Chemical Tracer Studies To Determine Water Saturation at Prudhoe Bay
- H.A. Deans (U. of Wyoming) | A.D. Mut (Exxon Production Research Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1997
- Document Type
- Journal Paper
- 52 - 57
- 1997. Society of Petroleum Engineers
- 5.3.4 Reduction of Residual Oil Saturation, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.4.1 Waterflooding, 5.6.5 Tracers, 2.2.2 Perforating, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 2 Well Completion, 1.11 Drilling Fluids and Materials
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The single-well chemical tracer (SWCT) method was used to measure resident water saturation in two wells in the Ivishak reservoir of the Prudhoe Bay field. The tests, performed in 1981, were in API Wells 406 and 488, both of which had been oil-base cored through the Ivishak. The results of the tracer test in Well API 406 were interpreted by conventional SWCT simulation procedures. A three-layer simulator model was used to match the tracer production profiles measured during the test. The average water saturation obtained from the simulator match was 16±3% PV, in good agreement with the Dean-Stark analysis of the oil-base cores. The tracer-test results from Well API 488 were more difficult to interpret because of the very low water saturation at this location in the Ivishak. The SWCT method requires a hydrolysis reaction to take place in the formation to produce a product tracer. In this case, the amount of water present was so small that very little product tracer was formed. The interpreted Sw for this well was 2±2% PV, again in reasonable agreement with the oil-base-core measured value of 3.8%.
Sw Determinations in the Ivishak
The estimation of hydrocarbon pore volume, VpHC in the Ivishak (commonly called the Sadlerochit) reservoir of the Prudhoe Bay field used the general relationship given in Eq. 1.
Because Sw was suspected to vary considerably within the oil zone, accurate estimation of VpHC demanded good measurements of Sw at a reasonable number of locations in the reservoir.
For reasons discussed by McCoy and Grieves,1 electric log accuracy was inadequate in some parts of the Sadlerochit. The primary method chosen to measure Sw was Dean-Stark analysis of samples from cores taken with oil-base drilling fluids.2 As an independent confirmation, in-situ measurements of Sw in two of the oil-base-cored wells by use of the SWCT method were performed.3
Tracer Methods for Measuring Sw.
The SWCT method was originally developed by Deans4 and Deans et al.5 to measure residual oil saturation, Sor, after waterflooding. Since 1968, this technique has been used in more than 200 sandstone and carbonate formations.5,6 Well-to-well tracer methods that use the same principles have been less frequently reported. Operational considerations make a single-well procedure much more practical.
As practiced, the SWCT method measures an average residual saturation over a relatively large volume of pore space. Typical depth of investigation is 10 to 20 ft away from the wellbore, which should be beyond the region of alteration caused by drilling and completion operations.
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