Title: Evolution of Permeability and Porosity In Sandstones
Speaker: by Philip H. Nelson, U. S. Geological Survey, Denver, Colorado
Date: April 6, 2001
Publication: The Outcrop, April 2001, p. 4
In many sandstone reservoirs, plots of core data show that the logarithm of permeability (k) is linearly proportional to porosity (Φ). Coherent trends usually are found if the data are derived from individual formations and/or depositional facies. I have compiled and replotted published permeability-porosity data, and extracted a trend line summarizing data for each formation. This collection of trend lines represents the evolution of permeability and porosity: from the high k, high Φ of sand packs and recently deposited sands, to the reduced permeability and porosity of consolidated sandstones, and finally to the low permeability, low porosity rocks typified as ‘tight gas sands’.
Sandstones of different compositions evolve along different paths on a log(k)-Φ plot. High-quartz sandstones (quartz arenites, with quartz > 95%) with quartz cement form the low-porosity bound for all sandstones, preserving relatively high permeabilities at low porosity (< 10%). Quartz arenites are typified by porosity values ranging from 4 to 10% at 1 millidarcy permeability and have the steepest slopes of all sandstones; that is, small porosity changes are associated with large permeability changes. The trend lines for sandstones with lower quartz content plot systematically at higher porosities for a given permeability value than quartz arenites. In other words, quartz arenites are the most efficient in terms of fluid flow.
Data from sand packs show that permeability is initially independent of porosity. Much of the pore space contributes to fluid flow. As rocks compact and undergo diagenesis, porosity is reduced, and a progressively smaller fraction of the pore space contributes to fluid flow as pore throats constrict. The lateral position of a data trend on a log(k)-Φ plot is determined by the fraction of pore space contributing to fluid flow. Fine-grained sandstones display lower porosity and permeability than coarser grained sandstones in the same formation. As diagenesis progresses, finer grained rocks lose pore size more rapidly than the next higher grain size, thereby broadening the range of log(k) values. The data fields of individual formations remain fairly coherent, although the slope, intercept, and degree of scatter of these log(k)-Φ trends vary from formation to formation. Framework grain composition and alteration products determine the trend and conversely, the character of log(k)-Φ relationships provide insight into rock character and fluid recovery.