One of the parameters needed to calculate in-situ water saturation from wireline logs is the resistivity of connate water, Rw. It is usually determined by measuring the resistivity and chemical composition of uncontaminated connate water produced from the formation or underline aquifer. If the formation does not produce any connate water, e.g., Deep Basin plays in Western Canada and tight gas reservoirs, or the produced water is contaminated it is difficult to determine accurate Rw necessary for water saturation calculation. This paper presents the results of a laboratory study of examining the validity of core based salinity determination. Controlled experiments were conducted on core samples, one Berea sandstone core, one tight sandstone core and one tight carbonate core with the latter two coming from gas producing formations in Alberta. The air permeability of the samples varies from 1 mD to 80 mD. Standard sandstone brine and carbonate brine of known salinity and compositions were used as the base fluids for comparison in the tests. Several methods, i.e., electrical properties measurement (back calculating Rw), de-ionized water flow through extraction, methanol flow through extraction, and extraction of core salts by leaching ground core, are compared and the pros and cons of each method are discussed. All of the methods tested in the study provided reasonably good results for the sandstone samples with little soluble minerals but poor results for the carbonate sample due to soluble minerals in the matrix. The effect of dissolution of soluble minerals on extracted salinity and individual ion concentrations are evaluated.
One of the parameters needed to calculate in-situ water saturation from wireline logs is the resistivity of connate water, Rw. It is usually determined by measuring the resistivity and chemical composition of uncontaminated connate water produced from the formation or underline aquifer. Log analysis using SP logs and interpretation from porosity and resistivity logs in aquifer are other possible sources of Rw values. If the formation does not produce any connate water, e.g., deep basin plays in Western Canada and tight gas reservoirs, or the produced water is contaminated it is difficult to determine accurate Rw necessary for water saturation calculation. Difficulty also arises if the salinity of connate water in a reservoir is not constant, leading to vertical and/or areal variation in Rw in the reservoir (McCoy, et al, 1994; Rathmell, et al., 1995; Rathmell, et al. 1999). In these cases core based salinity measurement provides an alternative and, sometimes, the only method to determine the Rw values.
Accurate measurements of core water salinity are based on the assumption that all of the chemical ions in connate water at reservoir conditions are still contained and remained the same in the core water at ambient conditions at surface. If the formation water is not at or near the solubility limits, i.e., soluble at reservoir temperature and pressure but insoluble at ambient conditions, it is possible to cut cores that retain in-situ formation water compositions by low invasion coring technology and oil based mud (OBM).
