Uncertainty analysis of pore pressure prediction in carbonate formation using conventional methods

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.