An Evaluation of Various Logging Methods for the Determination of Remaining Oil Saturation in a Mixed Salinity Environment

Abstract

SPE Members Abstract This paper describes several known log analysis methods and calculation techniques used on a number of wells for determining remaining oil saturation in a mixed or unknown water salinity environment. The results obtained from these various methods are compared with remaining oil saturations obtained from coring with sponge core. In zones where the porosity is greater than fifteen percent and the zone is well swept, the remaining oil determined from the carbon-oxygen ratio agrees very well with results obtained from sponge cores. Due to the shallow depth of investigation of the carbon-oxygen tool, care must be taken to insure that the reservoir zone of interest is free of filtrate. Introduction Over the past two years Aramco has made a considerable effort to evaluate various logging methods to solve the problem of determining oil saturation in a mixed salinity environment. Oil production from several fields in Saudi Arabia has production from several fields in Saudi Arabia has been supported by peripheral water drive since the early 1960's. The advance of the water flood has been monitored by logging in existing wells and by drilling evaluation wells in areas of scarce well control. The injected water salinity has varied from 8K ppm to 60K ppm TDS and has been significantly lower than the connote water salinity of 200K ppm TDS. The resultant flood front has often ppm TDS. The resultant flood front has often been a mixture of various waters with an unknown salinity. Calculation of remaining oil saturation by conventional methods then hinges upon the knowledge of water salinity. A great deal of effort has been spent to develop a method or methods which will either give us the in-situ water salinity or calculate water saturation independent of salinity. To evaluate some of these methods we chose wells in areas where some well-swept intervals occur and where it was felt that remaining oil saturation measurements determined from sponge core would represent the insitu reservoir oil saturation. In addition to the sponge core, the wells were logged with DIL-MSFL, LDT-CNL, EPT-MLL, and BHC with mud in the hole. RFT fluid samples were also obtained to determine the reservoir water salinity. After the wells were produced to clean out mud filtrate they were logged with PNL-DIL-Rxo and carbon/ oxygen logs. Remaining oil saturation was calculated from various combinations of log data and the advantages and disadvantages of the various methods are discussed. METHODS FOR CALCULATING WATER SATURATION With the exception of the carbon-oxygen ratio method, most other logging methods rely on knowing the formation water salinity to determine fluid saturation in the reservoir. When the salinity or resistivity (Rw) of the formation water is low (less than 50,000 ppm TDS) or highly variable, conventional saturation methods become unreliable. The carbonoxygen saturation method refers to the use of the measured carbon-to-oxygen ratio to calculate water saturation. Since the amount of oxygen in water is essentially independent of salinity, the method has a distinct advantage in salinity environments. Another advantage is the ability to log in either an open hole or cased hole environment. A major limitation of the carbonoxygen measurement is the shallow depth of investigation. Water saturations calculated using resistivity logs such as the deep induction or the deep laterolog require a continuous knowledge of formation water salinity throughout the reservoir. P. 495