Optimization and Establishment of Drillability in Gravel Formation

Abstract

Rock drillability is a comprehensive index used to evaluate the difficulty of drilling and is a basic factor to determine drilling efficiency. Proper evaluation of rock drillability is vital to achieving high-quality and efficient drilling. Currently, the method for determining the drillability of rocks is primarily based on laboratory micro-bit experiments to measure the core’s resistance to drilling. This method requires a large number of cores and is expensive, takes a long time to start the test, and often lags behind drilling. Moreover, some people analyzed the relationship between the actual drilling parameters and the mechanics’ specific energy of drill bit and established a method to evaluate the rock drillability of rock. Field application proves that the drillability model is poorly correlated with the actual drilling conditions. The value of the actual drilling energy is usually 100–1000 times that of the theoretically calculated mechanics specific energy. Through laboratory experiments, we study the strength and failure characteristics of different gravel sizes and gravel contents with different impact speeds. The matching relationship between rock mass and impact speeds is analyzed. Based on the experimental results, a phase similar to a drill bit and a drilling tool is proposed to support drillability assessment. Under the condition of the same gravel size, with the increase of gravel content, the average cutting speed gradually decreases under the condition of certain cutting force, which is approximately exponential; with the increase of gravel particle size, under the condition of certain cutting force, the average cutting speed rapidly decreases, and the gravel particle size has a decisive impact on the average cutting speed. The above research results can be used to establish a new drillability evaluation model and to optimize the drill bits, drilling tools, and drilling parameters.