Jiang Tingxue

Research on Lateral Scale Effect and Constitutive Model of Rock Damage Energy Evolution

The root cause of dynamic disaster such as mine rock burst and tunnel rockburst is that the over-limit of internal energy of engineering rock mass (coal mass) triggers sudden energy release. Rock is inhomogeneous medium composed of mineral particles with various sizes and different shapes after cementation. The damage failure process comes along with the energy assimilation and release. It has important significance on studying the stability. In this paper, lateral scale effect of rock was considered. Firstly, the impact of different aspect ratios on damage mechanics and energy evolution characteristic of rock was discussed by virtue of mesoscopic particle flow PFC2D software platform. After that, the constitutive model of rock damage based on energy features was analyzed. Research result indicates that: lateral scale affects uniaxial compressive strength of rock. With the increasing of aspect ratio, the uniaxial compressive strength of rock decreased and then increased, appearing “V” state; with the increasing of lateral scale of rock, various energies insides rock and energy absorption also appears increasing, but the energy release is unobvious; the fitted constitutive model based on friction energy parameter reflects stress–strain change characteristics of rock better and there is great difference between constitutive model of rock based on boundary energy feature and numerical curve.

Overview and prospect of fracture propagation and conductivity characteristics in deep shale gas wells

There are tremendous shale gas resources in China, however, many of them can’t achieve economical production due to a lower effective stimulated reservoir volume (ESRV). Up to now, very few researches have been conducting on the mechanisms of fracture propagation and conductivity, which are the two main controlling factors of the effective development of shale gas play. As we all know that with the increasing depth of shale gas wells, many parameters such as in-situ stress, stress difference between minimal horizontal stress and maximal one, rock mechanics, etc. are all changed a simultaneously, which results in different fracture propagation and conductivity mechanisms. Therefore, firstly, a comprehensive analysis on fracture propagation and its conductivity characteristics in shallow wells were conducted. The mechanism of single fracture propagation and multiple fractures’ propagation simultaneously in open hole wells were summarized in detail. Besides, the effect of casing completion and fracturing treatment parameters on single fracture initiation and propagation was analyzed, and the corresponding results for multiple fractures may take references to that in open hole wells. Based on above studies, the mechanism of conductivity of single fracture and complex one in the condition of multiple affecting factors were analyzed too. After that, the proppant transport pattern within the complex fracture and its transportation law in divergent fracture were simulated, which may be used to optimize treatment parameters of hydraulically fracturing and achieve the object of expected conductivity and maximal ESRV. Finally, according to the formation geologic characteristics of deep shale gas play, the future research tendency was indicated, such as the mechanism of fracture propagation in the mode of well pad with multiple horizontal wells and multiple clusters, and in the condition of mixed natural fractures pattern with both horizontal interface and high angle fractures, the mechanism of high conductivity of network fracture formed by rock self-supported, which has great significance for developing deep shale gas play economically and effectively.

The key parameters of proppant transport in complex fractures

The proppant transport in complex fracture has been a hot area of research. Via optimizing the pumping parameters, like pump rate and proppant type, the proppant packing efficiency will be improved in the branch fractures, by which the stimulation results will be enhanced. The research work on proppant transportation has just started, and the laboratory physical simulation experiments are the main methods. The researchers from Katherine Thomas Technology Center and Colorado School of Mines have carried out proppant transporting experiments in complex fractures separately. The major finding is the mechanism that the proppant flows from the main fracture turning into the branches: (1) the gravity effects of the proppant bed; (2) the fluid drag effect under the critical flow rate. However, only the descriptive results were obtained from the experiments. The further numerical models and calculations based on the experimental findings still can’t be found in the current reports. This paper was aimed to reveal the proppant transporting in complex fractures using numerical methods. By literature surveys, the fractional flow of fracturing fluid and critical condition of proppant diversion were proposed to be the key parameters of proppant transport in complex fractures. The calculations of fluid fractional flow in complex fractures and the critical condition of proppant diversion were derived and verified in this paper. The fracture unit model was abstracted and used to describe the complex fractures, by using which the complex fractures could be simplified into the combination of superior fracture and sub-fracture. The fracture unit model was compared with the model of limited entry fracturing for the fluid flow, formation condition, flow friction, etc. A calculation of fracturing fluid fractional flow in complex fractures was built referring to the calculation of limited entry fracturing. Furthermore, an improved calculation of fracturing fluid fractional flow in complex fractures was derived based on the electricity flow principle in the parallel circuit, which was also the mechanism of the limited entry fracturing. The calculation results were verified by the fluent simulation results. The improved calculation was proved to be more accurate, which had the error of −2.76%. Hence, the improved calculation will be used for the fluid distribution in the proppant transport in the complex fractures. Based on the fractional flow calculation, the critical flow rate of proppant initiation was proposed to represent the critical condition of proppant diversion. The calculation of critical velocity of proppant diversion has been derived. The calculation results were tested by the Colorado School of Mines experimental results. The average error was 8.18%, which indicated that the calculation could predict the critical velocity of proppant diversion. By synthesizing the fractional flow calculation and the propapnt diversion calculation, the proppant transport in branch fractures was predicted. Using the calculation results, a chart was drawn, in which the critical proppant diversion conditions were compared with the branch fracture flow rates intuitively under the experimental conditions of Katherine Thomas Technology Center. The 100 mesh proppant was able to enter the secondary and tertiary fractures. The 20/40 mesh proppant needed higher flow rate in the main fracture to be transported into the secondary fracture. The predicted results were in accordance with the Katherine Thomas Technology Center experimental results. Therefore, the key parameters theory and their calculations have been verified and could be used for further studies and as the theoretical foundation of the fracture design.