Summary
This paper describes the multidisciplinary approach taken to investigate and model complex water influx into a water-driven sandstone reservoir, taking into account vertical water flux from the lower sand as a suspected supplemental source.
The Khafji oil field is located offshore in the Arabian Gulf. Two Middle Cretaceous sandstone reservoirs are investigated to understand water movement during production. Both reservoirs are supported by a huge aquifer and had the same original oil-water contact. The reservoirs are separated by a thick and continuous shale so that the upper sand is categorized as edge water drive and the lower sand as bottomwater drive. Water production was observed at the central up structure wells of the upper sand much earlier than expected. This makes the modeling of water influx complicated because it is difficult to explain this phenomenon only by edge water influx.
In this study, a technical study was performed to investigate water influx into the upper sand. A comprehensive review of pressure and production history indicated anomalous higher-pressure areas in the upper sand. Moreover, anomalous temperature profiles were observed in some wells in the same area. At the same time, watered zones were trailed through thermal-neutron decay time(TDT) where a thick water column was observed in the central area of the reservoir. In addition, a three-dimensional (3D) seismic survey has been conducted recently, revealing faults passing through the two reservoirs. Therefore, as a result of data review and subsequent investigation, conductive faults from the lower sand were suspected as supplemental fluid conduits. A pressure transient test was then designed and implemented, which suggested possible leakage from the nearby fault. Interference of the two reservoirs and an estimate of supplemental volume of water influx was made by material balance.
Finally, an improved full-scale numerical reservoir model was constructed to model complex water movement, which includes suspected supplemental water from the lower sand. Employment of two kinds of water influx—one a conventional edge water and another a supplemental water invasion from the aquifer of the lowers and through conductive faults—achieved a water breakthrough match.
Introduction
The Khafji oil field is located in the Arabian Gulf about 40 km offshore Al-Khafji as shown by Fig. 1. The length and width of the field are about 20 and 8 km, respectively. The upper sandstone reservoir, the subject of this study, lies at a depth of about 5,000 ft subsea and was discovered in1960. The average thickness of the reservoir is about 190 ft. The reservoir is of Middle Cretaceous geologic age.
Underlying the upper sandstone reservoir is another sandstone reservoir at a depth of about 5,400 ft. It has an average gross thickness of about 650 ft and is separated from the upper sand by a thick shale bed of about 200 ft. Both reservoirs had the same original oil-water contact level as shown by the subsurface reservoir profile in Fig. 2.
Both sandstone reservoirs are categorized as strong waterdrive that can maintain reservoir pressure well above the bubblepoint. On the other hand, water production cannot be avoided because of an unfavorable water-to-oil mobility ratio of 2 to 4 and high formation permeability in conjunction with a strong waterdrive mechanism. In a typical edge water drive reservoir, water production normally begins from the peripheral wells located near the oil-water contact and water encroaches as oil production proceeds. However, some production wells located in the central up structure area of the upper sand started to produce formation water before the wells located in the flank area near the water level.
In 1996, we started an integrated geological and reservoir study to maximize oil recovery, to enhance reservoir management, and to optimize the production scheme for both sandstone reservoirs. This paper describes a part of the integrated study, which focused on the modeling of water movement in the upper sand. The contents of the study described in this paper are outlined as:diagnosis and description of the reservoir by fully utilizing available data, which include comprehensive review of production history, TDT logs, formation temperatures, pressures, and 3D seismic; introduction of fluid conductive faults as a suspected supplemental water source in the central upstructure area; design and implementation of a pressure transient test to investigate communication between the reservoirs and conductivity of faults; running of material balance for the two reservoirs simultaneously to assess their interference; and construction of an improved full-scale reservoir simulation model and precise modeling of complex water movement.
Brief Geological Description of the Upper Sand
The structure of the upper sand is anticline with the major axis running northeast to southwest. The structure dip is gentle (Fig. 3) at about3° on the northwestern flank and 2° on the southeastern flank. The upper sand is composed mainly of sandstone-dominated sandstone and shale sequences. It is interpreted that the depositional environment is complex, consisting of shoreface and tide-influenced fluvial channels.
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