Prevention and forecasting of rock burst hazards in coal mines

Rock bursts signify extreme behavior in coal mine strata and severely threaten the safety of the lives of miners, as well as the effectiveness and productivity of miners. In our study, an elastic-plastic-brittle model for the deformation and failure of coal/rock was established through theoretical analyses, laboratory experiments and field testing, simulation and other means, which perfectly predict sudden and delayed rock bursts. Based on electromagnetic emission (EME), acoustic emission (AE) and microseism (MS) effects in the process from deformation until impact rupture of coal-rock combination samples, a multi-parameter identification of premonitory technology was formed, largely depending on these three forms of emission. Thus a system of classification for forecasting rock bursts in space and time was established. We have presented the intensity weakening theory for rock bursts and a strong-soft-strong (3S) structural model for controlling the impact on rock surrounding roadways, with the objective of laying a theoretical foundation and establishing references for parameters for the weakening control of rock bursts. For the purpose of prevention, key technical parameters of directional hydraulic fracturing are revealed. Based on these results, as well as those from deep-hole controlled blasting in coal seams and rock, integrated control techniques were established and anti-impact hydraulic props, suitable for roadways subject to hazards from rockbursts have also been developed. These technologies have been widely used in most coal mines in China, subject to these hazards and have achieved remarkable economic and social benefits.

Simulation of directional propagation of hydraulic fractures induced by slotting based on discrete element method

Hydraulic fracturing(HF)technology can safely and efficiently increase the permeability of coal seam,which is conducive to CBM exploration and prevent coal and gas outburst.However,conventional HF fractures tend to expand in the direction of maximum principal stress,which may be inconsistent with the direction of fracturing required by the project.Therefore,the increased direction of coal seam permeability is different from that expected.To solve these problems,PFC2D software simulation is used to study directional hydraulic fracturing(DHF),that is the combination of slotting and hydraulic fracturing.The effects of different slotting angles(θ),different horizontal stress difference coefficients(K)and different injection pressures on DHF fracture propagation are analyzed.The results show that the DHF method can overcome the dominant effect of initial in-situ stress on the propagation direction of hydraulic fractures and control the propagation of fractures along and perpendicular to the slotting direction when θ,K and liquid injection pressure are small.When the DHF fracture is connected with manual slotting,the pressure will shake violently,and the fracturing curve presents a multi-peak type.The increase and decrease of particle pressure around the fracturing hole reflect the process of pressure accumulation and fracture propagation at the fracture tip respectively.Compared with conventional HF,DHF can not only shorten the fracturing time but also make the fracture network more complex,which is more conducive to gas flow.Under the action of in-situ stress,the stress between slots will increase to exceed the maximum horizontal principal stress.Moreover,with the change in fracturing time,the local stress of the model will also change.Hydraulic fractures are always expanding to the area with large local stress.The research results could provide certain help for DHF theoretical research and engineering application.

Studying the direction of hydraulic fracture in carbonate reservoirs:Using machine learning to determine reservoir pressure

Hydraulic fracturing(HF)is an effective way to intensify oil production,which is currently widely used in various conditions,including complex carbonate reservoirs.In the conditions of the field under consideration,the hydraulic fracturing leads to a significant differentiation of technological efficiency indicators,which makes it expedient to study the patterns of crack formation in detail.Studies were carried out for all wells,which were considered as the objects of impact,to assess the spatial orientation of the cracks formed.The developed indirect method was used for this purpose,the reliability of which was confirmed by geophysical methods.During the analysis,it was found that in all cases,the crack is oriented in the direction of the section of the development system element characterized by the maximum reservoir pressure.At the same time,the reservoir pressure values for all wells were determined at one point in time(at the beginning of HF)using machine learning methods.The reliability of the machine learning methods used is confirmed by the high convergence with the actual(historical)reservoir pressures obtained during hydrodynamic studies of wells.The obtained conclusion about the influence of the reservoir pressure on the patterns of fracture formation should be taken into account when planning hydraulic fracturing under the conditions studied.