The freezing and thawing of soils have been extensively researched, but little effort has been devoted to addressing the freezing evolution process of rock mass. In this paper, a novel approach for the multi-physics constitutive models considering the change of the elastic modulus and linear expansion coefficient were developed and employed. The proposed coupling model was successfully integrated into Abaqus to simulate the freezing process of rock mass with reasonable results. The simulation results indicate that the pore water will gradually freeze into the pore ice, which will fill the pore space of the sandstone, so that the overall volume of the sandstone increases significantly. Moreover, combined with previous physical experimental studies, it also can be concluded that low temperature freezing can significantly improve the pore structure and mechanical properties of frozen sandstone. Especially, the increased absolute porosity may not be recovered after the ice lenses melts, which may cause serious irreversible frost damage to the internal structure of the sandstone. The finite element module can be used to predict the change of temperature, unfrozen water content, ice lens content, pore structure and volumetric deformation of rock mass during freezing, which can be used to accurately evaluate the effect of frost heave on the internal structure of rock mass. The study provides a theoretical basis and reference for the design and maintenance of cold region engineering and cryogenic reservoir stimulation.

With the production of oil and gas from the reservoir for a long period of time, pore pressure will decline from the initial value to a lower level, which narrows the safety mud weight window, and consequently, makes it easier to generate the drilling induced fracture (DIF). In this paper, a new analytical model is proposed for predicting initiation pressure and corresponding initiation mode of DIF in the pressure depleted reservoir. The effect of pore pressure decline on stress field is considered. Formation around the borehole is divided into plastic zone and elastic zone according to the geomechanical parameters, and small deformation theory is adopted in both of the plastic zone and the elastic zone. For the plastic zone, the nonlinear constitutive relationship is captured using equivalent stress and equivalent strain. In addition, excess pore pressure theory is introduced to describe the pore pressure change during the drilling process owing to the formation of mudcake on the borehole wall. Then, the stress and pore pressure distribution in these two zones and the radius of the plastic zone are obtained. Meanwhile, the theoretical formula of initiation pressure and the corresponding initiation mode of DIF are derived. The reliability of the new model is validated by comparing the obtained results with other published models and the field measured data.

Hydraulic fracturing treatments are capable of enhancing the well production performance in low-porosity and low-permeability reservoirs because of the resulted improvement in communication between the formation and wellbore through created hydraulic fractures (HFs). In fact, not only the in-situ stress underground but also the pre-existing offset wells induced non-uniform pore pressure field will influence the HF propagation process. In this work, to investigate the HF propagation behaviors under the effects of adjacent production and injection wells, a new numerical model is developed within the framework of extended finite element method (XFEM) and cohesive zone method (CZM). The model can simultaneously simulate the HF initiation, propagation and reorientation, fracturing fluid flow and leakoff, formation fluid flow and rock deformation. Reliability of the new model is validated by comparing the numerically calculated fracture geometry with the experimental observation in the literature. The interesting and new results show that pore pressure field, stress field and fracture propagation speed can be altered around the pre-existing offset wells. Also, HF prefers to propagate toward injection wells when production and injection wells co-exist, and the fracture propagation trajectory will not be affected in the cases that only production or injection wells exist. The fracture will be shorter and wider when only injection wells exist or production and injection wells co-exist, while the pre-existence of only production wells leads to a longer and thinner HF. Moreover, a smaller pumping pressure can be applied to generate the HF when only production wells locate nearby, while a greater injection pressure is required to drive the HF forward when only injection wells pre-exist or injection and production wells co-exist. The obtained results provide new insights for understanding the fracture propagation behaviors in the field scale.

Hydraulic fracturing plays significant role in enhancing production performance of low-porosity and low-permeability reservoirs because the created hydraulic fracture (HF) can improve the communication between formation and wellbore. Therefore, understanding propagation behaviors of HF is critically important for the design of hydraulic fracturing treatment. Normally, HF tends to take a straight path in a symmetrical stress field when rockmass is homogeneous and isotropic. However, as a result of fluid injection or production through pre-existing wells, non-uniform stress field can be encountered during the fracturing treatment in which case HF will be deviated along the perturbed principal stress field in the formation. In this work, to investigate the effects of adjacent fractured and unfractured wells on HF propagation, a new numerical model is developed within the framework of extended finite element method (XFEM) and cohesive zone method (CZM). The model can simultaneously capture HF initiation, propagation and re-orientation, fracturing fluid flow and leakoff, formation fluid flow and rock deformation. The results indicate that HF is attracted towards pre-existing injection well with deviation angle being larger when the injection well is fractured, whereas HF tends to deviate away from offset production well with deviation angle being larger around fractured production well. Meanwhile, the pre-existence of injection well makes the HF wider, and longer HF can be created near the production well. The results also demonstrate that both breakdown pressure (BDP) and fracture propagation pressure (FPP) are increased around the pre-existing injection well while BDP and FPP are decreased near the offset production well. The obtained conclusions provide new insights for understanding the HF propagation problem in the field scale.

