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

Abstract

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.

Analysis of Stress Asymmetric Distribution Law of Surrounding Rock of Roadway in Inclined Coal Seam: A Case Study of Shitanjing No. 2 Coal Seam

“…e inclined coal seam reserves in the western region of China account for approximately 10.1% of the recoverable reserves, and there are high-quality coal seams with high mining value [5][6][7]. However, in the mining process of an inclined seam, the stress of the surrounding rock of the roadway is asymmetric owing to the influence of the inclination angle, which presents great difficulty in stability control and support engineering of the roadway [8][9][10][11][12][13][14][15]. Domestic and foreign researchers mainly focus on studying the asymmetric stress distribution characteristics of surrounding rocks in inclined seam roadways with large inclinations and steeply inclined seams, and the section forms are mostly rectangular and arch roadways.…”

Elastic-plastic analysis and shrinkage properties study of swelling rock borehole in coalmine

“…When the rock mass is disturbed, especially the change of environmental humidity and high ground stress conditions, the properties of the swelling rock will change greatly, resulting in volume swelling (Pi et al, 2018, Hawlader et al, 2005, Lu et al, 2016, Oldecop et al, 2012, Davison et al, 1990 and contraction (Yazdandoust et al, 2010, Estabragh et al, 2015. At present, coalmines generally adopt water-based gas control measures, such as coal seam water injection (Aguado et al, 2007), hydraulic fracturing (Liu et al, 2006), hydraulic flushing (Hao et al, 2016), and hydraulic slotting (Wang et al, 2017). When drilling into swelling rock strata, the swelling rock once affected by water expands and softens (Zhu et al, 2005, Vales et al, 2004, Yao et al, 2009, and the borehole diameter is reduced, which is easy to cause accidents such as holding the drill and clamping the drill.…”

Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

“…The mechanisms of fracture extension and the description of fracture morphology in CBM reservoirs have always been topical but difficult areas of research (Porcu et al 2013;Wang et al 2017;Wright et al 1995;Zhang et al 2016;Zhao et al 2016;Zou et al 2017). The uniquely developed internal cleat system (Abass et al 1990) makes a great difference in hydraulic fracture propagation mechanisms between CBM reservoirs and other conventional oil and gas reservoirs.…”

“…Scholars (Liu et al 2014;Tan et al 2017b) have also found that the propagation path of fractures followed certain principles, namely the least resistance and the most preferred propagation path. In addition, some researchers (Ding et al 2018;Huang et al 2017;Song et al 2014;Wang et al 2016aWang et al , 2017Yao et al 2018;Zhou et al 2016;Zou et al 2017) also analyzed the effects of the above factors by numerical simulation. The simulation results also proved that approach angle, horizontal principal stress difference and development of natural fractures are the main factors affecting the propagation orientation.…”