Reservoir characterization, porosity, and recovery efficiency of deeply-buried paleozoic carbonates: Examples from Oklahoma, Texas and New Mexico

Abstract

Oaplllary-preeeure data from the Early Ordovician Ellenburger Dolomite (west Texas and New Mexico)and the Late Ordovician-Early Devonian Hunton Group carbonates (Oklahoma) are used to calculate or infer petrophysical characteristics, such as median pore-throat size, pore-throat sille distribution, eerective porosity, and recovery efficiency (RE). For both data sets, porosity and RE are inversely related. A positive relationship between RE and poroeity has been reported by other workers, but the relative importance of these opposed trends is unknown.The ability to accurately predict which relationship will hold in a given reservoir unit would be of great value for predicting reservoir performance.RE is also inversely related to median throat size. This is a consequence of two controlling factors: rock characteristics and experimental procedure.Drainage (recovery) from small throats is more eCCicient than from large throats, and hysteresis limits recovery from large throats because throats filled at very low pressures early in the intrusion precess remain filled at comparable pressures upon extrusion.The experimental procedure also suppresses extrusion from large throats because the minimum pressure attained on extrusion is greater than the initial intrusion pressure, due to limitations of the apparatus.Reservoir rocks are classified in terms of their capillary-pressure curve form, because curve form is controlled by a variety of petrophysical factors which can be measured, and because curve form is strongly correlated with recovery efficiency.Steep-convex capillary-pressure curves correspond to samples with high REs, low porosities, small median throats, and high entry preaeures.Steep-concave curves correlate with low REs, high porosities, large median throats, and low entry preseures.Gently-sloping curves correspond to samples with moderate REs, intermediate median throat s_, poorly-defined entry pressures, platykurtic throat-sise distributions, and variable porosities.Polymodal curves result from polymodal throat-sise distributions, and exhibit variable REs and porosities.Steep-concave and steep-convex curves are interpreted in two quite different ways, as follows. Steep-convex curves indicate reservoir rocks with high REs, but low porosities and small throats, so that production is likely to be economical only under high pressures (or thick' oil columns) or from very large hydrocarbon pools.Conversely, steepconcave curves indicate porous reservoir rocks with large throats but probably poor primary recovery efficiency. These raerYoin will be economical even at low preseures and with short oil columns and small total reserves, but will probably need enhanced recovery to produce a significant proportion of reserves.This c1aseification may allow characterisation of an "ideal" capillary-preseure curve, which is characterised by a moderate entry pressure, intermediate median throat sise, good RE, moderate porosity, and leptolturtic throat-sise distribution.Capillary-pre88ure plots showing both cumulative and incremen...

Petrophysics of a dolostone reservoir: San Andres Formation (Permian), west Texas

“…In this study these parameters were used to empirically divide rocks into five distinct groups. Details of the techniques used in this study to classify rocks are different from those of previous workers (Brown, 1951;Murray, 1960;Stout, 1964;Wardlaw and Cassan, 1978;Wardlaw, 1980;Amthor et al, 1988). The basic philosophy of classification remains the same, however, and Groups 1-5 are directly correlatable to curves published by Murray (1960), Stout (1964) and Wardlaw (1980).…”

“…CP curves obtained by mercury porosimetry and centrifugation have been used as a basis for rock classification (Murray, 1960;Stout, 1964;Wardlaw and Cassan, 1978;Amthor et al, 1988). Such curves have also been used to obtain more precise determinations of the height of a column of oil above an unseen oil-water contact (Hobson, 1954;Arps, 1964;Stout, 1964;Jodry, 1972;Schmidt et al, 1977;Jennings, 1987), and to determine the thicknesses of transitional and oilbearing horizons, the potential for secondary migration, and trapping mechanisms.…”

Petrophysical characteristics of holocene beachrock

“…Their normalized throat sizes are large. These convex curves differ from those of Amthor et al (1988).…”

“…The extrusion curves are not subparallel to intrusion curves for this convex capillary-pressure curves. So, this kind of curve is different from any documented by Amthor et al (1988). Second, we found a series of concave curves that can be subdivided into three sub-categories based on the shapes of the curves and the intrusion volumes under high pressure.…”

Very high-pressure mercury porosimetry as a tool in reservoir characterization

“…Equations from Jennings (1987). Original equations from Purcell (1949) and Smith (1966). values are equal to or exceed maximum pressures attained in several published studies (Wardlaw and Taylor, 1976;Wardlaw and McKellar, 1981;Jennings, 1987;Ghosh et al, 1987;Amthor and Kopaska-Merkel, 1988). Table 2 lists rock samples from the Hunton Group and from the Ellenburger in West Texas.…”