Quantitative Evaluation Of The Risk Of Well Wall Instability In High Temperature And High Pressure Wells

With the growing demand for oil and gas resources in China,the oil and gas resources in conventional reservoirs can no longer meet the development needs,therefore,more and more high-temperature and high-pressure formations are being explored and developed.High temperature and high pressure formations have high temperature(usually greater than 150℃),high ground stress and possible abnormally high pressure in local areas,and narrow safety mud density window.During the drilling process in high temperature and high pressure formations,with the circulation of drilling fluid,the well wall temperature changes greatly,which will have a large impact on the collapse pressure and rupture pressure of the well wall,and is very prone to well wall instability.It is especially important to quantitatively evaluate the risk of well wall instability during drilling in high temperature and high pressure formations.In this paper,on the basis of the traditional collapse and rupture pressure calculation models,different collapse and rupture modes are studied and summarized,and different models of collapse pressure and rupture pressure are developed for different well types considering temperature variations.On this basis,a quantitative evaluation model of well wall destabilization risk under different modes was established,and a quantitative analysis and evaluation of the risk law of well wall destabilization under different influencing factors was carried out.The results show that there are various modes of collapse and rupture of high-temperature and high-pressure well walls,and the collapse mode rupture mode will change with the change of temperature field,which largely depends on the magnitude of temperature change and the change of borehole azimuth and well slope angle.In order to accurately evaluate the well wall collapse pressure and rupture pressure,different collapse modes and rupture modes should be selected at different stages.For straight wells,the rupture pressure no longer varies linearly with the change in well wall temperature,but may vary in two stages,and for horizontal wells,the collapse pressure and rupture pressure are positively correlated with the well wall temperature,but the change in collapse pressure is less than the rupture pressure;when the pressure in the well is low,the well wall temperature When the pressure in the well is low,the well wall temperature rises and the risk of collapse is high,except for some wells along the minimum horizontal main stress direction in the positive fault stress state,the overall risk of rupture is low,at this time,the change in well wall temperature has little effect on the risk of rupture,when the pressure in the well is high,the overall risk of well wall collapse decreases,at this time,the temperature has little effect on the risk of collapse,and the risk of rupture is negatively correlated with temperature;for horizontal wells,as the horizontal section is extended,the safety mud density With the increase of circulation time,the wellbore temperature field gradually reaches the equilibrium state,the well wall temperature changes less,and the well wall destabilization risk also changes less,and when the mud pump displacement increases,the well wall destabilization risk decreases with the increase of displacement.In the drilling process of high-temperature and high-pressure formations,accurate evaluation of well wall instability risk is important to ensure safe and efficient drilling and to solve the contradiction between source supply and demand.