Application of Oil-Base Mud Pressure Coring to the Determination of In-Situ Water Saturations
- G.S. Charlson (Vastar Resources Inc.) | R.A. DeRuiter (Arco E&P Technology) | A.P. Spence (Arco E&P Technology) | H.R. Warner Jr. (Arco E&P Technology)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- June 1997
- Document Type
- Journal Paper
- 105 - 110
- 1997. Society of Petroleum Engineers
- 1.2.3 Rock properties, 5.6.5 Tracers, 4.1.5 Processing Equipment, 1.6.9 Coring, Fishing, 5.3.4 Reduction of Residual Oil Saturation, 4.1.2 Separation and Treating, 5.8.3 Coal Seam Gas, 5.5.2 Core Analysis, 1.6 Drilling Operations, 5.2.1 Phase Behavior and PVT Measurements, 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials
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This paper describes a field project where pressure coring using oil base mud (OBM) was applied to determine whether or not a "surfacing" correction was needed in the use of OBM core water saturations (Sw) as accurate values for in-situ Sw. Six OBM pressure cores (2.75 inch diameter) were cut in two Prudhoe Bay field wells. One well was in the updip low Sw (generally less than 5%) gascap area and the other in the downdip higher Sw (25% to greater than 40%) portion of the oil column.
In this paper the modifications to the equipment and materials for this application of pressure coring are described. Also procedural changes required to handle successfully the pressure cores, both in the field and at the laboratory, are discussed. The extent to which the OBM had flushed the core was determined by tracers and by comparing the volume of gas released from the pressure core samples to the volume of gas released during laboratory simulated-OBM-filtrate flushing tests.
In the Prudhoe Bay field pressure cores were taken from two geologically different intervals: the Sag River Sandstone and the Ivishak (Sadlerochit) Sandstone. The results of the pressure core laboratory analyses are that: 1) for the Ivishak Sandstone, no water was expelled during thawing (equivalent to core depressurization during surfacing) for the full range of Sw (1 to 45%); but, 2) for the Sag River Sandstone, water (as much as 3% of the measured value) was expelled for 4 of 8 samples tested with all Sw values in the 25-40% range. The difference in water expulsion results is attributed to differences in pore structure between the two sandstones.
Except in the oil-water transition zone, water saturation (Sw) measurements taken on reservoir rock samples from cores cut with oilbase drilling muds (OBM) are typically considered to be reliable and quantitative measures of the in-situ Sw values and to be indicative of the vertical Sw distribution throughout the hydrocarbon column. When using OBM core Sw values quantitatively, one assumes that neither OBM filtrate invasion during coring nor the depressurization during core surfacing has altered the in-situ Sw values. Thus, the laboratory-measured Sw values are representative of in-situ values, other than the changes in Sw resulting from degassing the in-situ brine and temperature and pressure differences between surface and reservoir conditions. This paper describes a combination field and laboratory investigation of the alteration, if any, of in-situ Sw from the core surfacing process. A companion paper discusses the impact of OBM coring on OBM filtrate invasion and on in-situ Sw determinations.
In the Prudhoe Bay field on Alaska's North Slope, 27 OBM cores were cut to determine Sw variations both vertically and areally in the reservoir. This 225-square mile reservoir contains a 30 TSCF gas-cap overlying a 20+ BSTB oil column with a current maximum structural closure of more than 1000 feet. These OBM cores showed Sw values from less than 2%PV in the updip gas-cap area where there is no underlying aquifer to greater than 50%PV in the southwest downdip portion of the oil column. Also near the updlp limit of the aquifer, there is a rapid areal change in Sw (at constant subsea depth) generally perpendicular to the structural strike of the reservoir.
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