Determination of Fracture Initiation Locations during Cross-Measure Drilling for Hydraulic Fracturing of Coal Seams

Serious floor heave in gob-side entry retaining (GER) with fully-mechanized gangue backfilling mining affects the transportation and ventilation safety of the mine. A theoretical mechanical model for the floor of gob-backfilled GER was established. The effects of the mechanical properties of floor strata, the granular compaction of backfilling area (BFA), the vertical support of roadside support body (RSB), and the stress concentration of the solid coal on the floor heave of the gob-backfilled GER were studied. The results show that the floor heave increases with the increase of the coal seam buried depth, and decreases with the increase of the floor rock elastic modulus. The development depth of the plastic zone decreases with the increase of the c and φ value of the floor rock, and increases with the increase of the stress concentration factor of the solid coal. The development depth of the plastic zone in the test mine reached 2.68 m. The field test and monitoring results indicate that the comprehensive control scheme of adjusting backfilling pressure, deep grouting reinforcement, shallow opening stress relief slots, and surface pouring can effectively control the floor heave. The roof-floor displacement is reduced by 73.8% compared to that with the original support scheme. The roadway section meets the design and application requirements when the deformation stabilizes, demonstrating the rationality of the mechanical model. The research results overcome the technical bottleneck of floor heave control of fully-mechanized backfilling GER, providing a reliable basis for the design of a floor heave control scheme.

Section: Introductionmentioning

confidence: 99%

Floor Heave Mechanism of Gob-Side Entry Retaining with Fully-Mechanized Backfilling Mining

Gong

et al. 2017

“…Generally, in longwall top-coal caving faces with soft top-coal, the value of K d may is estimated between 1.2 and 1.3 and while in the longwall top-coal caving faces with hard top-coal, the value of the dynamic loading coefficient may be estimated to be in a higher range between 1.5 and 1.6 [27]. According to Figure 11d, the Equation (2) can be written in the following form, thus the ideal support capacity of the supports in LTCC-west8101 may be determined by Equation (3):…”

Section: Determination Of Reasonable Support Capacitymentioning

confidence: 99%

“…The pump pressure is 45 MPa, and the pump is stopped after 1 h of hydraulic fracturing or when the orifice water pressure is below 20 MPa. The relative parameters can be determined according to the research findings [20,27]. …”

Section: Hydraulic Fracturing For Hard Top-coalmentioning

confidence: 99%

“…Hydraulic fracturing is the most effective way to weaken the top-coal in situations in which explosives are not feasible due to high gas levels, such as in LTCC-west8101. Referring to the layout method of the fracturing drill holes on hard coal seams [20,27], hydraulic fracturing is conducted in LTCC-west8101. In the mining process, the top-coal is fractured in advance.…”

Section: Hydraulic Fracturing For Hard Top-coalmentioning

confidence: 99%

See 1 more Smart Citation

Mechanism and Prevention of a Chock Support Failure in the Longwall Top-Coal Caving Faces: A Case Study in Datong Coalfield, China

Zhu

et al. 2018

Longwall chock support failures seriously restrain the safety and high-efficiency of mining of extra thick coal seams, as well as causing a great waste of coal resources. During longwall top-coal caving (LTCC), the influential effect of the properties and the movement regulation of top-coal on strata behavior cannot be ignored, since the top-coal is the medium through which the load of the overlying strata is transferred to the chock supports. Taking Datong coalfield as an example, the mechanism of a chock support failure in the LTCC face was investigated. Research findings indicated that the hard top-coal and insufficient chock support capacity were primary reasons for chock support failure accidents. On account of the field-measured results, a new method to determine support capacity was proposed, which fully took the impact of the top-coal strength into consideration. The calculation revealed that the required support capacity had exceeded the existing production maximum, at about 22,000 KN. Since it was unrealistic to simply increase chock support capacity, other approaches, according to the theoretical analysis, were proposed, such as lowering the integrity and strength of the top-coal, and upgrading its crushing effect to weaken the support load effectively during the weighting period, which reduces the likelihood of chock support accidents occurring. Based on this, hydraulic fracturing for hard top-coal and optimization of the caving process (chock supports raised up and down repeatedly by manual operation before moving forward) were presented. The proposed solutions were successfully applied in LTCC-west8101 for subsequent mining and achieved substantial benefits. The above research provides valuable references and ideas for the control of strata behavior to ensure safe and highly efficient mining in extremely thick and hard coal seams with the LTCC method.

“…crossing interbeds, and is prone to divert along the bedding faces when the model blocks have thick and high-strength interbeds. With regards to hydraulic fracturing in geologic structures, the initiation location of hydrofracture near different types of geological faults was calculated by assuming typical in situ stresses for the faults by Lu et al [31]. Progressive development of opening-mode splay or branch fractures along a permeable fault in an elastic medium was studied numerically using a plane-strain hydraulic fracturing model by Zhang and Jeffrey [32].…”

Section: Analysis Of Propagation Modementioning

confidence: 99%

Study on the Propagation Laws of Hydrofractures Meeting a Faulted Structure in the Coal Seam

Wang

Xia

et al. 2017

Self Cite

Abstract:Hydraulic fracturing is an important technique for increasing coal seam permeability and productivity of CBM (coalbed methane). As a common type of faulted structure in the coal seam, the fault has a direct impact on the direction and scope of hydrofracture propagation, weakening fracturing effects. To study the propagation laws of a hydrofracture meeting a fault in the coal seam, based on a two-dimensional model of a hydrofracture meeting a fault, the combined elastic mechanics and fracture mechanics, the propagation mode, critical internal water pressure, and influencing factors were analyzed. A numerical simulation on the propagation laws of hydrofracture meeting a fault was conducted by using the coupling system of flow and solid in the rock failure process analysis (RFPA2D-Flow). The results show that the horizontal crustal stress difference, the intersection angle between hydrofracture and fault plane, and the physical mechanics characteristics of coal-rock bed are the main factors influencing fracture propagation. With a decrease of horizontal crustal stress differences, intersection angle and an increase of roof elasticity modulus, it is easier for the footwall hydrofracture to enter the hanging wall along the bedding plane, forming an effective fracture. When the stress difference is large and the dip angle of fault plane surpasses 45 • , the hydrofracture is easy to propagate towards the coal roof and floor by going through the fault plane. At this time, the coal seams of the footwall and the hanging wall should be fractured respectively to ensure fracturing effects, and the support of the roof and floor should be strengthened. The field experiment, theoretical analysis and numerical simulation were consistent in their results, which will contribute to the optimization of hydraulic fracturing and the prediction of hydrofracture in the coal seams containing faults.