Managing the Challenges of Voidage in a Mature Carbonate Water Flood

Abstract

Integrated field and well reviews followed by logical activities are being taken to address the voidage and declining pressure in Yibal. This is to ensure that target daily oil rates are achieved, and UR is maximized, in the largest oil producing asset in Oman, which delivers almost 15% of the total country production from a heavily faulted and fractured carbonate anticlinal structure, on stream since 1969. Water injection initiated in 1972 has gone through a series of stages; vertical oil phase injectors for lateral sweep, vertical aquifer injection for vertical sweep and later some horizontal deep aquifer injectors. The injection serves the dual purpose of pressure management and produced water disposal. The field has suffered negative voidage for quite some time, leading to rapid reservoir pressure depletion. Past studies have identified uneven vertical and lateral sweep due to uneven subsurface water distribution as a result of the complicated fracture and fault patterns as well as the subsurface hydro-dynamics and flow mechanisms. This has left the remaining oil in pockets that are becoming more difficult to produce. The high field average BSW has led to a situation where oil wells are closed in for surface facilities constraint and, or subsurface water injection capacity. Further, the uncertain water movement into producing wellbores makes production optimization activities risky. Recently, quite a number of injectors have been taken offline for integrity repairs and are only gradually coming back on stream after extensive and often expensive repairs. The implementation of a voidage replacement program which includes understanding the reservoir dynamics, identification of the worst hit low pressure areas, and prioritization of the short, medium and long term remedial activities is on going. Increased focus on gross offtake management, injection well stimulation, water shut off in producers, conversion of poor producers to injectors, and drilling of new injectors is showing returns in flattening of the decline rate of oil production. The issues around water injection as well as oil producer integrity are gradually being addressed. Several learning points for the implementation of the proper reservoir and voidage management have also been captured. The importance of human resources both in terms of numbers, level of experience and skills set, has also been recognized and the optimum balance of these is being worked on. Introduction The Yibal field (Fig 1), discovered in 1963 and brought on stream in 1969, currently produces oil (from the Shuaiba reservoir) and gas (from the Natih reservoir). Both streams are managed by different Asset teams.