Study on safe drilling cycle of soft mudstone of Minghuazhen Formation in an oilfield in Bohai Sea
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/cm3.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.
万方数据
维普
