A failure criterion for shale considering the anisotropy and hydration based on the shear slide failure model

A failure criterion fully considering the anisotropy and hydration of shale is essential for shale formation stability evaluation.Thus,a novel failure criterion for hydration shale is developed by using Jaeger’s shear failure criterion to describe the anisotropy and using the shear strength reduction caused by clay minerals hydration to evaluate the hydration.This failure criterion is defined with four parameters in Jaeger’s shear failure criterion(S_(1),S_(2),a andφ),three hydration parameters(k,ω_(sh)andσ_(s))and two material size parameters(d and l0).The physical meanings and determining procedures of these parameters are described.The accuracy and applicability of this failure criterion are examined using the published experimental data,showing a cohesive agreement between the predicted values and the testing results,R^(2)=0.916 and AAREP(average absolute relative error percentage)of 9.260%.The error(|D_(p)|)is then discussed considering the effects ofβ(angle between bedding plane versus axial loading),moisture content and confining pressure,presenting that|Dp|increases whenβis closer to 30°,and|D_(p)|decreases with decreasing moisture content and with increasing confining pressure.Moreover,|D_(p)|is demonstrated as being sensitive to S1and being steady with decrease in the data set whenβis 0°,30°,45°and 90°.

Drilling and completion technologies of coalbed methane exploitation: an overview

Coalbed methane(CBM)drilling and completion technologies(DCTs)are signifcant basis for achieving efcient CBM exploration and exploitation.Characteristics of CBM reservoirs vary in diferent regions around the world,thereby,it is crucial to develop,select and apply the optimum DCTs for each diferent CBM reservoir.This paper frstly reviews the development history of CBM DCTs throughout worldwide and clarifes its overall development tendency.Secondly,diferent well types and its characteristics of CBM exploitation are summarized,and main application scopes of these well types are also discussed.Then,the key technologies of CBM drilling(directional drilling tools,measurement while drilling,geo-steering drilling,magnetic guidance drilling,underbalanced drilling and drilling fuids),and the key technologies of CBM completion(open-hole,cavity and under-ream completion,cased-hole completion,screen pipe completion and horizontal well completion)are summarized and analyzed,it is found that safe,economic and efcient development of CBM is inseparable from the support of advanced technologies.Finally,based on the current status of CBM development,the achievements,existing challenges and future prospects are summarized and discussed from the perspective of CBM DCTs.

Anti-drilling ability of Ziliujing conglomerate formation in Western Sichuan Basin of China

The conglomerate rock is usually featured by strong heterogeneity,high abrasiveness,and poor drillability due to its complex composition and texture,which brought a huge challenge for drilling efficiency.In order to guide the drill bit selection and high-efficiency drilling,the physical,mechanical,and drillability characteristics were investigated for conglomerate rock that collected from the lower Jurassic Ziliujing formation in the Western Sichuan Basin of China.The mineral composition,SEM micro-structure,P-and S-wave velocities,uniaxial and triaxial compressive testing,drillability,abrasiveness were systematically tested and analyzed.The mechanical properties and anti-drilling ability of Ziliujing formation were proposed for a typical deep well of S-07,and the distribution characteristics were analyzed.The results indicated that the Ziliujing rock is rich-in quartz and clay minerals,due to the co-existing of strong quartz gravel and weak argillaceous cement,the Ziliujing rock shows strong heterogeneity.The relationships are roughly linear among UCS,drillability,and grinding weight loss with P-wave velocity.The Young's modulus,UCS,internal friction angle,drillability,and abrasiveness meet the Weibull distribution pattern,while only the Poisson's ratio meets the Kernel Smooth distribution pattern.Logging interpretation results reval that the Ziliujing formation has the Young's modulus of 38.61±17.08 GPa,the Poisson's ratio of 0.327±0.006,the internal friction angle of 49.21±11.00°,the drillability of 8.04±1.54,and the abrasiveness grade of 4.32±1.94.The mechanical properties and anti-drilling ability of logging interpretation are in good agreement with the experimental data.The Ziliujing formation is a kind of hard rock with strong heterogeneity,high strength,poor drillability,and medium abrasiveness.Based on the characteristics of Ziliujing formations,the SV516TAUL PDC bit with non-planar cutters was selected for the field application due to the good abrasion resistance,impact resistance,self-sharpening and thermal stability of the non-planar cutters.The field application shows that the average ROP of the new type drill bit in Ziliujing formation is 2.93 m/h,and the average footage is 225.9 m.Comparing with the traditional PDC bit,the ROP of the new drill bit with non-planar cutters has increased by 67.4%,and the footage has increased by 92.1%.The results of this paper can be utilized to guide the drill bit selection and high-efficiency drilling in conglomerate formation.

Evaluation on the anisotropic brittleness index of shale rock using geophysical logging

The brittleness index plays a significant role in the hydraulic fracturing design and wellbore stability analysis of shale reservoirs.Various brittleness indices have been proposed to characterize the brittleness of shale rocks,but almost all of them ignored the anisotropy of the brittleness index.Therefore,uniaxial compression testing integrated with geophysical logging was used to provide insights into the anisotropy of the brittleness index for Longmaxi shale,the presented method was utilized to assess brittleness index of Longmaxi shale formation for the interval of 3155e3175 m in CW-1 well.The results indicated that the brittleness index of Longmaxi shale showed a distinct anisotropy,and it achieved the minimum value at β=45°-60°.As the bedding angle increased,the observed brittleness index(BI_(2_β))decreased firstly and increased then,it achieved the lowest value at β=40°-60°,and it is consistent with the uniaxial compression testing results.Compared to the isotropic brittleness index(β=0°),the deviation of the anisotropic brittleness index ranged from 10%to 66.7%,in other words,the anisotropy of brittleness index cannot be ignored for Longmaxi shale.Organic matter content is one of the main intrinsic causes of shale anisotropy,and the anisotropy degree of the brittleness index generally increases with the increase in organic matter content.The present work is valuable for the assessment of anisotropic brittleness for hydraulic fracturing design and wellbore stability analysis.

Evaluation of shale fracture toughness based on micrometer indentation test

It is an important mechanical parameter for fracture toughness that affects hydraulic fracturing.Some methods such as indoor test core and logging data interpretation are commonly used to obtain fracture toughness.These methods,however,have their own limitations.Liu's method has focused the mechanical properties of shale including fracture toughness based on micro-/nano-indentation experiments.Zeng's method has pointed that the former method of calculating fracture toughness ignores the maximum holding stage in the indentation-displacement curves,they point out and correct the fracture stress intensity factor model,but the final calculation of the fracture toughness of the numerical difference is of two orders of magnitude.In this paper,micro-indentation experiments were carried out to further analyze and correct the model by collecting and analyzing Longmaxi shale cuttings.After Zeng and Liu's methods are combined,the irreversible elastic energy by energy area actually was measured.To some extent,the difference between the two kinds of fracture toughness is reduced.