Kansas Geological Survey, Open-file Report 2009-10
Alan P. Byrnes and others
KGS Open File Report 2009-10
Although prediction of future natural gas supply is complicated by uncertainty in such variables as demand, liquefied natural gas supply price and availability, coalbed methane and gas shale development rate, and pipeline availability, all U.S. Energy Information Administration gas supply estimates to date have predicted that Unconventional gas sources will be the dominant source of U.S. natural gas supply for at least the next two decades (Fig. 1.1). Among the Unconventional gas supply sources, Tight Gas Sandstones (TGS) will represent 50-70% of the Unconventional gas supply in this time period (Fig. 1.2). Rocky Mountain TGS are estimated to be approximately 70% of the total TGS resource base (USEIA, 2004) and the Mesaverde Group (Mesaverde) sandstones represent the principal gas productive sandstone unit in the largest Western U.S. TGS basins including the basins that are the focus of this study (Washakie, Uinta, Piceance, Upper Greater Green River, Wind River, Powder River). Industry assessment of the regional gas resource, projection of future gas supply, and exploration programs require an understanding of reservoir properties and accurate tools for formation evaluation of drilled wells. The goal of this study is to provide petrophysical formation evaluation tools related to relative permeability, capillary pressure, electrical properties, and algorithm tools for wireline log analysis. Detailed and accurate movable gas-in-place resource assessment is most critical in marginal gas plays and there is need for quantitative tools for definition of limits on gas producibility due to technology and rock physics and for defining water saturation.
Figure 1.1--Energy Information Administration prediction of future natural gas supply sources showing Lower 48 Unconventional sources will represent nearly 50% of consumption (Caruso, EIA, 2008).
Figure 1.2--Energy Information Administration prediction of future natural gas unconventional supply sources showing tight gas sandstones represent over half of unconventional supply (Caruso, EIA, 2008).
The results of this study address fundamental questions concerning: 1) gas storage, 2) gas flow, 3) capillary pressure, 4) electrical properties, 5) facies and upscaling issues, 6) wireline log interpretation algorithms, and 7) providing a web-accessible database of advanced rock properties. The following text briefly discusses the nature of these questions. Section 1.2 briefly discusses the objective of the study with respect to the problems reviewed.
1) Gas Storage--Issues with gas volume or storage are principally related to porosity, gas saturation, and fluid properties. Fluid properties have been well characterized in previous studies and gas saturation is defined by capillary pressure properties and wireline log response interpretation which are discussed separately. Routine (under no confining stress) porosity measurement in TGS requires careful quality control measures but is performed by commercial laboratories meeting quality standards. Although routine helium porosity is commonly measured, the influence of confining stress on porosity is not as thoroughly investigated. Further, the pore volume compressibility, or change in pore volume with change in net effective confining stress, has not been thoroughly studied for all Mesaverde rocks. This issue is important because it is necessary to know 1) how to correct higher routine porosity to reservoir (in situ) conditions, and 2) how in situ porosity changes with net effective stress increase associated with reservoir pore pressure decrease as the result of gas production.
2) Gas Flow--All assessments of gas resource are premised on assumptions concerning gas relative permeability and implicitly, the critical gas saturation (Sgc) or the minimum gas saturation at which gas flows. This saturation defines the beginning of the gas relative permeability curve. Some assessments assume that if gas is present its recovery is only a matter of price and/or technology. This premise is not valid for gas saturations less than or near critical saturation. Gas saturation less than or equal to Sgc can be achieved in nature by 1) highly local microscopic gas generation, such as from organic macerals, that have generated gas but the gas does not form a continuous phase across the pore system; 2) the rock has undergone water imbibition, either due to gas pressure decrease or water pressure increase, and the gas phase is trapped and represents a residual phase to water imbibition; 3) the gas entered the pore system under capillary pressure conditions existing during the gas entry, but the rock has since undergone further compaction or diagenetic alteration and now exhibits different capillary pressure properties; 4) the gas is actually mobile but is near Sgc rather than at a gas saturation (Sg) significantly greater than Sgc, where it would be interpreted that the gas phase is highly mobile. If Sgc is incorrectly interpreted to be low (e.g., Sgc = 2%) when it is high (e.g., Sgc = 30%), then for a measured gas saturation of Sg = 31%, for an incorrect gas relative permeability curve with Sgc = 2%, gas at Sg = 31% is incorrectly interpreted to be significantly more mobile than if Sgc = 30%, when the gas would be incipiently mobile. Limited research has been done in this area and published data can be interpreted to indicate that Sgc increases with decreasing permeability. This would eliminate some gas from being produced and from resource base estimates. Understanding the minimum gas saturation necessary for gas flow (Sgc) is fundamental to defining the tight gas sandstone resource and is particularly critical to quantify in marginal resources.
