Study On Gas Drilling Speed-up Technology For Continental Formation In Yuanba Area

In Sichuan Basin,Yuanba area is the deepest buried gas field in China at present,which is full of natural gas.However,the Penglaizhen-Shaximiao which is in the top of formation is prone to wellbore instability and has low collapse stress.The lower Ziliujing Formation-Xujiahe Formation has poor drillability,and also has compact rock,high strength and strong abrasiveness.Because of the special geological conditions,downhole faults and complex conditions frequently occur in the early period,which will increase the time of drilling in complex formations and delay drilling time,thus restricting the speed-up.In this paper,the structural characteristics,stratigraphic characteristics,characteristics of oil,gas and water are studied,and the specific problems faced by gas drilling in in Yuanba area are analyzed.When faced shallow water,the pump pressure rised,the torque increasesd,and also caused other problems,which is easy to occur complex situations.The sandstone and mudstone in the bottom of Shaximiao formation are frequently interbedded,Xujiahe formation has sandstone and shale interbedded,was mingled with coal seam,These formations has high formation pressure,high cohesion,which was easy to occur wellbore instability.Qianfoya,Ziliujing and Xujiahe Formation have many layers of gas reservoirs.Especially the Xujiahe Formation has high pressure in gas reservoirs.The formation has high gas production and is prone to underground combustion and explosion etc.After all the borehole instability is an urgent problem to be studied and solved.Firstly,the drillability in these formation was evaluated and analyzed,and the principle of speed-up in gas drilling was analyzed.In the process of gas drilling,the rock's stress of state in hole of bottom is changed from pressure to tension,the effect of cuttings holding is weakened,and the efficiency of rock's bit breaking is increased.Through calculation,it is concluded that rock drillability can be reduced by 1-4 grades under gas drilling environment,which has great potential for increasing drilling speed.Secondly,this paper evaluated the borehole stability in these continental strata,analyzed rock mechanics characteristics and the profiles of three pressure,and established stability evaluation model based on gas drilling cohesion.By calculating and analyzing the formation stability of the test well data,the wellbore conditions for gas drilling are met.Finally,according to wellbore stability and the situation of formation fluid production,the layers for gas drilling was optimized,and the wellbore structure was designed and gas injection parameters was optimized.The construction and process control schemes of gas drilling was worked out.This paper ensure the successful implementation of gas drilling in the continental strata of Yuanba area.And make full use of gas drilling,provide feasible technical support for safety and fast drilling in the continental strata of Yuanba area,and play a guiding role in promoting the exploration and development process in Yuanba area.