Study on fracture occurrence characteristics and wellbore stability of limestone formation

The development of micro fractures in the KT-II formation in a block of China National Petroleum Corporation (CNPC) overseas caused the downhole collapse and touching resistance repeatedly occurs, which seriously hindered the normal drilling process. To solve these problems, the occurrence of fractures in the underground was sorted out based on the field data, and the physicochemical and mechanical properties of the downhole cores were analyzed under different downhole conditions via systematic experiments to determine the main factors affecting the formation collapse and instability. Based on the main control factors of wellbore collapse and experimental data, the wellbore-formation coupling stress field and wellbore stability evaluation and prediction method for unstable KT-II formation were established. Results show that the unstable KT-II strata are mainly fractured limestone strata having a "thousand-layer cake" shape. Clay or mudstone intercalations with strong hydration and expansion capacity can be observed between cracks. Deviation angle, azimuth and fracture tendency have obvious influence on wellbore stability of KT-II fractured limestone formation. The straight hole runs vertically through the fracture plane, and the borehole wall exhibits good stability. When the deviation angle becomes larger than 50 degrees, the weak plane effect of fracture mechanics is prominent and the borehole wall will easily collapse along the crack. For the formation with horizontal fractures, the optimal wellbore stability can be observed when drilling is performed in the direction of the minimum horizontal principal stress. For limestone formation with a certain dip angle fracture, horizontal well is drilled in the direction opposite to the fracture tendency, i.e., the fracture tendency +180 degrees direction, the equivalent density of the collapse pressure is the lowest. The seepage and pressure penetration effects between wellbore and formation lead to an increase in the equivalent density of caving pressure by 0.08-0.1 g/cm(3). Improving the sealing characteristics of drilling fluid will improve the wellbore stability. The results of this study will provide guidance for achieving effective and safe drilling in fractured limestone formations.