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。

气体钻井能够就是大幅度降低井底压差，消除压持效应的影响，井底岩石应力状态发生转变，岩石的强度降低。钻头喷嘴的节流降温作用也增强了岩石易破碎能力。根据节流降温原理，在钻柱上适当位置设置节流装置进行气体节流降温，进一步降低喷嘴出口处的温度，更加有利于提高机械钻速，预防井下爆炸，而且可以利用地层能量给气体加热，增大循环系统的总能量，提高气体钻井循环系统的工作效率。采用钻柱内节流，再经过钻头喷嘴二次节流，可以使井底的降温效果增强。不同进出口形状及不同节流吼道组合的实验结果表明，节流前后的压力比对节流降温影响显著，而前后压差大小影响不显著，并且必须确保节流前后压比小于临界压比，否则易产生临界流动状态，而影响气体的流出和降温效果。根据井筒与地层之间传热模型为气体钻井携岩计算提供了更加精确的计算方法，计算的气体钻井最小注气量与现场实际施工更加吻合。在温度突然降低的情况下井底岩石径向应力在靠近井底附近应力值很大，切向应力在井底边界处应力最大。径向应力和切向应力都受到时间的影响，时间增加应力影响范围扩大。温差从30℃增加到180℃，机械钻速增加了22.4%，表明了随着井底温度降低，岩石抗钻性降低。

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