Anslysis Of Wellbore Stability Of XuWu Formation In Xin Chang

The gas resource quantity of Xinchang Xujiahe Formation Xuwu Member in West Sichuan is abundant, and the potential of exploring and developing is tremendous. But the geologic conditions of Xinchang Xuwu Member is complex, for instance, the lithologic character change frequently between the sandstone and the mudstone, the fracture and the microfracture are developed etc. These complicated factors will have an effect on the stability of wellbore while drilling. The analysis achievements and drilling countermeasurehave the importance leading meaning to efficient and safe construction.Based on the comprehension of basic geological characteristics of this area, this paper builds the rock mechanics calculation model and rockmechanics section, making definite the rockmechanics parameters of different lithology: the rank of poisson ratio is shale>sandy mudstone>shaly sand>sandstone; the rank of the other parameters such as elastic modulus、compressive strength、tensilestrength、cohesion、angle of internal frictionis sandstone>shaly sand>sandy mudstone>shale.The rock mechanics of these sub-members in Xuwu Member have little differences, these parametersare is influenced by the shale content. The value and direction of crustalstress of Xuwu Member is confirmed through model calculation and FMI paleomagnetism, the direction of maximum horizontal crustalstress is near NE, the azimuth of maximum horizontal crustal stress mostly located between 88°and 105°.The vertical、the horizontal and largest、the horizontal and least principal stresses are 2.40MPa/100m~2.43MPa/100 m 、 2.58MPa/100m~3.33MPa/100 mand 2.22 MPa/100m~2.36MPa/100 m. In addition, this paper uses numerical simulation technique to simulate the influence on crustal stress of several factors, such as the change of lithology on boundary surface、fault and so on.The simulation shows that the value and direction of crustal stress have slight change in the channel sand body with lithological change.The direction change less than 11°, the value change less than 10% and these two adjacent lithology changes associated with each other. The direction of crustal stress infault flank area will turn to the direction of fault plane and the deflection angle is big, the value of the crustal stress infault flank area will decrease, the peak will increase, and the change is relatively obvious.On the basis of logging information and core basic experiment data of Xinchang Xuwu Member, this paper analyze borehole stability combined with engineering geological data. According to the existing data, the formation fracture pressure is relatively high. The determination of collapse pressure is the emphasis of this paper. Besides,the borehole stability of vertical wells in research area is better, but the hole angle of highly-deviated wells and horizontal wells is relatively big, drilling toolsare not centered,the purifying effect of annularis bad, it’s easy to cause talus deposit. So highly-deviated wells and horizontal wells have high demands on borehole wall stability. This paper focuses on the optimization of rock damage criterion, forming a set of highly-deviated wells and horizontal wells borehole wall stability analysis method which give priority tocollapse pressure analysis. The paper builds characteristic section of borehole wall stability, gaining the collapse pressure and fracture pressure characteristics. It also analyzes the influence of factors including bedding weak plane、near wellbore formation pressure infiltration、drilling fluid density、drilling fluid activity、wellbore temperature change、lithological changes on the borehole wall stability, fully realizing the influence factor of highly-deviated wells and horizontal wells borehole wall stability of XinchangXuwu Member.The result shows that the borehole stability of high-angle directional wells and horizontal wells has obvious direction variations. And the borehole stability of shale and coal in the minimum horizontal crustal stress is the worst,so it is the key parameter to keep the borehole stable. The permeation of drilling fluid with weak structural plane of rock mass and(micro) cracks is the main factor to keep the layer stable. The change of temperature and activity of drilling fluid have a minimum influence on the borehole stability. By making borehole stable model with changes of lithology, we can obtain thatthe collapse pressure of pure mudstone formation is nearly 1.84g/cm3 equivalent density, the collapse pressure of the mudstone which is adjacent to sandstone is about 1.8g/cm3 equivalent density, the collapse pressure of the formation which is the juncture of sandstone and mudstone is approximately 1.89g/cm3 equivalent density, the collapse pressure ofthe sandstone in the mudstone is about 1.85g/cm3 equivalent density. It shows that the juncture of sandstone and mudstonehas have a certain influence on the borehole stabilityand the collapse pressure of the juncture of sand and shale is 0.05g/cm3 higher than that of pure mudstone.Therefore, enhancing the plug performance of drilling fluid, reducing the pressure infiltration of the wall, using oil-base drilling fluid, reducing the softening of the strength in shaft wall rock, and avoiding drilling in sand shale interface and so on are all conducive to maintain the stability of the borehole wall in the Xinchang Xuwu shale formation. This thesis forms a set of technical methods, which is suitable for the stability analysis of the borehole wall in Xinchang Xuwu sand-shale formation. And it is references for the stability analysis of the borehole wall in the similar complex geological conditions and the scheme optimizations of drilling engineering.