Saturation From Logs Laboratory Measurements of Logging Parameters
- H.C. Walther (Continental Oil Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 1968
- Document Type
- Journal Paper
- 251 - 258
- 1968. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 4.2.3 Materials and Corrosion, 5.8.7 Carbonate Reservoir, 5.3.1 Flow in Porous Media, 5.6.1 Open hole/cased hole log analysis, 5.5.2 Core Analysis, 5.2 Reservoir Fluid Dynamics, 1.14 Casing and Cementing, 5.8.5 Oil Sand, Oil Shale, Bitumen, 1.6.9 Coring, Fishing
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While logs provide the only in situ measurement of water saturation, the measurement is not direct. Calculated saturation depends upon the value assumed for the saturation exponent n. The literature furnishes evidence that n varies and that it depends upon the type of rock and fluids, wettability, confining pressure, etc. Hence, a laboratory technique was developed for measuring m and n under a confining pressure and at formation temperature. The technique involves mounting a sample in a Hassler-type sleeve, using an oil-brine flow system and making two- and four-electrode measurements. Results of measurements on Frio, Miocene and Berea sandstones and Cretaceous limestones are presented with applications to log analysis.
Decisions of whether or not to complete and how to complete, as well as reservoir engineering calculations, all require accurate water saturation data. Narrowing profit margins and development of marginal fields place strong demands on accuracy of data.
Logs provide the only in situ technique for determining water saturation. However, the measurement is not direct since the logs only measure formation resistivities. While equations relating resistivities to saturations were developed more than 20 years ago, these equations are too general for accurate application to all fields or formations. Also, the equations contain constants which are known to vary with rock type, stress, water resistivity, wettability, etc. Until more is known about these basic relationships, the recent advances in log quality and application of computers to log analysis cannot be fully utilized. The basic equations for resistivity log analysis are
where phi = fractional porosity F = formation factor (the ratio of the resistivity of a brine-filled core, Ro, to the resistivity of the brine, Rw) m = m factor or cementation factor a = an empirical constant
where Ro = resistivity of brine-filled core Rt = resistivity of core at saturation, Sw n = saturation exponent.
Constants a, m and n ideally should be determined for every reservoir. Currently, if no laboratory-measured values are available, the values a = 1.0 and m 2 are commonly used for limestone reservoirs, and a 0.62 and m = 2.15 are used for sandstone reservoirs. A value of 2.0 is assumed for n.
Values for n ranging from 1.0 to 4.3 for natural core, and up to 54 for synthetic cores, are reported in the literature (Table 1). Also, several references show that the measuring technique is very important and that many of the problems associated with relative permeability measurements might be expected. Even allowing for measurement problems, there is ample evidence to show that n does vary.
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