Saudi Arabia's Long Term Injectivity Test in a Field Characterized by a Tar Mat Zone Strives to Unlock Higher Reservoir Potential
Authors: Arukhe James Ohioma I. , Al-dhufairi Mubarak Audah , Ghamdi Selah Abdullah , Al-Harbi Badr Mohammed , Docherty Paul Rene 
Published: May 2011

Abstract
A carbonate field in Saudi Arabia is undergoing major development requiring water injection wells to provide peripheral matrix water injection as pressure maintenance scheme to support oil production. The field is characterized by a tar mat zone, which potentially could isolate the oil reservoir from the planned pressure support and serve as a barrier for the water injection. Therefore, the injection wells were geosteered horizontally right above the tar " barrier?? into the transition zone between the heavy and lighter oil, which poses a challenge in assuring adequate pressure support to the producers, without leaving pockets of relatively high oil saturation behind the waterflood front.
To address transmissibility uncertainty between producers and the injectors, long-term injection (LTI) pilot tests were designed utilizing one water injector and six observation wells to capture pressure signals. Building the surface facility to deliver the required test as planned was challenging, starting with the seawater as a source, to water treatment and ending with pump selection. This paper discusses the unique layout of the LTI surface equipment, a mini-plant by itself, and how operational challenges were overcome in the field. The authors highlight some operational issues related to the LTI test that had almost 90% efficiency from operating over 200 days and over 2 million barrels of injected filtered and treated seawater volume, as well as present valuable insights to demonstrate how a project of this scale was successfully executed and more value added to the development plans.
The unique equipment layout comprised twin sea-submerged, skid-mounted electrical submersible pumps (ESPs), 6?? hoses, filtration unit, a chemical treatment unit, eight 500 bbl storage tanks, and a horizontal pumping system (HPS). The layout of the surface facility components, their performance and the importance of continuous water injection in addressing the test goals are discussed. The injection well performance was monitored by integrating Joshi's equation to Hall Plot and slope analyses to provide means of more meaningful use of injection pressure and rate data. The synergy of the mini-plant components coupled with engineered performance monitoring tools were enablers in this test design to help unlock more reserves. Overall, the test was a great tool to qualify field development plan assumptions, indicating that less powered water injectors than initially planned, are required

Introduction
Production from the carbonate field started several years ago from reservoir A with fluid and rock compressibilities being the primary drive mechanisms. Nearly 20 years later, production started from a lower reservoir Bb, and from reservoir Aa six years later. Due to low demand, it was subsequently shut-in. In all three reservoirs, (Aa, Ba and Bb) a continuous tar mat underlay the oil column and posed an uncertainty as to the extent it was sealing and effectively separating the oil column in the reservoirs from the underlying aquifer. Water injection was considered a priori since the assurance for an adequate natural water drive from the aquifer is low. With the present major development by means of peripheral matrix water injection as the planned pressure support mechanism, the tar mat created a potential challenge in assuring adequate pressure support for the field during production. Without an effective communication between the tar mat layer and the oil zone, with water flooding, a potential risk yet remains of leaving relatively high oil saturation pockets behind the flood front.