![]() In this way, the OBM-core S w data define the S w values for the majority of the reservoir, whereas the P c/ S w data define the S w values in the interval just above the fluid contact and perhaps in areas of the field where P c data are available but OBM-core data are not. Alternatively, the OBM-core S w data can be used in combination with the P c/ S w data. For example, the OBM-core S w data can be used in combination with the resistivity logs to expand the data set used to include all wells and the whole of the hydrocarbon column. The technique chosen to calculate S w is often a hybrid that combines the use of two of these basic data sources. A key consideration when making calibrated S w calculations is the availability of special-core-analysis (SCAL) data on core samples from the particular reservoir that is, the number of laboratory electrical-property and P c/ S w core-plug measurements that have been made. Resistivity logs are run in all wells, so these data are available for making standard-log-analysis S w calculations. This is not a high incremental cost when OBM use is planned for other purposes. If no OBM cores have been cut, then this technique cannot be used unless funds are spent to acquire such data from one or more newly drilled wells. The choice of which S w-calculation approach to use is often controlled by the availability of the various types of data. This listing is the chronological order in which data are likely to become available, not in a ranked order based on the accuracy of the various methods. S w calculations using oil-based mud (OBM)-core-plug Dean-Stark water-volume determinations.S w calculations from laboratory capillary pressure/saturation ( P c/ S w) measurements by application of a model relating S w to various rock and fluid properties and height above the free-water level.S w calculations from resistivity well logs by application of a model relating S w to porosity, connate-water resistivity, and various rock electrical properties.S w in wellbores can be determined by the following primary methods: Techniques for calculating water saturation 5.3 Capillary pressure-derived water saturation values.5.2 Resistivity-log-derived water saturation values.4.5 Core, total, and effective systems compatibility.4.4 Adjustments to water saturation data from different methods.4.3 Routine OBM-core with resistivity-log data.4.2 Routine OBM core with capillary pressure data.4.1 Resistivity-log and capillary pressure data.4 Integration of water saturation data from different methods.3.3.1 Strengths and weaknesses of OBM-core values.3.3 OBM-core-plug Dean-Stark water-volume determinations.3.2.1 Strengths and weaknesses of capillary pressure-based calculations.3.2 Laboratory capillary-pressure/saturation measurements.3.1.6 Strengths and weaknesses of resistivity log-based calculations.3.1.5 Waxman-Smits-Thomas and dual-water models.3.1 Resistivity well logs and relational model.3 Application of each water saturation technique.2.3.2 Waxman-Smits-Thomas exponents and cation-exchange capacity.1 Techniques for calculating water saturation. ![]() A 10% pore volume (PV) change in S w has the same impact as a 2% bulk volume (BV) change in porosity (in a 20% BV porosity reservoir). In OOIP and OGIP calculations, it is important to remember the relative importance of porosity and S w. The challenge to the technical team is to resolve and to understand the differences among the S w values obtained using the different procedures, and to arrive at the best calculation of S w and its distribution throughout the reservoir vertically and areally. The complication is that often, if not typically, these different approaches lead to somewhat different S w values that may equate to considerable differences in the original oil in place (OOIP) or original gas in place (OGIP) volumes. Complexities arise because there are a number of independent approaches that can be used to calculate S w. Water saturation ( S w) determination is the most challenging of petrophysical calculations and is used to quantify its more important complement, the hydrocarbon saturation (1 – S w).
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