| dc.description.abstract | Permeability is a function of the primary rock properties of composition, texture, and sedimentary structures. For quartzose sandstones, the textural elements are the most important in determining both porosity and permeability. An empirical expression for permeability has been derived by Berg (1970). This equation assumes that permeability is largely a function of rectilinear pore size as determined by mean grain size, sorting expressed by deviation of grain size distribution, mean grain shape, and grain packing. The equation expresses permeability as a function of the square of the median grain size, of the fifth power of porosity, and of the phi percentile deviation. The equation was derived to approximate maximum permeability of well-sorted, highly-quartzose sandstones of 30 to 40 percent porosity.
Application of the equation to a variety of reservoir sandstones has yielded considerable success. The four major groups tested were Muddy, Frio, Frontier, and Woodbine-Paluxy sandstones. These sandstones represent a variety of depositional environments and exhibit considerable compositional and textural variation. The sandstones are medium grained to very fine grained and have average quartz contents ranging from 40 to 90 percent. Cement and matrix content ranges from less than 1 percent up to a maximum of 33 percent in some samples. Porosities range from 3 to 43 percent, and measured permeabilities range from 0.04 millidarcys to a maximum of 26.0 darcys. Sandstone samples were from subsurface cores with the exception of the Woodbine-Paluxy sandstones which were outcrop samples.
Consistent, reasonably accurate permeabilities were calculated for the clean, well-sorted, highly-quartzose, high-porosity Muddy Sandstone from Bell Creek field, Montana. The equation was also applied with good results to more poorly-sorted sandstones of less than 30 percent porosity which are not highly quartzose, such as the Frio 19B sandstone at Seeligson field. Furthermore, the equation was applied with some success to sandstones well below the porosity range for which it was derived. The Muddy and Frontier sandstones at Slick Creek field have porosities of 3 to 15 percent and a significant amount of matrix and cement. By including the effects of cement and matrix on the grain size distribution, good approximations for permeability were obtained. The Woodbine-Paluxy outcrop samples were found to yield acceptable results for higher porosities. Lower porosity outcrop samples contained variable amounts of cement as a consequence of weathering, and therefore, these samples yielded unreliable results.
The evaluation of Berg's equation has provided insight into the relationship between permeability and the primary properties of reservoir rocks. Such an equation may aid in future oil exploration and development by predicting permeabilities for sandstone reservoirs if properly applied. | en |
