Fifty Years of Wettability Measurements in the Arab-D Carbonate Reservoir
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 105114, "Fifty Years of Wettability Measurements in the Arab-D Carbonate Reservoir," by T.M. Okasha, SPE, J.J. Funk, SPE, and H.N. Al-Rashidi, SPE, Saudi Aramco, prepared for the 2007 SPE Middle East Oil & Gas Show and Conference, Bahrain, 11–14 March.
The Ghawar field in eastern Saudi Arabia contains carbonate reservoirs. Most wells in the field produce from the Arab-D reservoir, an Upper Jurassic limestone sealed by anhydrite. Oil production from the field started approximately 55 years ago. Water injection started in the 1970s. A current evaluation of Arab-D wettability takes into account a long historical record of wettability measurements and production. The results of the original measurements have changed slightly, but they show a strong consistency 50 years later. Local variations in wettability indicating mixed wettability and oil-wet tendencies were observed when tar was present in a significant amount and in areas high on structure.

Introduction
The wetting properties of carbonate reservoirs are fundamental to the understanding of fluid flow in all aspects of oil production and can affect the production characteristics greatly during waterflooding. Therefore, knowledge of the preferential wettability of reservoir rock is of utmost importance.
Carbonate reservoirs are heterogeneous in nature because of the wide spectrum of environments in which carbonates are deposited and the subsequent diagenetic alteration of the original rock fabric. These heterogeneities and the effects of wettability on residual-oil saturation, capillary pressure, electrical properties, relative permeability, and oil recovery encouraged research to characterize and evaluate wettability of carbonate reservoirs. In the past, many engineers assumed that most reservoir rocks were water-wet. Recent work showed many carbonate reservoirs are oil-wet. Other studies have shown that the wettability of carbonate rocks is oil-wet, neutral, or mixed. This paper summarizes a study of wettability for the Arab-D carbonate reservoir (Upper Jurassic) in Saudi Arabia.

Arab-D Reservoir
The Arab-D reservoir was discovered in 1948. Five additional areas were identified as parts of the Ghawar oil field. From north to south, they are Ain Dar, Shedgum, Uthmaniyah, Hawiyah, and Haradh. At the Arab-D level, the field is a north-northeast-trending composite anticline 230 km long by 30 km wide. The largest oil accumulations occur in the lowest grainstone cycle of the Arab formation, the Arab-D reservoir. The vertical oil column has a maximum thickness of 1,300 ft. The oil-saturated interval extends approximately 250 ft below the anhydrite that separates the Arab-D reservoir from overlying Arab-C carbonate beds.
X-ray-diffraction and X-ray-fluorescence analyses revealed that the most predominant mineral in the Arab D reservoir is calcite (80–100 wt%), while dolomite constitutes a second minor mineral (0–26 wt%), along with trace amounts (&lt;5 wt%) of quartz, pyrite, ankerite, and halite.