3) Capillary Pressure--While there is a some understanding of the influence of confining stress on permeability and porosity in tight gas sandstones, little work has been done for capillary pressure. In addition, most capillary pressure studies focus on the drainage capillary pressure curve and have not investigated or reported on the imbibition capillary pressure or on capillary pressure hysteresis where saturations change under a series of drainage and imbibition cycles beginning and ending at different initial and final saturations.
4) Electrical Properties--Extensive work has been done defining regional water composition, but there is less published work characterizing the effect of cation exchange (Waxman-Smits effect) on modifying standard Archie-calculated water saturations from wireline log response for Mesaverde rocks. In Mesaverde reservoirs diagenetic clays with high cation exchange capacity can be common and water salinities can often be fresh (<25,000 ppmw total dissolved solids). These conditions can lead to low resistivity for which the standard Archie analysis of wireline electric log response must be modified (e.g. Waxman-Smits, Dual Water approaches). Mesaverde studies published to date have focused primarily on the Mesaverde in the Multiwell Experiment (MWX) in the Piceance Basin and do not analyze other Mesaverde rocks. In addition, work has presented results for rocks with porosity generally greater than 6% porosity but little has been reported for rocks with porosity less than 6%. These rocks are generally considered to not be "pay" but reservoir flow simulation shows that these rocks represent storage for vertically adjacent beds where flow is significant. Therefore the accurate determination of water and gas saturation in these rocks is important to resource assessment. To measure this using wireline logs it is necessary to both understand the porosity exponent of these rocks and how electrical conduction changes with salinity.
5) Facies and Upscaling--Beyond investigating the above fundamental properties for representative lithofacies in the Mesaverde, it is necessary to know how critical gas saturation, capillary pressure, electrical properties, upscaling issues, and wireline log response and analysis change with more easily measured Mesaverde rock properties such as lithofacies, porosity, and permeability; and how flow properties, and particularly critical gas saturation, upscale with lithofacies bedding architecture. In addition, accuracy and variance of petrophysical relationships, such as permeability versus porosity, are premised on sampling, the scale of sampling, measurement methodology and accuracy and precision related to that, and the geostatistical or spatial distribution of the properties. Little published work is available that addresses how porosity or permeability change over short length scales (1-2 inches, 2.5-5 cm).
6) Wireline Log Interpretation--Petrophysical properties and relationships measured on core and at the core scale can provide critical reservoir characterization information, but core cannot reasonably, or economically, be obtained for most wells and over the entire Mesaverde interval of interest. For this reason, core are used to aid in calibration of wireline log response interpretation so that developed log algorithms can be used where core are unavailable. This requires that the wireline log response curves be correlated with core-measured petrophysical properties. These relationships can vary with such properties as rock lithology, petrophysical property, in situ conditions, log vendor, log vintage, log traces available in the logging suite, and the log algorithms developed and used. Algorithms can sometimes be developed that meet required accuracy and precision quality standards but that require a suite of input logs that are unavailable for historical wells and/or prohibitively expensive for new wells. Determining the number of unique lithofacies classes and the criteria for defining classes can involve four principal criteria: (1) maximum number of lithofacies recognizable using the available petrophysical wireline log curves and other variables; (2) minimum number of lithofacies needed to accurately represent lithologic and petrophysical heterogeneity; (3) maximum distinction of core petrophysical properties among classes; and (4) the relative contribution of a lithofacies class to storage and flow.
7) Data access--The body of data concerning TGS advanced rock properties is extensive but few companies have been able to devote the time or resources to compiling the data and make the data digitally accessible. A well-designed internet-accessible database is needed to provide access to the library of data, query the data with respect to user-defined relational issues, and provide a framework for future data input through XML linkage.
Kansas Geological Survey, Energy Research
Placed online Dec. 15, 2011
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