Log Analysis of an Oil – Bearing Water Productive Vuggy Dolostone: Permian Glorieta Dolostone SE New Mexico
G.B. Asquith, Department of Geosciences Texas Tech University
During drilling through the Permian Glorieta Dolostone in southeast New Mexico bright yellow-green to dull yellow gold fluorescence was noted along with from 28 to 561units of gas. Later nine rotary sidewall cores were taken in the Glorieta. Oil saturation (So) from core analysis ranged from 12.7% to 33.1% with fluorescence over 40% to 100% of the sidewall cores (Martin and Asquith, 2004). Thin section analysis of the nine sidewall core revealed the presence of vuggy and intercrystalline porosity. Log analysis indicated that Sw(archie) < Sw(ratio) and that PHI(nd) > PHI(sonic) = PHI(rxo) thus revealing that the Glorieta is addition to being a vuggy carbonate is also water – wet [i.e. PHI(sonic) = PHI(rxo)].
In a water-wet vuggy reservoir with fluorescence over 40% to 100% of the cores the non-wetting oil must be in the matrix porosity and therefore oil MUST be filling the vugs [i.e. Sw(vug) = 0.0 and BVW(vug) = 0.0]. Archie water saturations calculated by the following method ranged from 31.5% to 69.4% and averaged 50.9%:
Sw(archie) = [BVW(matrix) + BVW(vug)]/PHI(nd)
BVW(matrix) = Sw(matrix) * PHI(sonic)
Sw(matrix) = [(1/PHI(sonic)^2)*(Rw/Rt)]^0.5
BVW(vug) = 0.0 [Water – Wet]
A DEW Plot (Rt/Rw versus Rxo/Rmf) of the Glorieta indicated that the Glorieta should be water productive. The initial production of an offset well (0.25mi) at the same subsea depth from correlative zones was 300bwpd with a slight show of gas which supports the data from the DEW PLOT. The question is with water saturation from 31.5% to 69.4% why is the Glorieta potentially water productive.
A cross plot of PHI(sonic) versus core derived permeability for the nine rotary sidewall cores indicated the presence of Lucia(1995) petrophysical classes 1, 2 and 3. An assumed height above the free-water level of 400’ together with PHI(sonic) and the appropriate Lucia(1995) petrophysical class were used to calculate irreducible water saturations (Swirr) of the matrix porosity. The relative permeabilities (Kro and Krw) for the matrix porosity were obtained by combining Sw(archie) of the matrix porosity, Sw at ROS (1.0-So), and the Lucia(1995) derived values of Swirr with the Jones(1945) relative permeability equations.
The relative permeabilities of water (Krw) ranged from 0.09 to 0.69 and for oil (Kro) ranged from 0.0 to 0.08. Thus even though the vugs MUST contain oil the relative permeability to oil in the matrix intercrystalline porosity is so low that predominantly only water can flow. Remember, to produce oil from the vugs the oil must flow through the matrix intercrystalline porosity.