Study On Anti - Condensation Reservoir Damage Of Gas Drilling In Deep Dense Condensate Gas Reservoir

Compared with conventional drilling technologies, gas drilling has some advantages on the exploration and evaluation of deep tight gas reservoir, enhancing productivity and alleviating reservoir damage.so gas drilling technology has been one of the best way to explore and exploit deep tight gas reservoir, which has been used in Xinjiang autonomous region and Sichuan province. However, when drilled by gas, phase change of condensate gas would occur and condensate liquid would drop out for the change of temperature and pressure when seeping in reservoir and flowing in wellbore, which can cause formation damage. And this phenomenon has influences on drilling and completion process, which has been proved in an oilfield. Prior to this, research related to condensation rule in gas drilling is seldom. This paper is based on external causes which influence condensate gas reservoir phase and the main works are as follow:(1) Wellbore temperature of gas drilling has been studied. Wellbore temperature distribution models under different conditions have been established based on heat transfer and energy conservation theories, including drilling reservoir before, blowing while drilling and after stopping gas injection mathematic models. Then gas temperature distribution of wellbore under different conditions have been acquired by solving mathematic models combining corresponding boundary conditions(2) Wellbore flowing law of gas drilling and formation seepage after uncovering reservoir have been revealed. As flowing rule of wellbore would change after uncovering reservoir by gas drilling and reservoir gas rapidly getting into wellbore under huge pressure difference, wellbore pressure and flowing speed of gas along well depth have been obtained through constructing and solving steady flow model of gas in the wellbore according to mass and momentum conservation models. Moreover, pore pressure of formation changes with time and radial distance has been obtained by establishing formation seeping model after reservoir has been uncovered.(3) Variation of condensate gas phase in the formation after condensate reservoir has been uncovered by gas drilling has been analyzed. Based on analysis of formation pressure change with time and radial distance and assumption of isothermal reservoir, the law of phase variation of condensate gas and saturation distribution of retrograde condensate liquid in the formation and factors of influencing phase variation have been analyzed. Furthermore, alleviating damage by increasing the wellhead pressure has been put forward. Degree of reservoir damage caused by retrograde condensation behavior was analyzed qualitatively by calculation of skin factor. Impacts of retrograde condensation behavior on the subsequent completion has also been discussed.(4) Condensation law of condensate gas in the wellbore while gas drilling has been analyzed. Condensation law of condensate gas in the wellbore has been studied through research on gas temperature and pressure. Then critical speed for carrying condensate liquid by gas while gas drilling has been obtained based on fluid carrying model. The results show that temperature affect phase variation of condensate gas in the wellbore more than pressure when it is in low pressure conditions. In addition, bottom hole liquid would occurred if condensate liquid cannot be carried out of ground under high wellhead pressure.Research achievements of this paper have positive significances on evaluation of exploiting deep tight condensate gas reservoir and safely and rapidly uncovering reservoir with gas drilling.