井下节流气体钻井井筒流场和温度场研究

Gas drilling can avoid formation loss, shale hydration and reservoir damage, and increase the drilling speed, but have some disadvantages such as lower strength of drill string for high temperature, borehole deviation and downhole fire or explosion. the efficient rock fragmentation mechanism of gas drilling was analyzed by thermal stress increase because of the bit nozzle throttling gases cooling rock. According to the theory of great temperature drop before and after gas throttling, the project puts forward a proposal of adding a downhole choke in the drilling string during gas drilling to make the gas temperature drop in advance, and increase the gas in the bit nozzle temperature drop range, improve the efficiency of rock fragmentation, and cool the downward flow gas in drill string, reduce the downhole explosion risk, absorb formation heat to increase the driling circulation system total energy, further increase the drilling efficency. Downhole choking technology is widely applied in the gas well production, and the theory is relatively perfect for downhole throttling technology. But there are still few studies about downhole choking technology applied to drilling operation. Project intends to study the feasibility of application downhole throttling technology to gas drilling based on the combination of theoretical analysis, numerical simulation and experiments, and establish a coupling calculation model of flow field and temperature field under the downhole throttling conditions, discover the objective law of the pressure, flow velocity and temperature distribution within wellbore, determine the matching relations of the downhole throttling throat diameter and the bit nozzle diameter. Finally the target of building model and method of speed the drilling penetration and raise efficiency is realized by applying the downhole throttling to gas drilling.

气体钻井能够避免井漏、泥页岩水化膨胀和储层污染，提高钻井速度，但是也存在下部钻柱温度过高，钻柱强度降低，井眼易倾斜和井下爆炸等问题。有文献从钻头喷嘴对气体节流降温增大井底岩石热应力的角度，分析了气体钻井高效破岩机理。本项目根据气体节流降温理论，提出在气体钻井的钻柱内增设节流装置，使气体在向下流动过程中提前降温，既可增大气体在钻头喷嘴处的降温幅度，提高破岩效率，又可冷却钻柱、降低井下爆炸风险，还可吸收地层热量，增加循环系统总能量，提高钻井效率。井下节流技术在气井采气作业中应用较多，研究较为完善，但是在钻井方面还鲜有研究。项目拟采用理论分析、数值模拟和室内实验相结合的方式，针对在气体钻井中实施井下节流可行性进行研究。建立井下节流条件下流场和温度场耦合物理模型和数学模型，研究井筒内压力、流速和温度分布规律，确定节流器喉道直径与钻头喷嘴直径匹配关系，构建井下节流气体钻井提速提效理论和方法。