Mechanical model of water inrush from coal seam floor based on triaxial seepage experiments

In order to study the mechanism of confined water inrush from coal seam floor,the main influences on permeability in the process of triaxial seepage experiments were analyzed with methods such as laboratory experiments,theoretical analysis and mechanical model calculation.The crack extension rule and the ultimate destruction form of the rock specimens were obtained.The mechanism of water inrush was explained reasonably from mechanical point of view.The practical criterion of water inrush was put forward.The results show that the rock permeability "mutation" phenomenon reflects the differences of stress state and cracks extension rate when the rock internal crack begins to extend in large-scale.The rock ultimate destruction form is related to the rock lithology and the angle between crack and principal stress.The necessary condition of floor water inrush is that the mining pressure leads to the extension and transfixion of the crack.The sufficient condition of floor water inrush is that the confined water’s expansionary stress in normal direction and shear stress in tangential direction must be larger than the internal stress in the crack.With the two conditions satisfied at the same time,the floor water inrush accident will occur.

Mutual impact of true triaxial stress, borehole orientation and bedding inclination on laboratory hydraulic fracturing of Lushan shale

Unconventional resources like shale gas has been the focus of intense research and development for two decades. Apart from intrinsic geologic factors that control the gas shale productivity (e.g. organic matter content, bedding planes, natural fractures, porosity and stress regime among others), external factors like wellbore orientation and stimulation design play a role. In this study, we present a series of true triaxial hydraulic fracturing experiments conducted on Lushan shale to investigate the interplay of internal factors (bedding, natural fractures and in situ stress) and external factors (wellbore orientation) on the growth process of fracture networks in cubic specimens of 200 mm in length. We observe relatively low breakdown pressure and fracture propagation pressure as the wellbore orientation and/or the maximum in situ stress is subparallel to the shale bedding plane. The wellbore orientation has a more prominent effect on the breakdown pressure, but its effect is tapered with increasing angle of bedding inclination. The shale breakdown is followed by an abrupt response in sample displacement, which reflects the stimulated fracture volume. Based on fluid tracer analysis, the morphology of hydraulic fractures (HF) is divided into four categories. Among the categories, activation of bedding planes (bedding failure, BF) and natural fractures (NF) significantly increase bifurcation and fractured areas. Under the same stress regime, a horizontal wellbore is more favorable to enhance the complexity of hydraulic fracture networks. This is attributed to the relatively large surface area in contact with the bedding plane for the horizontal borehole compared to the case with a vertical wellbore. These findings provide important references for hydraulic fracturing design in shale reservoirs.

Effect of stress-dependent microannulus aperture on well leakage

Debonding at the cement-casing interface is recognized as a principal failure mechanism leading to CO_(2) leakage in wells.This detachment gives rise to a microannulus,which notably possesses greater permeability than undamaged cement,undermining its sealing efficacy.Conventionally,the permeability of the microannulus is regarded as a uniform value throughout the well.However,fundamentally,a microannulus is one type of fracture,and its gap or aperture size is affected by the effective stress.In this work,we developed a unique experimental apparatus.This equipment facilitates the curing of cement inside a steel casing,the formation of a microannulus between the casing and the cement,and the investigation of the fluid flow dynamics along the microannulus under laboratory-replicated in situ conditions.The microannulus was formed by injecting fluid from one end of the setup,and receiving similar amount of fluid on the other end signified the development of the leakage channel.Additionally,strain gauges affixed to the casing’s external surface yielded key information on the microannulus’s opening and closure.We observed a noticeable decline in microannulus hydraulic aperture(or permeability)in relation to effective stress and an exponential equation fits their relationship.Our findings also indicate a distinct behavior when comparing liquid CO_(2) with water.Specifically,it is easier for liquid CO_(2) to create the microannulus.However,the hydraulic aperture range for this microannulus(0.7-6 mm)is considerably smaller than that created by water flow(2-17 mm).Finally,we integrated the stressdependent microannulus aperture size into the combined analysis of well mechanical integrity and well leakage.The outcomes consistently demonstrated that when factoring in the stress-dependent aperture sizes,the leakage rates are 3e5 times compared to a fixed aperture model.The traditional assumption of a constant aperture significantly underestimates fluid leakage risks.

Experimental investigation on deformation characteristics and permeability evolution of rock under confining pressure unloading conditions

Deformation behavior and hydraulic properties of rock are the two main factors that influence safety of excavation and use of rock engineering due to in situ stress release.The primary objective of this study is to explore deformation characteristics and permeability properties and provide some parameters to character the rock under unloading conditions.A series of triaxial tests with permeability and acoustic emission signal measurement were conducted under the path of confining pressure unloading prior to the peak stress.Deformation behavior and permeability evolution in the whole stress–strain process based on these experimental results were analyzed in detail.Results demonstrate that,under the confining pressure unloading conditions,a good correspondence relationship among the stress–axial strain curve,permeability–axial strain curve and acoustic emission activity pattern was obtained.After the confining pressure was unloaded,the radial strain grew much faster than the axial strain,which induced the volumetric strain growing rapidly.All failures under confining pressure unloading conditions featured brittle shear failure with a single macro shear rupture surface.With the decrease in deformation modulus during the confining pressure unloading process,the damage variable gradually increases,indicating that confining pressure unloading was a process of damage accumulation and strength degradation.From the entire loading and unloading process,there was a certain positive correlation between the permeability and volumetric strain.