Borehole instability was frequently encountered during shale gas drilling.Most conventional models are not applicable to layered formation's wellbore stability analysis on account of anisotropic strength character...Borehole instability was frequently encountered during shale gas drilling.Most conventional models are not applicable to layered formation's wellbore stability analysis on account of anisotropic strength characteristic.In this study,an empirical equation for predicting anisotropic strength was implemented in the Mogi–Coulomb criterion to describe variations of cohesive strength and friction angle of shale formations.A collapse pressure model and its appropriate solution method for layered shale formations were proposed.The impact of different strength criteria and rock anisotropy type on rock strength and collapse pressure was investigated.The analysis indicated that the predicted strength of our modified criterion was usually higher than the weak plane failure criteria.The collapse pressure calculated by the modified Mogi–Coulomb criterion was lower than the weakplane failure criteria.Furthermore,it was more consistent with real mud weight.Additionally,the anisotropy type of rock notably influences wellbore stability.More significant anisotropy coefficients correspond to higher strengths,which results in smaller collapse pressure values.Improper anisotropy coefficients can over-or underpredict the collapse pressure.Reasonable estimates of collapse pressure of anisotropic rocks can be made through the modified Mogi–Coulomb criterion using limited experimental data and the anisotropy rock type.展开更多
The confined compressive strength(CCS)plays a vital role in drilling optimization.On the basis of Jizba's experimental results,a new CCS model considering the effects of the porosity and nonlinear characteristics ...The confined compressive strength(CCS)plays a vital role in drilling optimization.On the basis of Jizba's experimental results,a new CCS model considering the effects of the porosity and nonlinear characteristics with increasing confining pressure has been developed.Because the confining pressure plays a fundamental role in determining the CCS of bottom-hole rock and because the theory of Terzaghi's effective stress principle is founded upon soil mechanics,which is not suitable for calculating the confining pressure in rock mechanics,the double effective stress theory,which treats the porosity as a weighting factor of the formation pore pressure,is adopted in this study.The new CCS model combined with the mechanical specific energy equation is employed to optimize the drilling parameters in two practical wells located in Sichuan basin,China,and the calculated results show that they can be used to identify the inefficient drilling situations of underbalanced drilling(UBD)and overbalanced drilling(OBD).展开更多
基金This work was financially supported by the National Natural Science Foundation of China(No.51774248)Sichuan Province in China Key Science and Technology Foundation(No.2019YFH0166).
文摘Borehole instability was frequently encountered during shale gas drilling.Most conventional models are not applicable to layered formation's wellbore stability analysis on account of anisotropic strength characteristic.In this study,an empirical equation for predicting anisotropic strength was implemented in the Mogi–Coulomb criterion to describe variations of cohesive strength and friction angle of shale formations.A collapse pressure model and its appropriate solution method for layered shale formations were proposed.The impact of different strength criteria and rock anisotropy type on rock strength and collapse pressure was investigated.The analysis indicated that the predicted strength of our modified criterion was usually higher than the weak plane failure criteria.The collapse pressure calculated by the modified Mogi–Coulomb criterion was lower than the weakplane failure criteria.Furthermore,it was more consistent with real mud weight.Additionally,the anisotropy type of rock notably influences wellbore stability.More significant anisotropy coefficients correspond to higher strengths,which results in smaller collapse pressure values.Improper anisotropy coefficients can over-or underpredict the collapse pressure.Reasonable estimates of collapse pressure of anisotropic rocks can be made through the modified Mogi–Coulomb criterion using limited experimental data and the anisotropy rock type.
基金The work is supported by the Open Fund(Number:PLN1421)of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation Southwest Petroleum University,SWPU Science&Technology Fund(Number:2013XJZ029)Sichuan Youth Science&Technology Foundation(Number:2014JQ0045)Natural Science Foundation of China(Number:51134004).
文摘The confined compressive strength(CCS)plays a vital role in drilling optimization.On the basis of Jizba's experimental results,a new CCS model considering the effects of the porosity and nonlinear characteristics with increasing confining pressure has been developed.Because the confining pressure plays a fundamental role in determining the CCS of bottom-hole rock and because the theory of Terzaghi's effective stress principle is founded upon soil mechanics,which is not suitable for calculating the confining pressure in rock mechanics,the double effective stress theory,which treats the porosity as a weighting factor of the formation pore pressure,is adopted in this study.The new CCS model combined with the mechanical specific energy equation is employed to optimize the drilling parameters in two practical wells located in Sichuan basin,China,and the calculated results show that they can be used to identify the inefficient drilling situations of underbalanced drilling(UBD)and overbalanced drilling(OBD).
