Shearwater (UK Block 22/30b): managing changing uncertainties through field life

Abstract

Development ofthe ShearwaterField, adeepgas-condensateaccumulation inthe HPHT region ofthe UKCS, beganin1997 withthe initiation ofdevelopment drillingandconstruction oftopside facilities. Itformed part ofthe major development ofthe CentralNorthSea HPHT gasplaythroughoutthe late1990s,togetherwith othergasdevelopments across Elgin/FranklinandE rskine.Ass uch, Shearwaters houldb eviewed, together withtheseotherstructures,aspart ofanexpansion ofthe gassector int he NorthSea inanarea ofchallenging subsurface conditionsandlimited development experience to constrainp roduction uncertainty. Fieldsanction wasbased on the understandinga ndmodellingofk eys ubsurface uncertainties. Development experience since thenhast ested thatinitials ubsurface viewandlessons learnt will provide ak eyanalogueto help constrain the development ofa ny futureH PHT gasaccumulations. Thisp aperdescribest he subsurface modelfor the Shearwaters tructurea ndh ow recent development experience andproduction performance hasconstrained the reservoircharacterization uncertainty ofthe FulmarFormation.DVD: CoredisplayD1isr elevant to thischapterandcanbe viewed on the accompanyingDVD.The ShearwaterFieldrepresents anintegralp art ofthe recent development ofthe CentralNorthS ea high-pressure/hightemperature(HPHT)p etroleum system,providingb othg ascondensatef rom its Jurassic reservoirs anda lso aprocessingh ub facility for neighbouringd evelopments. Located 250kmeast of AberdeeninBlock 22/30b ( Fig.1 ),the fieldwaso riginally discovered in1 991byexploration well 22/30b-4re, which encountered richg as-condensateh ydrocarbons atr eservoir pressureso f15100 psia tt he crest ofthe structure.Subsequent appraisalin1 992 (22/30b-11) and1996 (22/30b-15s1) further confirmed the presence ande xtent ofac ommercialv olume, sufficient for fieldd evelopment sanction in1 997. Drilling constraints associated withsubsurface HPHT conditions,together withsignificant concerns regardingthe ability to drill into a partially depleted reservoir,meant thatall ofthe development wells needed to be drilled vertically inthe reservoirandbeforefirst gasw asp roduced.Ino rdert os atisfyt hesefi eldd evelopment constraints ac omprehensivereviewo fsubsurface uncertainties anddevelopment options wasundertaken. Thisformed the focus of the initialfieldgeologicalmodelling, constrainingthe numberand placement ofd evelopment wells andtheircompletion interval (Blehaut etal .1 999). Asix-well drillingc ampaignfollowed, between1997 and1999,beforeinstallation ofthe platform facility in2 000. Fivewells weretargeted att he FulmarFormation,with the remainingwell havinga nappraisalelement to test structural definition att he crest ofthe field(to distinguishb etweent he wedge, monoclinea ndslump models ofB lehaut etal .( 1999)). Thisappraisalw ell encountered ag as-condensateP entland Formation section andproved astructurec omparableto the wedge modelofB lehaut etal . ( 1999).Drillinga fterp roduction start-up wasalways considered unlikely becauseofthe riskst oholestability. However,awell failureresulted...

Faults and fault properties in hydrocarbon flow models

“…The third reported case of fault seal breakdown is discussed by Gilham et al. (2005), and concerns a fault in the Shearwater High Pressure/High Temperature reservoir in the Central North Sea.…”

“…2005). Dynamic evidence for initial fault sealing and later breakdown in Shearwater derives from interpretation of a P/Z plot (Gilham et al. 2005).…”

“…(2007), and a detailed discussion of the pitfalls associated with it are given by Dake (2001). Gilham et al. (2005) interpreted a sharp change in linear slope in the Shearwater P/Z plot, but a close examination of the plot indicates that a curved trend may be as likely.…”

“…The third reported case of fault seal breakdown is discussed by Gilham et al (2005), and concerns a fault in the Shearwater High Pressure ⁄ High Temperature reservoir in the Central North Sea. The gas-water contact is the same across the fault, and evidence that the fault was a static seal at the onset of production was interpreted from different fluid compositions measured in samples acquired from wells drilled on each side the fault.…”

“…A simple summary of this type of analysis is provided by Zijlstra et al (2007), and a detailed discussion of the pitfalls associated with it are given by Dake (2001). Gilham et al (2005) interpreted a sharp change in linear slope in the Shearwater P ⁄ Z plot, but a close examination of the plot indicates that a curved trend may be as likely. This would imply that a low transmissibility (but non-sealing) fault is present throughout the period considered, and does not require any changes in the fault properties (Dake 2001;Zijlstra et al 2007).…”