Investigation of Fluid Conductive Faults and Modeling of Complex Water Influx in the Khafji Oil Field, Arabian Gulf
- N. Nishikiori (Arabian Oil Co.) | Y. Hayashida (Arabian Oil Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2000
- Document Type
- Journal Paper
- 401 - 407
- 2000. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 5.1.2 Faults and Fracture Characterisation, 4.1.2 Separation and Treating, 5.1 Reservoir Characterisation, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.5 Reservoir Simulation, 5.2 Reservoir Fluid Dynamics, 5.6.5 Tracers, 2.2.2 Perforating, 5.6.3 Pressure Transient Testing, 3.3.1 Production Logging, 5.1.5 Geologic Modeling, 5.6.4 Drillstem/Well Testing, 2.4.3 Sand/Solids Control, 5.1.1 Exploration, Development, Structural Geology, 5.5.8 History Matching, 4.3.4 Scale
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This paper describes the multidisciplinary approach taken to investigate andmodel complex water influx into a water-driven sandstone reservoir, taking intoaccount vertical water flux from the lower sand as a suspected supplementalsource.
The Khafji oil field is located offshore in the Arabian Gulf. Two MiddleCretaceous sandstone reservoirs are investigated to understand water movementduring production. Both reservoirs are supported by a huge aquifer and had thesame original oil-water contact. The reservoirs are separated by a thick andcontinuous shale so that the upper sand is categorized as edgewater drive andthe lower sand as bottomwater drive. Water production was observed at thecentral upstructure wells of the upper sand much earlier than expected. Thismakes the modeling of water influx complicated because it is difficult toexplain this phenomenon only by edgewater influx.
In this study, a technical study was performed to investigate water influxinto the upper sand. A comprehensive review of pressure and production historyindicated anomalous higher-pressure areas in the upper sand. Moreover,anomalous temperature profiles were observed in some wells in the same area. Atthe same time, watered zones were trailed through thermal-neutron decay time(TDT) where a thick water column was observed in the central area of thereservoir. In addition, a three-dimensional (3D) seismic survey has beenconducted recently, revealing faults passing through the two reservoirs.Therefore, as a result of data review and subsequent investigation, conductivefaults from the lower sand were suspected as supplemental fluid conduits. Apressure transient test was then designed and implemented, which suggestedpossible leakage from the nearby fault. Interference of the two reservoirs andan estimate of supplemental volume of water influx was made by materialbalance.
Finally, an improved full-scale numerical reservoir model was constructed tomodel complex water movement, which includes suspected supplemental water fromthe lower sand. Employment of two kinds of water influx—one a conventional edgewater and another a supplemental water invasion from the aquifer of the lowersand through conductive faults—achieved a water breakthrough match.
The Khafji oil field is located in the Arabian Gulf about 40 km offshoreAl-Khafji as shown by Fig. 1. The length and width of the field areabout 20 and 8 km, respectively. The upper sandstone reservoir, the subject ofthis 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 isof Middle Cretaceous geologic age.
Underlying the upper sandstone reservoir is another sandstone reservoir at adepth of about 5,400 ft. It has an average gross thickness of about 650 ft andis separated from the upper sand by a thick shale bed of about 200 ft. Bothreservoirs had the same original oil-water contact level as shown by thesubsurface reservoir profile in Fig. 2.
Both sandstone reservoirs are categorized as strong waterdrive that canmaintain reservoir pressure well above the bubblepoint. On the other hand,water production cannot be avoided because of an unfavorable water-to-oilmobility ratio of 2 to 4 and high formation permeability in conjunction with astrong waterdrive mechanism. In a typical edgewater drive reservoir, waterproduction normally begins from the peripheral wells located near the oil-watercontact and water encroaches as oil production proceeds. However, someproduction wells located in the central upstructure area of the upper sandstarted to produce formation water before the wells located in the flank areanear the water level.
In 1996, we started an integrated geological and reservoir study to maximizeoil recovery, to enhance reservoir management, and to optimize the productionscheme for both sandstone reservoirs. This paper describes a part of theintegrated study, which focused on the modeling of water movement in the uppersand. 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, formationtemperatures, pressures, and 3D seismic; introduction of fluid conductivefaults as a suspected supplemental water source in the central upstructurearea; design and implementation of a pressure transient test to investigatecommunication between the reservoirs and conductivity of faults; running ofmaterial balance for the two reservoirs simultaneously to assess theirinterference; and construction of an improved full-scale reservoir simulationmodel 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 runningnortheast to southwest. The structure dip is gentle ( Fig. 3) at about3° on the northwestern flank and 2° on the southeastern flank. The upper sandis composed mainly of sandstone-dominated sandstone and shale sequences. It isinterpreted that the depositional environment is complex, consisting of shoreface and tide-influenced fluvial channels.
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