Safe drilling technologies for horizontal wells of coal-rock gas in Sichuan-Chongqing area and their application to JT1H well
In order to safely and efficiently develop coal-rock gas reservoirs of the Permian Longtan Formation in Sichuan Basin and ensure the effective substitute of natural gas in Sichuan-Chongqing area,it is necessary to ascertain develop-ment difficulties in coal-rock gas wells with this formation as the target and to solve frequent accidents and complexities.Thus,taking JT1H well,a key exploration one deployed by PetroChina Southwest Oil&Gasfield Company as an objective,we discussed its sticking points in engineering and geology,including fractures developed within coal rocks,poor drillabili-ty resulted from complex lithology,large variation in wellbore trajectory,and inconvenient control on drilling-fluid perfor-mance.Then,complexity treatment was analyzed from four aspects of wellbore trajectory,drilling-fluid system,drilling pa-rameters,and precise sticking prevention.Results show that(i)the five-stage trajectory is adopted,which reduces the dog-leg degree with effect to satisfy the need of safe drilling for a horizontal interval over 1000 m long;(ii)giving full play to the geology-engineering integration and predicting coalbed thickness,downhole lithology and formation dip can help optimize engineering measures and lesson complexities;(iii)a method which may control the key drilling parameters in horizontal in-tervals like weight on bit(WOB),rotation speed and displacement is established to diminish coalbed scour and formation collapse;and(iv)lithologic elements are analyzed to detect high-density and high-hardness coal gangue in time and avoid sticking.In conclusion,the compound drilling technologies of efficient PDC bit+rotary steering tool+screw improve the rate of penetration and ensures the drilling rate.With the cumulative thickness of 219 m and the horizontal interval of 1160 m,there are 6 coalbeds of Longtan Formation safely drilled in JT1H well by virtue of these technologies.To deal with abnormality in drilling parameters,to adopt shale shaker,all-in-one machine and centrifuge with effect,and to devel-op coal-ash mechanical defoaming may cut down pipe sticking.In addition,promoting safe drilling technologies is of guid-ing significance for the further study on coal-rock gas development in Sichuan-Chongqing area.
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