Water Salinity of First Eocene Reservoir: Unique Behavior and Influence on Reservoir Engineering Calculations

Abstract

Summary. The salinity of the produced water from the First Eocene reservoir of the Wafra field was studied through its history. The change in the salinity of the initially produced water (from about 500 to 20,000 ppm NaCl) was attributed to meteoric water that might have entered the reservoir through outcrops west of the field. The correct value of the interstitial water salinity (23,000 ppm) that should be used in estimating the original oil in place (OOIP) by the volumetric method was determined by three different approaches. In addition, a technique to overcome the complex behavior of aquifer salinity in calculating the volumetric OOIP for the First Eocene reservoir is outlined. A study of the change in the produced water salinity of the First Eocene reservoir with time proved that water is dumping from an upper water- bearing zone into the reservoir. Analysis of temperature and noise logs confirmed that this upper water dumping, which apparently has supported the reservoir pressure, occurred behind casing in many deeper wells penetrating the First Eocene reservoir. Introduction History. The Wafra field is located in the Partitioned Neutral Zone between Kuwait and Saudi Arabia, as shown in Fig. 1. This field comprises several major and minor oil accumulations at different depths. The shallowest reservoir in the Wafra field is the First Eocene, which is found at a very shallow depth, and its name reflects the geologic age of the producing limestone zone. The productive area of the Wafra field comprises several productive area of the Wafra field comprises several reservoirs so that, at one location, one deep well can penetrate more than one reservoir. penetrate more than one reservoir. Drilling was begun in the Partitioned Neutral Zone in Dec. 1949 by a Getty Oil Co. and American Independent Oil Co. joint operation at the location of Exploratory Well is on the eastern flank of a large gravity anomaly. This exploratory well encountered a wet sand member at a depth very comparable to its depth in a big reservoir some 19 miles [30 km] north. Drilling activities were continued and seismic surveying was adopted in 1950 in an effort to determine precisely the subsurface structure shown by the gravity precisely the subsurface structure shown by the gravity anomaly. Following the unsuccessful results of Exploratory Wells 2 and 3, drilling operations were suspended awaiting additional geophysical work. The Wafra field was discovered in April 1953 when Exploratory Well 4 proved to penetrate commercial oil accumulation in the proved to penetrate commercial oil accumulation in the Burgan sand. Cores from this and a subsequent well confirmed the presence of oil accumulations in beds overlying the Burgan sand, especially in Eocene' limestones. The well designed to test the productivity of Eocene limestones, spudded on Dec. 31, 1953, proved conclusively that a significant oil accumulation existed in the Copyright 1987 Society of Petroleum Engineers First Eocene limestone at a very shallow depth. The last First Eocene well was completed in Jan. 1963. Currently, the First Eocene reservoir contains more than 200 completions, the majority of which are partially penetrating openhole completions. penetrating openhole completions. Geology. It is known from the stratigraphic column of the Wafra field that the First Eocene reservoir is overlain by the Rus and Dammam formations. The Rus formation is an anhydrite rock that represents a cap rock for the First Eocene reservoir. The Dammam formation is mainly a water-bearing formation with a salinity of about 16,000 ppm. The First Eocene reservoir rock has been described as a marly dolomitic limestone of various porosities. porosities. The First Eocene reservoir consists of two structures known as "main area" (bigger structure) and "west Wafra" (smaller structure). Although the First Eocene limestone in the west Wafra structure is appreciably higher than that in the main area, it is particularly important to note that little or no oil is present in the western flank of the west Wafra structure. present in the western flank of the west Wafra structure. In addition, logs taken from wells penetrating the First Focene limestone suggest a strongly tilting oil/water contact (OWC). The OWC at the eastern flank of the reservoir is about 200 ft [61 m] deeper than at the western flank of the reservoir. These observations imply that oil accumulation in the First Eocene limestone is under hydrodynamic condition. JPT P. 1119