Research On The Mechanisms Of Borehole Collapse Instability For Horizontal Wells In Shale Gas Reservoirs

Shale gas,a kind of unconventional natural gas,occurs in free or absorption state in shale or mudstone formations,exists in nearly all basins.As the depth of burial and gas saturation vary greatly,shale gas can be very different in industrial value.Because of the influence of the shale gas production in North America,the Chinese government is also taking effective measures to speed up the exploration and development of unconventional natural gas resources,including shale gas resources.The most effective technologies for development of shale gas are horizontal well drilling and multi-stage fracturing,the borehole collapse instability is one of the complicated problems encountered in the drilling engineering,there is a lot of research on borehole stability at home and abroad,the control technology of borehole stability has also made a great progress and the phenomenon of borehole instability is greatly reduced.However,the drilling of long-distance horizontal boreholes in the unstable and hard brittle shale gas reservoir,which is a new challenge for drilling engineering,which makes borehole collapse become a very serious problem in horizontal wells,it's easy to cause the borehole problems and drilling accidents take place,such as pipe sticking,pipe packing,etc.,even abandonment of the whole well,it also can significantly influence the drilling cycle,drilling costs,cementing quality and fracturing effectiveness,resulting in huge economic loss,and direct impact on the development of shale gas.Therefore the mechanics of borehole collapse instability is a major technical bottleneck for the drilling of long-distance horizontal boreholes,it's also a key question needs to be answered for safe and effective shale gas production.So,needs to be combined with the actual situation of shale gas reservoir,research on the mechanics of borehole collapse for the borehole instability problems of horizontal wells in shale gas reservoir,so that guide the establishment of rational technical measures,which provided the theoretical basis for the needs of horizontal wells drilling safe,high-quality and effective.According to the study of the dissertation,the major knowledge and results are as follows:(1)Carried out research on the geological characteristics and microscopic instability mechanism.Analyzed the geological characteristics of CN-WY Demonstration Zone for shale gas development in the Sichuan basin,using the geomechanical analysis method analyzed the shale outcrop,core and imaging logging data,illustrated the type,origin and characteristic of weak-planes,and revealed the influence mechanism of macro structure of shale rock on the borehole instability.Tested the mineralogical composition and the physicochemical property of the shale,analyzed the microscopic structures and features of shale before and after soaking by using the casting thin sections,the scanning electron microscope and the CT scanning technology,and a quantitative evaluation method based on CT scanning technology was first proposed to research the meso-damage characteristics of shale hydration,and explained the influence mechanism of microstructure of shale rock on the borehole instability.(2)The study of rock mechanics test for shale gas reservoir was carried out.The characteristics of compressive strength,compressive deformation,typical failure mode,and the anisotropic strength of shale rock were understood based on the uniaxial compressive strength tests and conventional triaxial compression tests.The characteristics of shearing strength,shearing deformation,shearing failure mode,and the anisotropic strength of shale rock were also understood based on the direct shear tests.Which lays a foundation for the description method of anisotropy,the establishment of the constitutive model and the establishment of the strength criterion for shale rock.(3)A novel mechanical evaluation method of borehole stability for shale gas reservoirs was proposed.In view of the borehole stability problems for the hard brittle and weak water sensitive shale rock under the condition of oil-base drilling fluid,based on the Bradley's elastic solution of stress distribution around borehole,and combined with the true triaxial strength criterion(Mogi-Coulomb criterion,Hoek-Brown criterion)and the breakout width model,a mechanical evaluation method of borehole stability was proposed under the condition of the breakout is permitted to a certain degree,and the optimization method for well path was proposed based on the analysis of borehole stability.Finally,the rule of borehole stability and the optimal well path under the conditions of five types of typical fault mechanism were analyzed.(4)The pore pressure transmission model around the borehole for shale gas reservoirs was proposed.Analyzed the mechanism of water transport in the shale formation,constructed the description method of anisotropic permeability and diffusivity for shale with bedding well developed,and proposed the pore pressure transmission model of anisotropic shale on the basis of the non-equilibrium thermodynamic theory.Conducted the pressure transmission tests for anisotropic shale by a new scheme of the pressure transmission experiment,proposed the inversion method for testing data by the pressure transmission tests and the finite difference method,and the ion diffusion coefficient,the hydraulic diffusion coefficient and the membrane efficiency along different direction has been obtained,the pressure transmission model also been verified by comparing the calculated results and measured data.(5)The chemical-mechanical coupling model and evaluation method of borehole stability for shale gas reservoirs was proposed.Illuminated the interaction of chemical-mechanical coupling and combed the characterization methods of elastic parameters for transverse isotropic shale,the evolution equation of damage variable has been fitted based on the CT scanning tests and the damage mechanics theory,the damage constitutive model of the shale hydration and the control equation of stress distribution around the borehole were developed,and the effective stress distribution model around the borehole under the conditions of the chemical-mechanical coupling was solved by the complex-function theory and the superposition principle.The strength theory of single weak plane and the analysis methods of rock strength with multiple weak planes were used to describe the characteristics of anisotropic strength,and the criterion of wellbore stability was proposed by combining with the nonlinear weakening laws of strength parameters for shale hydration.Finally,the solving process and method of the chemical-mechanical coupling model was also developed,which has formed the chemical-mechanical coupling evaluation method of borehole stability for shale gas reservoirs.(6)The borehole instability law of horizontal wells for shale gas reservoir was studied by the chemical-mechanical coupling evaluation method of borehole stability.Three kinds of the typical instability model around the borehole,such as the failure model of shear across the intact rock matrix,the failure model of shear along weak-plane and the compound failure model,was analyzed and contrasted with typical laboratory tests results,field tests results and downhole imaging logging data,the correctness of the analysis results of this paper were verified.The transfer law of pore pressure,the law of stress distribution,the distribution law of failure region,the law of wellbore stability for arbitrary inclined wells and their evolution law under the conditions of oil-based and water-based drilling fluid were analyzed and researched systematacially,so as to reveal the mechanism of borehole collapse instability for horizontal wells in shale gas reservoir,which provides the scientific justification for the establishment of rational technical measures.(7)The rationality and the veracity of the mechanical evaluation method and the chemical-mechanical coupling evaluation method of borehole stability has been verified by the application of several field case studies.The mechanical evaluation method of borehole stability has been applied in four field case studies,such as the Sichuan basin,Tarim basin and Ordos basin,and the rationality and the veracity of this method has been verified,the results also shows that the predicted results of this new method are more coincident with the actual situation by combined with the true triaxial strength criterion and the breakout width model.The chemical-mechanical coupling evaluation method of borehole stability also has been used to verify the rationality and the veracity by the borehole instability accidents of a horizontal well in CN-WY Demonstration Zone in Sichuan basin,the analysis results are consistent with the actual situation.The research results in this dissertation give complete and deep knowledge of the mechanism of borehole collapse instability of shale reservoir,propose and improve the mechanical evaluation method and the chemical-mechanical coupling evaluation method of borehole stability.These will provide the theoretical foundation for the establishment of rational technical measures of horizontal wells in shale gas reservoirs,which will promote the progress of the research and application of borehole stability theory of horizontal wells in shale gas reservoirs.