Taha M. Okasha

Wettability Studies at the Pore Level of Saudi Aramco Reservoirs

The wettability of rocks is of critical importance in understanding reservoir dynamics. Several studies of wettability evaluation on Saudi Aramco reservoir rocks were reported, but to our knowledge none of them have attempted to study wettability

Wettability Evaluation of the Unayzah Reservoir in Central Saudi Arabia

Unayzah Formation in Central Saudi Arabia is an important source of light (48-52 API gravity) sulfur frees (0.02-0.07%) crude oil (Arabian Super Light). In 1989, the first oil discovery was made at Hawtah. Subsequently, a number of other oil discoveries were made at various fields (Umm Jurf, Ghinah, Nuayyim, Hazmiyah, etc.). Oil production from Central Saudi Arabia fields started in August 1994. As part of special core analysis program to support reservoir management in development of these fields, intensive SCAL studies were completed and crucial data required for field simulation were provided to proponents. This paper summarizes the results of a set of consistent methods used in determining wettability of Unayzah reservoir. Both quantitative methods (Amott, USBM, and contact angle) and qualitative method (oil/water relative permeability) were used to evaluate the variation of wettability in Central Saudi Arabia fields. In this work, direct wettability tests were carried out on a large number of native core plugs from Unayzah reservoir. Moreover, several waterflood tests were conducted at reservoir conditions on composite cores using recombined reservoir fluids. Data from relative permeability curves were used for comparison with wettability results obtained from direct wettability tests. Amott and USBM wettability indices indicated that Unayzah reservoir core material represented a range of wettabilities, but tended towards neutral to slightly to strongly waterwet. Contact angle results for rock material recovered from Unayzah reservoir showed water-wet characteristics for both oil-brine/rock system and oil-brine/quartz system. Moreover, the values of wettability indices are in agreement with relative permeability results which reflect a behavior of water-wet core material based on Craig’s rule of thumb. INTRODUCTION AND BACKGROUND Saudi Aramco has an interest in several central area fields that produce from Upper Carboniferous and Lower Permian Unayzah reservoir. The Unayzah formation is a heterogeneous, friable, and fluvial to eolian clastic rock which contains detrital clays. Oil production from Central Saudi Arabia fields started in August 1994. As part of special core analysis program to support reservoir management in development of these fields, intensive SCAL studies were completed and crucial data required for field simulation were provided to proponents.