| Hydraulic fracturing is one of the key technologies for low-permeability reservoir recovery.The permeability of the low permeability formations can be improved by fracturing crack in response to a combination of geostress and water injection pressure.The stimulation effectiveness of reservoir formation is mainly determined by the formation of a developmental crack network and interconnection,which is closely related to the geostress,lithology,and pore structure of reservoir rock.Sandstone is one of main unconventional reservoir rock and stacked by multi-stage gravels.Generally,it performs rapid accumulation,dramatic variation on the thickness and lithology in the longitudinal deposition.Consequently,The structural and compositional maturity of the rock are very low.Consequently,the reservoir rock is highly heterogeneous.In the reservoir stimulation,the initiation and propagation of the hydrofracturing crack was affected by the heterogeneous gravels as well as geostress conditions and the injection pressure,which caused multiple crack initiation and torsion,bifurcation in the crack propagation evently leading to complex spatial distribution and propagation pattern of the crack.As a result,the reservoir is usually difficult to be enhanced desired,the recovery rate is low and the waste of resources is serious.Thus,understanding the mechanisms and factors that govern the initiation and propagation behavior of hydrofracturing cracks has become a pivotal issue in enhancing hydraulic fracturing stimulation of unconventional gas reservoirs.The in situ detection of the hydrofracturing cracks is limited by the shortages of the technologies existed,implemented difficultly and costly.As a result,It is difficult to understand and accurately describe the mechanism of the crack propagation,the characteristics of the crack spatial distribution and the influence of geological environment.Taking into account the above issues,this thesis intend to study the mechanism of coupling effected of gostress ratio and material heterogeneity on the crack initiation and propagation both experimentally and numerically.In the experimental studies,the natural rock core is rarely obtained,and quantity limited,thus the mechanical properties and grading characteristics of natural rock was tested.Accordingly,we constructed the artificial rock like specimens for triaxial hydrofracturing experiments.meanwhile,by means of high-resolution microfocus computed tomography,image processing,we acquired the spatial distribution of the hydrofracturing crack and probed the effects of geostress difference and material heterogeneity on the characteristics of hydrofracturing cracks,such as initiation,propagation and distribution.The mechanism of crack initiation and propagation in the heterogeneous model was revealed by employing the maximum circumferential tensile stress criterion and finite element simulation.Fractal theories were applied to characterizing the 3D hydrofracturing crack networks of both heterogeneous and homogeneous rock samples.However,the experimental results illustrated the final crack network pattern after the experiments,neither the whole hydrofracturing process.Therefore,we employed numerical simulation method to show the crack initiation and propagation process.In the numerical simulation,the numerical model was reconstructed in terms of the artificial physical model used in the experiment.The reconstruction models was meshed by MIMICS software and the meshes was classified in accordance of the compositions of the model.Furthermore,we simulated the hydraulic fracturing process by setting the stress boundary condition according to the geostress conditions in the experiment to obtain the crack initiation and propagation process based on the CDEM algorithm.Finally,in order to we carried out the hydrofracturing tests based on transparent materials as well.There are two advantages for this testing method: the first one is that the transparency of the transparent material itself can be used to show the spatial morphologies of the cracks,which get rid of the dependence on the precision of the experimental equipment and complicated image processing.The second one is that the homogeneity of the material can be ensured in this way excluding other factors that complicated the model,such as porosity,natural fracture and defect.It is helpful to focus on the main factors concerned that effect crack initiation and propagation.However,it can not employ the heterogeneity into the model based on the traditional transparent materials and production methods such as the PMMA material used in our study.Therefore,we construted the transparent samples containing heterogeneous structure by 3D printing technique to carry out the hydrofracturing test.The main conclusions are as follow:(1)Both horizontal geostress difference and material heterogeneity has significant influence on the characteristics of hydrofracturing cracks in heterogeneous reservoir rock.When the degree of horizontal geostress difference is higher,that is,the geostress ratio reaches 1:1.7,it is more likely to have a double-wing single crack than multiple twist cracks,no matter the tested rock model is heterogeneous or homogeneous.The horizontal geostress difference is the predominant factor that influences the fracture pattern of reservoir rocks.In contrast,if the degree of horizontal geostress difference is lower,that is,the geostress ratio is below 1:1.7,multiple twist cracks rather than a single crack occurs.In the context of this sort,the distributed gravels remarkably influence the initiation and propagation of hydrofracturing cracks,which implies that material heterogeneity plays the predominant role in affecting the crack initiation and propagation performance.The geostress ratio 1:1.7 appears to be a threshold value at which the heterogeneity has great influence on crack initiaiton and propagation.The fractal characterization of the 3D hydrofracturing crack network of rock models verifies the distinct contribution of geostress difference and material heterogeneity to controlling the crack initiation and propagation in heterogeneous reservoir rock.The fractal analysis results are consistent with the analysis of fracture patterns of the tested rock models.The fractal dimension of the hydrofracturing crack network is positively related to the degree of material heterogeneity and negatively related to the degree of geostress difference.(2)The analysis of the stress states in the neighborhoods of the wellbore and the initial crack using fracture mechanics theory and finite element methods quantitatively elucidates the mechanisms through which horizontal geostress difference and material heterogeneity induce and control the hydrofracturing crack initiation and propagation.The analytical results are in good agreement with the experimental measurement.(3)Material heterogeneity had significant influence on crack initiation.Cracks emerged at various azimuths around the vertical wellbore,and microcracks coalesced to form main branches during the propagation process.Heterogeneous media provided different resistance in different crack propagation trajectories,which resulted in different crack volumes.Conversely,the crack initial and propagation in the homogeneous model are more simple.The geostress ratio controlled crack propagation.A geostress ratio of 1:1.7 appeared to be the threshold below which material heterogeneity rather than geostress difference played the dominant role in governing crack initiation.Meanwhile,the numerical simulations were verified by the experimental results.(4)The transparent 3D printing specimens revealed the hydrofractuirng crack pattern directly.When the gravel is weak,the crack prefered to break through the gravels and go advance.However,the crack would like to bifurcate when it enters the harder regions from softer areas,which provided experimental data for understanding hydrofracturing crack distribution pattern of glutenite.This paper has the following innovations:(1)we considered the coupling effect of material heterogeneity and geostress ratio on hydrofracturing crack initiation and propagation and employed fractal dimension method to despict the crack complexity;(2)we employed the algrithm that combined finite element method and discrete element method to simulate the hydrofracturing crack initiation and propagation process;(3)The 3D printing technology was used to construct the transparant samples with heterogeneous gravels inside to implement hydrofracturing tests and directly observe the crack morphology in the heterogeneous medium. |
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