constant volume reservoir and assumes that rock and water expansion are negligible and that there is no net movement of gas into or out of the reservoir volume of interest. A material balance plot of P/Z vs. Gp for a volumetric, depletion drive gas reservoir generates a straight line of slope -(Pi / ZiG i) with an intercept of Pi / Zi for Gp=0. Extrapolation of the straight line to the Gp axis i.e. P/Z=0, yields the OGIP. Many gas or low yield gas condensate reservoirs do not yield linear plots but instead may curve upwards or downwards2. The constant volume assumption is then not valid. In addition to bad data, an upwards or downwards curving P/Z plot may be due to a number of reasons inclusive of water influx, drainage/leakage into the reservoir, subsidence/compaction drive, expanding oil rim (upwards curving) or retrograde condensation, drainage/leakage out of the reservoir, competitor overproduction and/or overpressured reservoir (downwards curving). Retrograde condensation is usually not significant enough to result in downwards curvature13. Downward curving plots usually plot more accurately as a continuous curve but are generally approximated by two (2) linear segments with distinct slopes that yield a good fit to the end points of the data set with a poor match in the area of intersection9. The slope of the early life data (the first slope) extrapolates to an Apparent Gas In Place (AGIP). The late life data plots with a higher slope and extrapolates to the OGIP. Dependent on the severity of the downward curvature, extrapolation of early life data may yield grossly inflated estimates of the GIP with consequent deleterious effects on the ability of the Operator to efficiently manage and produce the field. This is particularly important where long-term contractual arrangements are in force for the supply of gas from the field as pertains for the majority of Trinidad's East Coast gas reservoirs. Kiskadee Field The Kiskadee Field is located 40 kilometres off the eastern coast of Trinidad in approximately 270' of water. The field was discovered in 1977 with the drilling of two (2) exploration wells that tested gas-condensate from the primary reservoir sands at 15,500' subsea (Kiskadee Sand). The sands are mildly overpressure (0.55 psi/ft), Pliocene age, shallow marine, deltaic deposits with an average porosity of 25% and permeabilities within the cleaner sands of between 50 to 100 mD. The field lies along the gently dipping, northwest plunging structural nose of a major (Cassia) anticlinal structure. Primary structural elements are northwest trending normal synthetic faults, downthrown to the northeast, that divide the field into four (4) productive fault blocks (FBs) that are as shown in Figure 1.
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