渤海某油田明化镇组软泥岩安全钻井周期研究

Study on safe drilling cycle of soft mudstone of Minghuazhen Formation in an oilfield in Bohai Sea

渤海某油田明化镇组发育活性软泥岩,作业过程中频繁出现起钻遇卡、下钻遇阻、倒划眼以及憋压蹩扭矩等复杂情况,大位移井井壁稳定问题突出,急需开展软泥岩地层安全钻井周期研究。基于泥岩矿物组成分析和室内岩石力学试验,建立了泥岩力化耦合模型及强度参数变化模型,并以大位移井A1井为例开展了安全钻井周期分析研究。结果表明:1)该油田泥岩伊/蒙混层含量达57%,改进型PEC钻井液条件下膨胀率达5.69%,泥岩单轴抗压强度随浸泡时间的增加而快速降低,高的伊/蒙混层比的岩石矿物组成是造成油田井壁失稳的根本原因;2)井周应力差最大值出现在地层内靠近井壁的某一位置处,坍塌破坏首先在地层内部而非井壁上发生;3)随着钻孔时间的延长,坍塌压力先减小后增大,井眼短时间内坍塌风险较低,但维持井壁稳定所需的钻井液密度迅速增加。以A1井为例进行安全钻井周期研究,当钻井液密度保持在1.25 g/cm^(3)时安全钻井周期为9~38 h,沿最小水平主应力方向钻进最为安全。该研究成果对类似油田的大位移井安全钻井周期研究有一定借鉴意义。

Active soft mudstones are developed in Minghuazhen Formation of an oilfield in Bohai Sea.Complex situations such as stuck tripping,blocked tripping,reverse reaming,pressure and torque holding,and so on frequently occur during the operation.The problem of wellbore stability of extended reach wells is prominent,and it is urgent to study the safe drilling cycle of the soft mudstone formation.Based on the mineral composition of mudstone and the indoor rock mechanics test,the author established the mechanical-chemical coupling model and the strength parameter change model of mudstone and analyzed the safety drilling cycle with the extended reach well A1 as an example.The results show that:1)The illite-smectite mixed-layer content of mudstone in the oil field is 57%,the expansion rate of the modified PEC drilling fluid is 5.69%,the uniaxial compressive strength of the mudstone decreases rapidly with the immersion time,and the high illite-smectite mixed-layer ratio of rock mineral composition is the fundamental reason for the wellbore instability of the oil field;2)The maximum differential stress around the well occurs at a certain position in the formation near the wellbore,and the collapse failure first occurs in the formation rather than on the wellbore wall;3)The collapse pressure first decreases and then increases with the drilling time of the hole.The risk of hole collapse in a short time is low,but the density of drilling fluid required to maintain the wellbore stability quickly increases.The author took well A1 as an example to study the safe drilling cycle.The safe drilling cycle is 9~38 h when the drilling fluid density is kept at 1.25 g/cm 3.It is the safest to drill along the direction of the minimum horizontal principal stress.The research results have certain reference significance for the study of safe drilling cycle of extended reach wells in similar oilfields.