Constraint of maximum stress using wellbore failures affected by hydrocarbon production at Katakai gas field, Japan

摘要

The Katakai gas field, located in Niigata, northern Japan, produces hydrocarbon from a volcanicrnreservoir deeply sheeted over four thousand meter depth. The reservoir rock indicates inherently lowrncapability of fluid transport in well production tests and core flooding tests. Therefore, wellrncompletion strategies are aimed to maximize contact area to the reservoir rock employing a longrnopen-hole interval, multi branched holes, and hydraulic fracturing. To attain success in theserncompletions, it is essential to control mud weight to maitain a stable stress condition around wellbore.rnIn-situ stress field is evaluated by wellbore stability analysis including rock strength experiments,rndipole sonic logs, caliper logs and extended leak-off tests. Maximum stress magnitude is constrainedrnby observed differences in occurrences of wellbore failures between the two wells located closelyrnwith each other. One well showed severe wellbore failures, whereas the other well maintained stablernwellbore walls in spite that the same mud weight were used in both wells. The differences arerninterpreted to be caused by formation pressure variation associated with gas production. Thernformation pressure declined by the depletion is able to stabilize wellbore.rnThe evaluated maximum stress magnitude ranges from 107MPa to 123MPa at 4500m depth,rnindicating a strike-slip faulting stress regime. The stress regime is consistent with the result derived byrnhydraulic fracturing experiment in the well located 2.3 km away from this study area. But, thernmagnitude is smaller than that of the hydraulic fracturing experiment by approximately 20MPa.rnThrough the study, the following four possible factors are emerged. The first is definition of arnthreshold value of breakout width for promoting succesive wellbore failures. The second is arndifference in applying methods. The third is a variation of the local tectonic stress field in thernreservoir. The last is stress redistributions due to reservoir deformation accompanied by the reservoirrndepletion.