Abstract

The Ghawar field in Eastern Saudi Arabia contains the largest accumulation of carbonate reservoirs in the world. The majority of 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 1970’s.  Long before water injection was considered for the reservoir, the evaluation of wettability was considered essential.

Our present day evaluation of Arab-D wettability takes into account a long historical record of wettability measurements and production history.  The procedures, results and caveats of the original measurements have changed slightly but they also show a strong consistency fifty years later.  Wettability indices obtained from initial tests, Amott, and USBM methods generally indicate neutral to slightly oil-wet character for cores processed and tested in a preserved state.  Comparisons with restored state cores did not indicate major differences.  Over the years fluids used in coring operations and core preservation have shown little impact on the observed results. 

Local variations in wettability indicating mixed wettability and oil-wet tendencies can be observed when tar is present in a significant amount and in areas high on structure. The combination of methods from advanced SEM observations, to qualitative contact angle measurements, to relative permeability results all point to a common wettability value.

Introduction

It has become an evident that about 50 % of the world proven oil reserves are contained in carbonate reservoirs.¹ 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 water flooding. So, knowledge of the preferential wettability of reservoir rock is of utmost importance to petroleum engineers and geologists. Due to this importance, many reviews of wettability and its effect on oil recovery have been conducted.^2-4

Carbonate reservoirs are heterogeneous in nature due to the wide spectrum of environments in which carbonates are deposited and subsequent diagenetic alteration of the original rock fabric. These heterogeneities and effect of wettability on residual oil saturation, capillary pressure, electrical properties, relative permeability, and oil recovery encouraged many researchers to perform various studies to characterize and evaluate wettability of carbonate reservoirs. In the past, many engineers assumed that most reservoir rocks are water-wet. The reasons for this conviction are the work of Leverett⁵ and test methodology of determination of wettability after thoroughly cleaning cores that were likely to have been contaminated and exposed to air. The paper published by Treiber et.al.⁶ was the major breakthrough in showing that the large numbers of carbonate reservoirs are oil-wet. Consequently, various studies showed that the wettability of carbonate rocks is oil-wet, neutral or mixed.^7-9

This paper provides detailed study and survey of wettability evaluation for Arab-D carbonate reservoir (Upper Jurassic), Saudi Arabia. The wettability results presented in this paper combine data obtained from various quantitative and qualitative methods over fifty years using preserved and restored core material. The studied areas are Uthmaniyah, Hawiyah, and Haradh.

Arab-D Reservoir

The Ghawar field in Eastern Saudi Arabia contains the largest accumulation of carbonate reservoirs in the world. The majority of wells in the field produce form the Arab-D reservoir, an Upper Jurassic limestone sealed by anhydrite. The Arab-D reservoir was discovered in 1948. Following further separate discoveries along the structure’s main axis, five areas were quickly identified as parts of giant Ghawar oil field (Fig.1): from north to south they are Ain Dar, Shedgum, Uthmaniyah, Hawiyah, and Haradh. At the Arab-D level, the field is NNE-trending composite anticline 230 km long and about 30 km wide.¹⁰ The largest oil accumulations occur in the lowest grainstone cycle of the Arab Formation, the Arab-D reservoir. The vertical oil column reaches a maximum of 1,300 ft. The oil-saturated interval extends about 250 ft below the anhydrite that separates the Arab-D reservoir from overlying Arab-C carbonate beds (Fig. 1).