Research On Borehole Stability Of Underbalanced Drilling In Northeast Sichuan

Because of the formation,during the construction of conventional drilling fluid in Northeast Sichuan,the ROP is slow,the drilling cycle is long,and it often encounters complex downhole conditions such as well wall collapse,instability,drilling fluid loss,and reservoir contamination.The use of underbalanced drilling technology can increase speed and efficiency,reduce leakage,and protect the reservoir,but the stability of the borehole wall is an unavoidable problem.Underbalanced drilling technology itself is not conducive to borehole wall stability and may cause downhole complexity.The dynamic process of borehole diameter change is analyzed by establishing the dynamic simulation calculation method of borehole wall instability,so as to optimize drilling parameters.It is very important for the successful implementation of underbalanced drilling in northeast Sichuan.Samples of Xujiahe formation in northeastern Sichuan are collected to determine formation parameters.Applying paleomagnetic core orientation technology to orientate the rock samples in northeast Sichuan;The servo controlled rock mechanics triaxial test machine is used to test the sample in differential strain to determine the numerical value and direction of each stress in the original formation,the measurement results show the order of the three geostress components in the Xujiahe formation in northeast Sichuan:the maximum horizontal geostress is the largest,and the vertical geostress is the middle,σ_H>σ_V>σ_h,The maximum principal stress direction is N86°E;Conventional rock mechanics parameters were obtained by triaxial experiment,the experimental data show that the average mudstone density of Xujiahe formation is 2.51g/cm³,the average cohesion is 29.07MPa,the average angle of internal friction is 32.54°;Developed a set of shale/drilling fluid mechanics-chemical coupling experiment equipment to determine the permeability and membrane efficiency of rock samples,Core activity was measured at approximately 0.93;The permeability and membrane efficiency of rock samples were determined by using mud shale mechanical-chemical coupling experimental equipment,the average permeability of the measured rock samples is 3.11×10^-9μm²,the average membrane efficiency is 0.185.Secondly,we studied the mud shale mechanical-chemical coupling mechanism research,Dynamic simulation research and field test verification of borehole wall failure process.Through the experiment of strength change rule under the condition of drilling fluid immersion,it shows that the mudstone will not change greatly after its strength drops to a certain degree;The pore pressure distribution model,stress distribution calculation model,rock strength change model and plastic collapse failure calculation model are established;Finally,through the field experiments of three Wells,the chart of wall failure time of target block was drawn,the model was solved and verified,and good results were obtained.The accuracy of failure cycle prediction chart is verified by field test analysis of three Wells,Underbalanced/controlled pressure acceleration applications were performed in four Wells,Yuanba xx3,Yuanba xx1,Yuanba xx5 and Yuanba xx4.The total length of the four Wells was 2,847m,and the average ROP was 1.44m/h,more than 1.8 times that of conventional drilling methods.The acceleration effect is very significant.It can be seen that the application of liquid phase underbalance drilling technology in Yuanba block can obviously improve the ROP,shorten the drilling cycle,and achieve the expected purpose of safe and accelerated drilling construction.