Research On The Wellbore Instability In Shale Formation Induced By Unloading

Shale gas,as a kind of relatively clean energy,has become an unneglected resource with advantages of wide distribution scope,substantial deposits and long productive life.Wellbore is unstable in the course of drilling horizontal wells,especially in formations with weak bedding planes which have strong strength anisotropies.Wellbore instability in shale formations has been the technology bottleneck for developing shale gas in a safe and high efficient way.The pressure posing on wall rock provided by drilling fluid replace the pressure provided by formation rock,which would make a main stress reduce,and stress path is unloading pressure.Unloading mechanical tests are carried out to get the mechanics and deformation parameters of rock.It is more precise to study the failure behavior of rock from the view of energy.Therefore,unloading tests and studying the failure behavior of rock from the view of energy can make wellbore instability prediction more precise.This paper copes with the wellbore instability to reduce the cost of exploration and development in oil and gas fields and increase economic efficiency.In view of this,triaxial compression tests have been carried out to study the effects of the orientation angle between the major principal stress direction and the weak bedding plane on the strengths of weak bedding shales.Based on energy dissipation and energy release principles for shale,energy strength criterion for unloading has been established.This paper focuses on the weak bedding shales and discusses their strength heterogeneity,and the model of wellbore stability has been established with considerations of in-situ stresses,the occurrence bedding planes and wellbore track,especially anisotropic strength behavior of bedding shales.According to the study of the dissertation,the major achievements and knowledge are as follows:(1)In order to investigate the failure of rock,Longmaxi formation shales are taken as the research object,and triaxial unloading tests have been carried out.Specimens at orientation angles ?=0° were used in triaxial compressive tests.And based on strength data of triaxial compressive tests,triaxial unloading tests have been carried out for specimens at different orientation angles ?(0°,15°,30°,45°,60°,75°and 90°).Mechanical properties and elastic parameters and failure mode have been obtained.The effects of bedding plane on modulus of elasticity and Poisson's ratio and peak strength have been researched.Elasticity modulus and Poisson's ratio obtained from the triaxial compression tests and triaxial unloading tests are similar.There were differences between the cohesive forces and friction angles obtained from the triaxial compression tests and triaxial unloading tests.The cohesive force obtained from triaxial unloading tests is higher than it of the triaxial compression tests,and the cohesive force of former is lower than it of the latter.Elasticity modulus increases with angle ?,and Poisson's ratio is irrelevant to angle ?.Triaxial unloading tests data shows the shape of curve between peak compressive strength and orientation angle is‘U-shaped',and the bedding planes of weakness caused the strong strength anisotropies.The maximum strength is found to be ?=0°and ?=90°,and the minimum value is about 60°.The maximum strength is about two times higher than the minimum strength,which reveals weak bedding planes have significant influence on shale strength.(2)Energy plays an important role during rock deformation and failure,and the nature of rock failure is the abupt energy release of rock.The strain energy density function has been proposed considering the transverse isotropic characteristics of shales.It is analyzed the influences of bedding planes on energy and evolution characteristics of energy.The results show that the variation of energy is distributed into three stages:energy-accumulating stage and energy-dissipating stage and energy-releasing stage.The work produced by axial force is stored in the form of elastic energy at the energy-accumulating stage;the work produced by axial force are consumed by the formation and extension of cracks at the energy-dissipating stage,which would lead to strength decrease,and dissipated energy rapidly increases and elastic energy remains about the same:Elastic energy release results in rock failure when strength of rock fell below a certain point.And bedding plans have great effects on energy,when the normal direction of bedding planes and the orientation of maximum principal stress are both in the same direction,the total energy and elastic energy and dissipated energy are max,and the minimum of them are at orientation angle ?=60°.(3)Energy failure criterion of shale have been proposed based on energy dissipation and energy release principles,and the shear strain energy density function haves been built.The rock would failure when quantities of the volumetric energy reached a certain critical threshold;rom this energy strength criterion for shale has been built.What's more,this strength criterion was verified used with the experimental data and true triaxial test data from literature.The result of the verifications shows,the energy strength criterion is similar to M-C criterion when the intermediate main stress is neglected,and the energy strength criterion is more accurate than M-C criterion when the intermediate main stress is considered.And the energy strength considers the effects of bedding planes on strength of rock,which reflect the impact of much higher bedding plans on peak strength of a transversely isotropic rock.(4)Based on the poroelasticity theory and rock strength theory,this paper proposed a wellbore instability analysis model based on rock mechanics parameters and energy strength criterion and stresses model around a wellbore.The applicability on wellbore instability analysis model of energy strength criterion was evaluated.This article,systematically analyzes the combined effects of borehole trajectory,occurrence of stratification,strength of weak plane on wellbore instability.The results show that energy strength criterion is of great applicability in wellbore instability analysis for horizontal wells to reveal the combined effects of borehole trajectory,occurrence of stratification,strength of weak plane on wellbore instability.All those can provide theoretical support for optimization of mud density and wellbore trajectory design.