深井欠平衡环空岩屑对井底压力影响的研究

大庆油田在长垣东部利用深井欠平衡钻井技术进行深层气勘探，取得了较好的勘探效果，及时发现和保护了气层，但许多欠平衡井都是在接单根后效才能点火成功，表明在钻进过程中井底未能达到欠平衡状态。通过对井底压力影响因素和深层欠平衡钻井情况的分析，提出了环空岩屑影响井底压力的计算方法并编制计算软件，计算表明实际钻井过程中环空岩屑对井底压力的影响很大，在5000 m左右的深井低密度欠平衡钻井中，环空岩屑影响井底压力高达3~4 MPa，甚至更高，比大庆油田深井欠平衡钻井动负压值控制范围0~2 MPa要高很多，造成井底过平衡，影响勘探效果，对储层产生伤害。因此，在深井欠平衡钻井设计以及现场施工中应充分考虑环空岩屑对井底压力的影响，以提高欠平衡钻井井下负压控制水平。

INFLUENCE OF ANNULAR CUTTINGS ON BOTTOM-HOLE PRESSURE FOR UNDER-BALANCED DRILLING OF DEEP WELLS

The under balanced drilling techniques of deep wells are used for exploration of deep gas reservoirs in east Changyuan of Daqing oilfield. Good exploration results have been achieved. The gas bearing formations are found and protected timely. But many wells don’t produce gas until making a connection after under balanced drilling, which indicates the well bottom is not in the under balanced state during drilling. With analysis of the effecting factors of bottom hole pressure and the situation of under balanced drilling of deep wells, the calculating method and software are developed for the influence of annular cuttings on bottom hole pressure. The calculating results show there is big influence of annular cuttings on bottom hole pressure during drilling. When under balanced drilling with low density mud for a deep well of 5000 m., the influence of annular cuttings on bottom hole pressure reaches up to 3－4 MPa., even more, which is much higher than the controlling range 0－2 MPa. of the dynamic under balanced pressure for under balanced drilling of deep wells in Daqing oilfield, makes the well bottom over balance, affects the exploration results and pollutes the reservoirs. So the influence of annular cuttings on bottom hole pressure should be considered sufficiently when design and operation of under balanced drilling of deep wells to improve the controlling level of bottom hole under balanced pressure for under balanced drilling.