Investigation of Fateh Mishrif Fluid-Conductive Faults
Authors: Trocchio James T. 
Published: December 2007

Adjacent well production histories indicated that several faults within theFateh Mishrif reservoir have fluid-conductive natures, Example historiesdisplayed high formation water cuts in upstructure wells that could beexplained only by aquifer influx along nearby faults. Subsequent data gatheredin these wellsincluding the results from pressure-transient tests, productionpressure-transient tests, production logging, and radioactive-tracersurveys-verified fluid flow paths. This paper demonstrates how specificdata-gathering paper demonstrates how specific data-gathering operations andrecent reservoir simulation studies revealed the effect of fluid flow alongfault planes on overall reservoir performance. The paper presents effects offluid-conductive presents effects of fluid-conductive faults ondevelopment-well placement, waterflood pattern balancing, and workoverevaluations.

Introduction

Fateh field is in the Arabian Gulf about 60 miles [97 kml offshore Dubai. The Fateh Mishrif reservoir, at 8,300 ft [2,530 m] subsea (ss), is a limestoneformation with an average thickness of 250 ft [76.2 m] formed during the MiddleCretaceous Age, Upthrust from an underlying salt dome created a dense networkof high-angle normal faults. Subsequent erosion resulted in the doughnut-shapedstructure shown in Fig. 1. The Mishrif is surrounded by an aquifer, but waterinflux is inhibited by a tarmat, or viscous oil layer, at the base of thereservoir.

Fluid-conductive fault sections are indicated by dashed fault-plane lines inFig. 1 and designated according to nearby platform names. Little information isavailable to describe the actual physical characteristics of these faults, butFig. 2 provides a very simple conception. The faults are actually composed ofmultiple failure planes that do not necessarily extend throughout the lateraldirection of the faults. Before oil migration, water movement through the faultplanes resulted in calcite crystallization, as planes resulted in calcitecrystallization, as limited core data and drilling cuttings proved. After theoil reservoir formed, proved. After the oil reservoir formed, further faultmovement created crushed limestone sections into which the oil moved. Waterfrom the aquifer traveled along these fluid-filled areas as the reservoir wasproduced. Vertical fluid movement is restricted produced. Vertical fluidmovement is restricted because the Mishrif is bounded by shaly formations thatslightly deform to seal the faults.

We identified five different fault sections as fluid conductive. Eachsection is discussed in detail according to three levels of recognition orstudy. First, a suspicion stage develops during which nearby well productiondata suggest fluid flow, usually water, along a highly conductive path. Forexample, a high formation water cut in a newly completed upstructure wellimplies flow along the fault. Next, an investigation stage begins, involvingsuch data-gathering activities as pressure-transient tests, radioactive-tracerstudies, and production logging, which serve to confirm the fluid-conductivefault suspicion. Finally, a management stage handles the fault's influence onreservoir production. Injection pattern balancing, production. Injectionpattern balancing, reservoir modeling for development planning, and wellworkovers are involved in planning, and well workovers are involved in thisfinal stage.

Fault B

Suspicion Stage. Fig. 3 illustrates the Fateh Mishrif structure in the FaultB area. Open well symbols designate offset wells pertinent to the study of thisfluid-conductive fault. Well completion dates are also provided.

A fluid-conductive fault was suspected adjacent to Well B-5 becauseformation water was produced within 6 months of well completion in Feb. 1971. For comparison, offset Production Well B-6, which was completed 37 ft [11.3 m]lower, did not produce water until 1986. However, Well produce water until1986. However, Well B-4, positioned adjacent to the fault plane 200 ft [61 m]updip of Well B-5, has not shown any influence from the fault. Therefore, ahigh-conductivity path was presumed to extend from the aquifer to slightlypresumed to extend from the aquifer to slightly updip of Well B-5.

Investigation Stage.

A 1981 pressure-buildup (PBU) test on Well B-4indicated the presence of a barrier-i.e., a sealing fault. This barrier wasphysically possible because the total Mishrif thickness in Well B-4 is only 66ft [20.1 m], probably close to the fault-block offset. TheMiller-Dyes-Hutchinson (MDH) buildup plot displayed the characteristic doublingof slope, as seen in Fig. 4. With an intersection time of 26 hours, thedistance to linear discontinuity was calculated at 205 ft [62.5 m], whichcorresponds to the distance between Well B-4 and Fault B.

Previous PBU surveys on Well B-5 indicated neither a sealing fault nor anextensive linear flow period. Extensive linear flow was apparent on PBU testsfor other producers adjacent to fluid-conductive faults, as described later inthis paper. We concluded that the Well B-5 completion interval does notintersect the fault directly, but is heavily influenced by fluid flow along thefault, as production data show.

An extensive radioactive-tracer survey began in the Fateh Mishrif reservoirin Feb. 1979.

JPT

P. 1038