Transformation effect and energy control mechanism of hydraulic fracturing in deep coal reservoirs
FANG Xiaojie
WU Caifang
SHEN Jian
ZHANG Hewei
QU Jing
GAO Bin
LI Geng
Abstract：The hydraulic fracturing process involves energy input,release,and dissipation.Investi-gating the energy evolution mechanisms and stimulation effects during fracturing processes can pro-vide theoretical foundations and technical support for the optimization of fracturing techniques in deep coal reservoirs.Taking the No.2 coal seam in the southern Yanchuan block in the eastern margin of the Ordos Basin as the research object,by optimizing the physical simulation method of true triaxial hydraulic fracturing,combined with numerical simulation and mathematical modeling,the stress distribution and fracture initiation and propagation characteristics of hydraulic fracturing in deep coalbeds are analyzed,revealing the evolution mechanism of input energy and its control effect on the fracturing effect.The results show the following:The optimization method for the physical simulation of true triaxial hydraulic fracturing,in which the borehole is filled with NaCl powder,re-inforced with anti-pull-resistant glue,and buffered with rubber gaskets on the outer side of the sample,has good reliability,which greatly reduces the probability and end effect of the sample breaking during stress loading and meets the needs of the physical simulation of hydraulic fracturing in deep coal.As the injection rate increases,the fracture creation effect changes from a small-scale,low-fracture width complex fracture network to a large-scale,medium-fracture width complex frac-ture network and then to ultralarge scale,high-fracture width simple fractures.A injection rate of 9 mL/min can form a fracture system with a high scale and fracture complexity.The fracture pres-sure point is the turning point from slow to fast expansion of the fracture zone.Comprehensively considering the effects of in situ stress and water pressure,a model of hydraulic fracturing stress dis-tribution is constructed.The calculation revealed that the transformation of circumferential stress from compressive stress to tensile stress is the driving force for the initiation of fracturing fractures.In the deep normal faulting stress regime,fractures not only form in the plane where the vertical stress and the maximum horizontal principal stress are located but also preferentially initiate near the wellbore parallel to the vertical stress direction.The energy of hydraulic fracturing physical simula-tion of fracture initiation comes from the accumulated input energy during the water pressure in-crease stage(stage Ⅱ).It consists of pressure energy and kinetic energy,and pressure energy is the main body of energy.The injection rate exhibits a quadratic relationship with both the input en-ergy and the dissipated energy,while it follows a logarithmic relationship with the energy input rate.Increasing injection rate can increase the energy input rate,but the energy input efficiency de-creases.There is a Gaussian distribution between the fracture rate and the energy input rate.When the energy input rate in Stage Ⅱ is between 0.70 and 1.15 J/s,the effect of physical simulation fracturing is the best.
Keywords：deep coalbed methanehydraulic fracturingphysical simulationnumerical simula-tionenergy evolutiontransformation effect
Publication Date：2026-01-30
Online Publishing Date：2026-01-28(First online date of this platform, not the publication date of the document)
Pages：20( 193-212 )