Study On Wellbore Stability Of Shale Formation With Multi Weak Plane

Borehole stability research has been a hot topic in drilling engineering in recent years.Sidewall instability is mainly formation is drilled formation after borehole has broken the original ground stress equilibrium,the hole circumferential stress secondary distribution,in order to maintain the original ground stress equilibrium,projects through to the injection well hole drilling fluid,drilling fluid to form fluid column pressure to balance the geostress and injection density too high can't maintain balance of formation;When the density of the injected drilling fluid is too low,the drilling fluid enters the formation through the micro-fractures,and the physico-chemical interaction causes the strength of the shale to decrease and the borehole wall to collapse.Hole collapse of traditional computing model is generally based on a combination of a single weak plane and Mohr-Coulomb criterion is studied,and the shale formation due to weak micro cracks,joints and bedding face considerable development,the traditional model can't accurate characterization of sidewall instability,therefore,this article in view of the shale formation more weak surface properties,the establishment of a revised more weak surface strength criterion,A prediction model of borehole wall collapse and instability under the action of multiple weak planes was established to analyze the influence of weak plane group number,occurrence of weak plane,shale strength weakening and collapse period on borehole wall instability.The results show that the more weak surfaces,the greater the collapse pressure,the narrower the safe drilling azimuth range and the greater the drilling difficulty.With the change of drilling azimuth,the weak plane inclination area where the maximum collapse pressure is in different weak plane groups is transformed.The effect of multiple weak surfaces on the collapse pressure of borehole wall is more significant in the late drilling period.Example analysis shows that the model can predict the collapse pressure more accurately,which is of guiding significance to shale drilling engineering.Second,because in the process of drilling borehole wall instability can lead to a complex pattern in borehole,serious can cause leakage,sticking and other drilling accidents,therefore,according to different engineering geological condition choose different failure criterion calculation of safe drilling fluid density window to prevent sidewall instability problem is an important academic value.In this paper,the applicability of three different rock failure criteria,M-C,Mg-C and D-P,under five different fault mechanisms is studied,and the variation of borehole collapse pressure with the trajectory of borehole under different fault mechanisms is discussed.The research results not only provide scientific basis for the optimization of failure criteria under different engineering geological conditions,but also have certain guiding significance for the optimization design of drilling hole trajectory,which can guide drilling operation more effectively.