Three laboratory methods were developed to measure matrix gas permeability (Km) of Devonian shale cores and drill cuttings at native water saturations. The first method uses pulse pressure testing of core plugs with helium. The second, new method uses pulse pressure testing of core chips or drill cuttings with helium. These methods gave comparable results on 23 companion shale samples from two wells, with Km = 0.2 to 19 × 10(−8) md. The third, new method uses degassibility of core plugs with helium and methane, and yielded Km higher by a factor of 3 to 10. Most of the core plugs tested showed multiple microfractures that remain open at reservoir stress, and these dominate conventional flow tests. These microfractures are parallel to bedding, are coring induced, and are not present in the reservoir. Knowledge of Km is important in computer simulation modeling of long term Devonian shale gas production, and has been a key to understanding the nature of the natural fracture network present in the reservoir.
The Devonian shale is a commercially productive gas reservoir over a wide area of the Appalachian basin. Properties of the shale, especially permeability, have been the subject of a significant amount of research. Permeability results from previous measurements of Devonian shale have ranged from less than 0.01 to 800 microdarcies (1 × 10(−5) to 0.8 md). Two factors have made these measurements of permeability unreliable. First, pulse test experiments have been generally limited to measuring permeability greater than 0.01 d. Second, laboratory tests show that even when the shale cores are loaded to reservoir stress one or more coring-induced microfractures are usually present that remain partially open. These microfractures dominate flow, so observed permeability exceeds true matrix permeability by several orders of magnitude.
Devonian shale reservoir bulk gas permeability in the Appalachian basin is generally greater than 10 d based on well flow rates (typically greater than 20 to 30 MCF/D), so natural fractures must play a dominant role in gas productivity. Since matrix permeability (Km) appears to be very small, why is it important? Figure 1 shows the predicted cumulative production from a typical Devonian shale gas well based on 3D computer modeling. Table 1 shows the fracture spacing, aperture, and other reservoir properties used in the model. These properties are considered typical of those found in Pike County, Kentucky, based on Gas Research Institute study (discussed below). In Figure 1, for Km greater than 10(−6) md (0.001 d), recovery is independent of Km, i.e., productivity is controlled only by fracture properties. For values of Km less than 10(−9) md, recovery is too low to be commercial. In the range 10(−9) to 10(−6) md, Km is an important factor. Our studies show Km is in this range.
In 1991, GRI began a research program involving special coring, logging, and testing in the Devonian shale section of two wells in Pike County, Kentucky: Ashland FMC No. 69 and No. 78.