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Uncertainty analysis of pore pressure prediction in carbonate formation using conventional methods
WANG Zizhen1, WANG Ruihe1, SHAN Xun1, ZANG Yanbin2, ZHANG Rui1, LI Mingzhong3
(1.School of Petroleum Engineering in China University of Petroleum, Qingdao 266580, China;2.Research Institute of Petroleum Engineering, SINOPEC, Beijing 100101, China;3.Downhole Operation Company, Huabei Petroleum Engineering Company, SINOPEC, Zhengzhou 450042, China)
Abstract:
The origin of abnormal pore pressure in carbonate rocks is different from that of clastic sedimentary rocks. P-wave velocity and porosity have poor correlation in carbonate rocks, which results in high uncertainty in pore-pressure prediction using common methods. Based on the effective media theory, the P-wave velocity in carbonates with different types of pore structure was calculated, and then the deviation of pore-pressure predicted with conventional methods was analyzed. The results show that under the same stress condition, mineral composition and porosity, the pore-pressure of carbonate formation with crack type porosity tends to be over-estimated, which can cause lost circulation and reservoir damage during drilling. On the other hand, the pore-pressure of carbonate formation with moldic or vug type porosity can be under-estimated, which may lead to well kick or even blowout. For carbonate formations with both crack and moldic types of porosity, whether the predicted pore pressure is under-or over-estimated depends not only on the relative volume fraction of cracks and vugs, but also on their absolute volumes. Therefore, there are safety concerns when conventional pore-pressure prediction method and casing program design method are used during drilling carbonates with complicated pore structure.
Key words:  carbonate rocks  pore structure  pore-pressure prediction  P-wave velocity  pressure deviation