The Application of Pulsed Neutron Decay Time Logs To Monitor Waterfloods With Changing Salinity

Abstract

A combination of a pulsed neutron decay time log (TDT) TM) and an induction log has been successful in monitoring the early stages of a low- salinity waterflood. This technique has proved successful in determining the salinity of the mixture of flood-water and formation water so that water saturation can be determined behind the floodfront. Introduction During the early development of the giant Ghawar and Abqaiq fields of Saudi Arabia, Arabian American Oil Co. (Aramco) decided that the most efficient means of maintaining reservoir pressure would be through a peripheral water injection system. Accordingly, injection wells were located downdip from the original oil/water contact and as far away as 5 km from the nearest producing well.Initially, the sources of injection water were massive sandstone aquifers of the Wasia and Biyadh formations, which overlie the entire Ghawar and Abqaiq structure. Water salinities from these aquifers vary from 5,000 to 18,000 ppm. Water from the oil-producing zone of the Arab-D reservoir has salinities of about 200,000 ppm. In addition to the Wasia and Biyadh source, seawater with salinities of about 57,000 ppm now is being used in some areas.The producing interval, the Arab-D reservoir, typically is a calcarenitic-to-aphanitic limestone with some dolomitic stringers. Porosity is 20% on the average but can be as high as 30 to 32% in the upper part of the reservoir. Permeabilities range from a few part of the reservoir. Permeabilities range from a few millidarcies to over 1 darcy. Normally the producing wells are completed openhole with 7-in. casing set to the top of the reservoir. Usually the reservoir section is drilled out below the 7-in. casing approximately 200 to 250 ft below the casing shoe, several hundred feet above the present oil/water contact. Two-in. tubing, which will act as a kill string, normally is run just above the casing shoe, and the well is produced up the casing annulus.Most injection wells are completed openhole, the same way as producers. Injection rates tend to vary from area to area, ranging from 5,000 to more than 50,000 B/D.As early as 1960, a feasibility study made by Aramco successfully determined the rate of oil/water contact advance and calculated the oil displacement efficiency of waterflooding in the Arab-D reservoir. For this study, observation wells were drilled into the oil/water contact to observe fluid changes with time. The wells were completed openhole with a highly resistive fluid such as fuel oil in the wellbore. Initially the induction and sonic logs were run through the reservoir section. Subsequent induction logging was done at regularly scheduled periods to observe changes in oil saturation with time.The study assumed, at the time, that the advancing water at the oil/water contact would be entirely Arab-D formation water. Therefore, the values of water saturation could be calculated from induction and sonic logs using the classic Archie equation: (1) JPT P. 957