Managing seismic positioning uncertainty in HP/HT developments

Two high-pressure/high-temperature (HP/HT) fields were developed successful ly in the deeper parts of the U.K. central North Sea-the Heron with 12,600 psi reservoir pressure and 340 degrees F and the Shearwater with 15,400 psi reservoir pressure and 370 degrees F. The fields share the following probl ems frequently encountered in HP/HT development. A very small drilling margin between reservoir pore pressure and fracture p ressure. All development webs must be drilled before production begins because drill ing into depleted reservoirs is considered high risk. Development wells need to be drilled crestally to delay production of highl y saline water that could cause severe scaling. Identifying development-well locations in both fields is complicated by the presence of large seismic positioning uncertainties. Although the overburd en is relatively flat, escaping gas over the crest of the fields and lithol ogical changes introduce large raybending effects leading to lateral seismi c positioning uncertainties as large as 985 ft at top reservoir level.