Determination of Oil Saturation After Waterflooding in an Oil-Wet Reservoir The North Burbank Unit, Tract 97 Project

Abstract

Laboratory and field techniques for measuring oil saturation were applied to an oil-wet reservoir. Log-inject-log and core measurements confirmed a 3.1 saturation exponent. Ultimate residual oil saturations determined by log-inject-log, core analysis, and microlaterolog procedures using laboratory-determined cementation and saturation exponents were in good agreement. Introduction In evaluating the prospects for commercial application of enhanced recovery processes, one of the most important factors to be considered is the quantity and distribution of oil present in the pore space at the time the process is to be applied. Waterflooding tends to leave low oil saturations in high-permeability zones and high saturations in tighter zones. Where expensive processes are to be applied, it is vital that the oil saturation distribution be established as accurately as possible. We have avoided the term "residual oil" because it is not precisely defined in the literature. Instead, the term "ultimate residual" is used to describe thoroughly flushed sand near the end-point of the relative permeability curve, and "current oil saturation" refers to the oil permeability curve, and "current oil saturation" refers to the oil saturation at some defined point in the production history of a reservoir. It is difficult to determine the oil saturation in a waterflooded reservoir for tertiary recovery purposes because there is some mobile oil present and significant changes can occur during drilling or coring. During the past few years, the rapid expansion of field testing of past few years, the rapid expansion of field testing of tertiary recovery processes using expensive chemical fluids has accentuated the importance of upgrading the accuracy of determination of oil saturation after secondary recovery. The industry has responded with several useful techniques, including special coring, logging, and tracer procedures. A number of excellent papers have been published giving details of these methods. This paper reports on the application of several techniques to a paper reports on the application of several techniques to a strongly oil-wet reservoir - the North Burbank Unit Tract 97 area. Nature of the Reservoir The North Burbank reservoir, which lies at a depth of about 3,000 ft, is a relatively clean, well consolidated sandstone of a type that occurs in many fields of north-eastern Oklahoma. The sand averages 47.2-ft net thickness, 16.8-percent porosity, and 50-md permeability. The stock-tank oil has a gravity of 39 degrees API. Tract 97, where the tertiary pilot is being run, is one of the most homogeneous areas of the North Burbank field. There are frequent, thin, shale laminae, but these do not interfere markedly with vertical fluid movement. Dispersed shales occur occasionally in the sand column, but water-sensitive clays have not been observed in the North Burbank reservoir. It long has been known that the North Burbank reservoir is strongly oil-wet. During the development of the waterflood, at least two wells on each quarter section were cored and, at frequent intervals, a wettability index designated as the Amott-Harvey "relative displacement index" has been run. This technique, which is similar to the Amott wettability measurement, compares the tendency of a permeability plug at irreducible oil to imbibe oil with the tendency of the same plug at irreducible water to imbibe water. The results are expressed as numbers ranging from − 1.0 (oil-wet) to + 1.0 (water-wet). Details of the procedure are given in Ref. 8. There is always some variation from specimen to specimen within a well, and from well to well, but the average over the North Burbank Unit is -0.7, strongly oil-wet. JPT P. 491