Study On Casing And Bit Series For Complex Formation In Ying-Qiong Basin

Due to the high temperature and high pressure(HTHP)and complex geological structure as well as narrow safety density window of drilling fluid in Ying-Qiong basin in the South China Sea,the traditional casing and bit series has become increasingly difficult to meet the complex field environment and requirements,so it is necessary to determine reasonable casing-borehole clearance so as to further design a new series of casing bits in order to prevent well leakage,overflow and even blowout caused by surge pressure during casing operation.On the basis of fully investigating the traditional casing and bit series in Ying-Qiong Basin of South China Sea,combining with the unconventional casing bit series which has been successfully used in other areas,this paper preliminarily determines two sets of casing bit series by analyzing the geological conditions in the study area and the characteristics of complex downhole accidents.The calculation model of surge pressure in wellbore during casing running is established,and the influence of casing running speed,drilling fluid density,drilling fluid rheological parameters,casing and borehole clearance on annulus pressure in casing running and cementing process is calculated and analyzed.Combining the preliminary determined casing and bit series and the influence of different factors on the surge pressure,in order to ensure that the surge pressure at the bottom of the well during casing running and the ECD in the wellbore during cementing will not leak the bottom of the target layer and the casing shoe of the layer before the target layer,the reasonable casing-hole clearance under ultimate state was calculated.On the basis of the above,through optimizing the standard casing and bit series and wellhead equipment,and combining with the applicability analysis of expansion tube technology in Yingqiong Basin,the final recommendation scheme of casing and bit series is formed.The results show that the influence of casing velocity,drilling fluid density,drilling fluid fluidity index and drilling fluid consistency coefficient on surge pressure is positively correlated,while the influence of casing-hole clearance on surge pressure is negatively correlated.For the specific stratigraphic environment with narrow density window in Ying-Qiong Basin,the unilateral casing-borehole clearance in target formation shall be maintained at 15.0 mm-19.55 mm,10.85 mm-13.15 mm and 13.0 mm-19.55 mm respectively in two schemes to ensure safe casing running and completion of cementing.The results and recommended two sets of casing and bit serious can be used for safe drilling and reasonable control of casing running parameters in high temperature and high pressure complex formations in Ying-Qiong Basin of the South China Sea.