A Systematic Approach to Leak-Off Pressure Prediction in Unconventional Resources –A Case Study from Junggar Basin, China

Mud loss is one of the most severe incidents encountered while drilling. As the focus of exploration has moved to the deeper and more complicated formations, mud loss happens more frequently, causing safety incidents and lowering the drilling efficiency. The lost mud invades into formations, makes harm to the reservoir and affects the accuracy of formation evaluation. Large numbers of studies have proved that the key strategy to deal with mud loss is to prevent it and follow with remedial action. The prerequisite of such measurements is locating the formation of mud loss. Loss pressure is the highest pressure that formation can hold before mud loss happens. It is important for deciding mud weight and critical for fractured formations. Therefore, an innovative ring-cracking loss model is created by integrating minimum horizontal principal stress loss model, statistical loss model, and critical crack width loss model after analyzing the loss mechanism of fractured formation. These four models are applied to the fractured formation of block X, and the results are compared to the actual downhole pressure.The result from ring-cracking-based loss model is less than the actual downhole pressure. This model is suitable for field application and can guide the design of mud density.