Innovative Enhancement of an Existing Peripheral Waterflood in a Large Carbonate Reservoir in the Middle East

Abstract

The Wafra Ratawi, a Lower Cretaceous Oolitic reservoir, located in Kuwait, has been on production since 1956. Production remained flat from about 40 producers until 1990. The field was shut-in in 1990 during the Gulf War. An aggressive drilling program was initiated after 1992 when the field was brought back on-line, followed by a 26-injector peripheral waterflood in July 1998. Production peaked in 2Q 2002. After approximately two years, production dropped in 4Q 2003 with declines approaching 25% AED. The Ratawi Asset Management Team (AMT) investigated the causes of the decline and observed the following:○Reservoir pressure decline near the crestal and the southern regions of the field○Voidage Replacement Ratio (VRR) < 1.0○Water breakthrough especially in the first ring of wells inside the peripheral injection loop○Poor conformance due to high permeability streaks resulting in water breakthrough in wells inside the first ring of producers The Ratawi AMT concluded that a significant portion of the injected water is being cycled out by the first ring of producers that had experienced breakthrough, resulting in poor Voidage Replacement Ratio (VRR), and the consequent depletion in the center of the reservoir. A 20MM$ project was initiated for the enhancement of the existing peripheral waterflood. The project entailed the drilling 4 new horizontal injectors near the crestal region and converting 1 vertical producer to a horizontal water injector near the southern region---the areas of maximum pressure depletion. The injection target was deep, just above a vertically sealing barrier in the reservoir. Intent of the "deep" injection was to implement a gravity stable, bottom water drive to supplement the aquifer in this relatively heavy oil reservoir. To augment the existing waterflood, the Pressure Maintenance Plant (PMP) needed to be de-bottlenecked to capacity of 250 MBWIPD from the nominal 190 MBWIPD. An interesting aspect of this project was the relatively novel dumpflood. De-bottlenecking the PMP would be completed almost a year after the first new injector had been drilled. To accelerate water injection, and to circumvent long lead times associated with injection lines and plant de-bottlenecking, dumpflooding was identified as an option. A dumpflooding source was identified, assessed, and the dumpflood was implemented successfully. In the dumpflood, water from the overlying Zubair aquifer was used to "dump" a total of approximately 60 MBWIPD into the Ratawi. This is a "first" in the Partitioned Neutral Zone, Kuwait. Peripheral injection projects are hard-pressed to maintain reservoir pressure in the center of large reservoirs. Along with the dumpflood, the novelty of this project lies in the use of supplemental horizontal injection to simulate a bottom-water drive in the central portions, which augments the peripheral injection. This paper discusses the genesis of this project through the inception, execution and operation/evaluation of the dumpflood and powered injection phases. Introduction The Wafra field, discovered by Aminoil and Pacific Western (later Getty Oil) in 1954 is located in the Partitioned Neutral Zone - an area between the Kingdom of Saudi Arabia and Kuwait. The mineral rights in this region are shared equally between the two countries. The field is now operated by Saudi Arabian Texaco (SAT), a 100% owned subsidiary of Chevron, and Kuwait Gulf Oil Company (KGOC). SAT operates the field on behalf of the Kingdom of Saudi Arabia under a concession agreement signed in 1949. Out of the six reservoirs in the Wafra field, the most prolific is the Lower Cretaceous Ratawi Oolite.