Liquid nitrogen (LN2) fracturing is a promising new technology for unconventional reservoir simulation because it can effectively solve problems related to low permeability, low brittleness, and water shortage. The present work conducted a series of permeability and strength property-related experiments to evaluate the effect of LN2 cooling on the permeability and mechanical characteristics of anisotropic shale. The main findings of the study are as follows: (1) The influence of the bedding direction on the permeability of anisotropic shale cannot be eliminated by LN2 cooling. LN2 cooling could effectively increase the initial natural damage and the pore space of anisotropic shale, possibly increasing the volume of reservoir stimulation and provide more channels for the seepage and migration of oil and gas. (2) After LN2 cooling, the strength and brittleness of shale are obviously reduced, leading to the decrease in the ability of shale to resist deformation and failure, thereby helping to decrease the initiation pressure of reservoir stimulation. (3) The brittleness of shale will markedly increase during cryogenic fracturing, thus helping to form more complex fracture networks. Based on the present research, LN2 fracturing has obvious advantages compared with hydraulic fracturing in increasing the volume of reservoir stimulation. The results of this study are instructive for understanding the synergistic mechanism of LN2 fracturing and evaluating the effectiveness of reservoir simulation.

Frost heaving plays an important role in improving the internal structure and mechanical behavior of rock mass, but little effort has been devoted to addressing this concern. In this paper, a series of pore structure, uniaxial compression experiments and mesoscopic numerical analyses were conducted to explore the frost heaving mechanisms and mechanical behaviors of rock mass. In these tests, the compactness, P-wave velocity, compressive strength, elastic modulus and brittleness of frozen sandstone increased significantly; and the permeability and permeability coefficient decreased by several orders of magnitude with temperature dropping. The experimental results indicate that cryogenic freezing can significantly improve the internal structure and strength characteristics of sandstone. In reservoir simulation, it may be instructive for forming complex fracture networks, which helps to provide more channels for oil and gas seepage and migration, thus improving the fracturing performance. In addition, the meso-damage constitutive model were successfully integrated into Abaqus to simulate the damage evolution of rock mass, which has quite promising future for solving the trans-scale progressive failure of rock mass. The study provides a basic reference for the design and maintenance of cold region engineering and cryogenic reservoir stimulation.

The most striking feature of liquid nitrogen (LN2) fracturing is to drastically reduce the temperature around the rock when LN2 is injected into the reservoir. The strong thermal gradient can significantly induce the internal thermal stress of the rock, which results in severe damage. In this study, the permeability, ultrasonic wave, and triaxial compression test were performed to investigate the influence of LN2 treatment on pore structure and mechanical properties of stratified coal. In the pore structure tests, the increasing range of permeability of LN2-treated samples is 100.3-149.6%, and the P-wave velocity decreases by 3.1-4.6%. In the triaxial compression tests, the compressive strength and elastic modulus of LN2-treated samples decrease by 11-39.6% and 18.2-32.1%, respectively. The experimental results indicate that LN2 treatment increases the connectivity of coal pores, improving its flow conductivity and permeability, which can effectively enhance the fracturing effect. Meanwhile, it also reduces the ability of coal to resist deformation and rupture, which contributes to the reduction of initiation pressure in the reservoir simulation. In addition, the analysis further suggests that the super low temperature thermal stress can greatly promote the generation of secondary cracks and the formation of complex fracture networks inside coal. The study provides an important reference for tight reservoir simulation of cryogenic fracturing.

Hydraulic fracturing is an effective way to stimulate the production rate for reservoirs with low permeability. However, the infiltration of the fracturing fluid will damage the permeability of the reservoir matrix and the conductivity of the proppant pack. The commonly used HPG and CMHPG were selected to study the gel-breaking performance of the fracturing fluid with different mass concentration, and further to study the damage to the matrix and the proppant pack. Results showed that: The gel-breaking performance of CMHPG is better than that of HPG, the viscosity and the residue content of gel broken solution are significantly reduced; the solid phase damage is the main factor that causes the matrix damage. The smaller the reservoir permeability, the greater the damage rate caused by the fracturing fluid; the greater the amount of the thicker, the greater the amount of residue in the broken gel, and the greater the damage to the conductivity of the proppant pack.

The petrological records of mantle-magmatic CO2 leakage,which occurred in Honggang anticline,southern Songliao basin,can be revealed by studing on spatial distribution and the occurrence of dawsonite cements and also the characteristics of fluid inclusions in dawsonite-bearing sandstones.In Honggang anticline,dawsonite is widespread in Qingshankou Formation,in Quangtou Formation and Yaojia Formation come second.Most of the dawsonite-bearing sandstones do not contain CO2,while some of the dawsonite-bearing layers contain oil.There is a weak positive association between the content of dawsonite cements and bitumen,which distribute in both oil and dry layers.Two different diagenetic associations,belong to pre-injection and post-injection of CO2,respectively,can be distinguished in dawsonite-bearing sandstones.The first phase hydrocarbon inclusions which are mainly liquid hydrocarbon inclusions,can be found in the pre-injection association,while in the post-injection association,the second phase hydrocarbon inclusions develop,which are liquid-gas phases inclusions by primary.The CO2 leakage on a large-scale can be recorded by the existences of dawsonite cements occurrence on non-CO2 gas reservoir,and the bitumen.Among them,the bitumen can be the result of separating of the light-heavy components which induced by the decreased of the strata pressure when the CO2 leaked.As a consequence,the channel for CO2 leakage is Honggang fault,while the leakage time should be early than the second section of Mingshui Formation.