页岩欠平衡钻进可行性探讨

The Feasibility Study of Underbalanced Drilling in Shale

摘要: 随着地层孔隙压力较高的页岩气的开发，在确保页岩井壁稳定的同时，如何提高页岩层的钻井效率和确保井下工具的正常运转，成为降本增效的关键，其中措施之一就是在保证井下安全的情况下降低钻井液密度。由于品质均一的页岩层本身具有低孔低渗的特点，页岩层内地层流体靠负压差进入井筒的能力有限，故在欠平衡状态下也很难造成溢流和井涌的风险，这一点在研究区块的探井实钻中得到证实。同时，利用实钻数据及科研成果对井壁稳定性加以分析，认为在欠平衡模式下该区的水平段高压页岩层依然可以保持良好的井壁稳定性，使得使用欠平衡模式进行水平段钻进具有了一定的可行性。

Abstract: With the development of shale gas reservoirs with higher pore pressure, how to improve the drilling efficiency in the shale layer and ensure normal operation of downhole tools at the same time has become the key of lowering cost. One of the measures was to reduce the drilling fluid density while ensuring drilling safety. Because the uniform shale bed itself was of low porosity and low permeability, the ability of formation fluid in the shale bed getting into the wellbore un-der negative pressure difference was limited, therefore it was difficult to induce the risk of over-flow and well kick under the underbalanced condition, which was confirmed in the drilling of ex-ploration wells in the study block. The borehole stability is analyzed based on the real drilling data and research results; the result indicates that in the horizontal section with high pressure shale in underbalanced condition can still maintain the borehole stability, so it is feasible to drill the horizontal sections in underbalanced mode.