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Hydraulic fracturing is the most effective technology to increase the productivity of low permeability CBM wells and the main way to increase single well production is to increase the length of hydraulic fractures as much as possible. In order to increase single well production, multi-micro-fracturing stimulation technology is put forth by this paper. The experiments of the migration and precipitation of proppant are carried out in ideal artificial hydraulic fracture and the Multi-micro-fracturing is simulated by Meyer. The pilot comparison experiment also carried out to verify the effects of Multi-microfracturing on the increase of single-well production. The conclusions stated that the sanding times has great impact on the migration distance of sand in hydraulic fractures and the more the sanding times are, the longer the sand bank is. Conventional fracturing technology results in the accumulation of sand near the wellbore, however, multi-micro-fracturing technology can facilitate the further migration of sand. The three-times fracturing technology can form much longer effective fractures and larger effective stimulation volume of coal seam and can stop the hydraulic fracture penetrating into the floor slab of coal seam effectively. So the three-times fracturing technology gets gas production up to 4200m3/d, 4 times of that of Conventional fracturing. So multi-microfracturing technology can significantly improve single well production and three-times fracturing technology is appropriate for the CBM formation in Southern Qinshui Basin. [ABSTRACT FROM AUTHOR]

In order to optimize wellbore shear failure criteria, this article, comprehensively, evaluated the application characteristics of Mohr Coulomb criterion, Drucker-Prager criterion and modified Lade criterion. At present, the research on the stability of the wellbore is mainly based on the classic Mohr-coulomb criterion, but lots of experiments show that the intermediate principal stress still has a huge impact on the rock damage. In order to accurately describe the influence of intermediate principal stress on the borehole wall stability, this paper applies the modified Lade criterion to wellbore stability analysis for isotropic stratum. Results indicated that Mohr-Coulomb criterion, which is linear and neglects the effect of intermediate stress, underestimates the strength and estimates the highest minimum mud pressure required to stabilize the wellbore. The Drucker-Prager criterion can provide a better estimation compared with Mohr-Coulomb. However, the modified Lade criterion that considering the effect of intermediate stress is the best failure criterion for determination of safe mud weight window. The in-situ stress distribution and the actual space state of the borehole also largely determine the stability of the borehole. The wellbore collapse pressure cloud pictures showed that the changing regularity and sensitivity of wellbore collapse pressure with deviation angle and azimuth angle is different. The research can be used to optimize borehole trajectory, avoid drilling accident-prone areas and improve drilling efficiency. [ABSTRACT FROM AUTHOR]

Typically, the required mud weight is determined by a wellbore stability analysis in which the rock material is assumed to be linear elastic, homogeneous, and isotropic in mechanics and strength behaviors. This traditional stability analysis, however, may lead to erroneous results particularly for a well drilled into fractured carbonate formation, and ignoring the strength anisotropy associated with planes of weakness such as foliations, or schistosity presented in carbonated rocks. Therefore, based on the porous elastic mechanics model of borehole stability and the single weak-plane strength theory, borehole stability model for inclined wells in fractured carbonate formations is established. Based on those, this paper quantitatively analyzed the effects of attitude of fissures and system researched the hole collapse pressure under different well track along with the change of weak plane occurrence characteristics. The basis for wellbore stability during drilling and production was provided. The results showed that collapse pressure is controlled by weak plane occurrence characteristics. Compared with traditional hole stability analysis model, the method of this article can provide a scientific calculated means and methods for solving the carbonate borehole instability. [ABSTRACT FROM AUTHOR]

Shale wellbore instability is a major cause of high cost and long time drilling and completion for shale gas. The effect of working fluid soaking on shale crack propagation and mechanical properties was analyzed to optimize drilling technology and protect wellbore stability. Response surface methodology is applied to study the influencing factors under working fluid soaking. The results show that radial holes play a certain guiding role in the directional propagation of hydraulic fractures in low permeability reservoir. Fractures start to crack and propagate a certain distance along radial holes, and then gradually divert to the maximum principal insitu stress direction of reservoir. The expansion of shale cracks after fluid immersion can be divided into two stages: the rapid formation of cracks and the slow extension of existing fractures; the shale rock strength is reduced after soaking of different fluids, and the shale rock mechanical parameters are immersed by drilling fluid. Significant changes have occurred, showing a decrease in elastic modulus and an increase in Poisson's ratio. After soaking with the same drilling fluid, the longer the immersion time, the greater the reduction in shale rock strength. [ABSTRACT FROM AUTHOR]