Contributions of non-tectonic micro-fractures to hydraulic fracturing—A numerical investigation based on FSD model

Shale gas has been discovered in the Upper Triassic Yanchang Formation, Ordos Basin, China. Due to the weak tectonic activities in which the shale plays, core observations indicate abundant random non-tectonic micro- fractures in the producing shales. The non-tectonic micro-fractures are different from tectonic fractures and are characterized by being irregular, curved, discontinuous, and randomly distributed. The role of micro-fractures in hydraulic fracturing for shale gas development is currently poorly understood yet potentially critical. Two-dimensional computational modeling studies have been used in an initial attempt toward understanding how naturally random fractured reservoirs respond during hydraulic fracturing. The aim of the paper is to investigate the effect of random non-tectonic fractures on hydraulic fracturing. The numerical models with random non-tectonic micro-fractures are built by extracting the fractures of rock blocks after repeated heating and cooling, using a digital image process. Simulations were conducted as a function of:(1) the in-situ stress ratio;(2) internal friction angle of random fractures;(3) cohesion of random fractures;(4) operational variables such as injection rate; and(5) variable injection rate technology. A sensitivity study reveals a number of interesting observations resulting from these parameters on the shear stimulation in a natural fracture system. Three types of fracturing networks were observed from the studied simulations, and the results also show that variable injection rate technology is most promising for producing complex fracturing networks. This work strongly links the production technology and geomechanical evaluation. It can aid in the understanding and optimization of hydraulic fracturing simulations in naturally random fractured reservoirs.

Numerical simulation and response analysis of microspherical focused logging in inclined micro-fractured formation

Tight and unconventional reservoirs have become the focus with the progress of petroleum exploration and development.Micro-fractures in these reservoirs can effectively improve reservoir permeability,and well-developed micro-fractures can serve to directly improve productivity.Compared with the centered electrical well logging method,the Micro Spherical Focused Logging(MSFL)is more suitable for microfracture identification due to its high resolution and near borehole wall measuring method.In this study,an anisotropic model is used to depict micro-fractured formation.First,a forward model with microfractured formation,borehole,logging instrument and surrounding rock is established.Subsequently,MSFL responses under different micro-fracture porosity,resistivity,dip angle and borehole radius,are calculated based on the finite element method(FEM).Finally,the MSFL responses under different microfracture parameters are analyzed with the response laws clarified.

Quantitative study on hydrocarbon expulsion mechanism based on micro-fracture

The significance of source rocks for oil and gas accumulation has been indisputably acknowledged.Moreover,it has been gradually realized that there is difference between hydrocarbon generation capacity and hydrocarbon expulsion capacity,and this has prompted research on hydrocarbon expulsion efficiency.However,these studies dominantly highlight the results of hydrocarbon expulsion,and investigation into the corresponding process and mechanism is primarily from a macroscopic perspective.Despite its wide acceptance as the most direct hydrocarbon expulsion mode,hydrocarbon expulsion through micro-fractures is still not sufficiently understood.Therefore,this study obtains observations and performs experiments on two types of source rocks(mudstones and shales)of the Chang 7 oil group of the Yanchang Formation in Ordos Basin,China.Microscopy reveals that organic matter is non-uniformly distributed in both types of source rocks.Specifically,mudstones are characterized by a cluster-like organic matter distribution,whereas shales are characterized by a layered organic matter distribution.Thermal evolution simulation experiments demonstrate that the hydrocarbon generation process is accompanied by the emergence of micro-fractures,which are favorable for hydrocarbon expulsion.Moreover,based on the theories of rock physics and fracture mechanics,this study establishes micro-fracture development models for both types of source rocks,associated with the calculation of the fracture pressure that is needed for the initiation of fracture development.Furthermore,the relationship between the fluid pressure,fracture pressure,and micro-fracture expansion length during micro-fracture development is quantitatively explored,which helps identify the micro-fracture expansion length.The results indicate that the development of micro-fractures is commonly impacted by the morphology and distribution pattern of the organic matter as well as the mechanical properties of the source rocks.The micro-fractures in turn further affect the hydrocarbon expulsion capacity of the source rocks.The results of this study are expected to provide theoretical and practical guidance for the exploration and exploitation of tight oil and shale oil.

Porothermoelastic response of an oil sand formation subjected to injection and micro-fracturing in horizontal wells

Stimulation of unconsolidated formations via horizontal wells has seen its vast implementation in the recent development of heavy oil reservoir to save the time and cost of preheating the reservoir before the steam-assisted gravity drainage(SAGD)process.A mathematical approach was proposed in this research that fully couples the hydraulic,mechanical and thermal responses of unconsolidated sandstone formations and also applies failure criteria for describing either shear dilation or tensile parting mechanism that generates microcracks.The approach was implemented to predict the porothermoelastic response of a pair of SAGD wells subject to injection and subsequent micro-fracturing using hot water.It was found that the predicted bottom hole pressures(BHPs)match closely with the field observed data.An elliptical dilation zone developed around the dual wells with relatively high pore pressure,porosity,permeability and temperature,implying good interwell hydraulic communication between both wells.The activation of microcracks dramatically accelerated the dissipation of pore pressure across the entire formation depth and also facilitated heat convection in between the dual wells,though to a lesser extent.In summary,the approach provides a convenient means to assist field engineers in the optimization of injection efficiency and evaluation of interference among multiple horizontal wells.

A composite material model for investigation of micro-fracture mechanism of brittle rock subjected to uniaxial compression

A two phase model of rock was proposed in order to investigate the mechanism of brittle fracture due to uniaxial compression, in which rock was considered to be a composite material consisting of hard grains and colloids. The stress state in colloid region near grains was calculated using Finite Element Method (FEM). The influence of the tensile stresses on the crack initiation and failure process of brittle rock subjected to uniaxial compression was investigated by numerical experiments. The FE results show that tensile stresses are induced easily in the neighboring area of hard grains with the maximum value near grain boundaries. The distribution of tensile stresses depends on the relative position of hard grains. The cracks initiated just near the boundary area of hard grains, which was governed by tensile stress. These results clearly reveal the micro fracture mechanism of brittle rock loaded by uniaxial compression. It can be concluded that the failure mode of brittle rock under uniaxial compression is still tensile fracture from the point view of microstructure. However, since the wide colloid region is still under compressive stress state, further propagation of boundary cracks through this region obviously needs more external load, thus causing the uniaxial compressive strength of rock much higher than its tensile strength obtained via Brazilian (splitting)

3D random Voronoi grain-based models for simulation of brittle rock damage and fabric-guided micro-fracturing

A grain-based distinct element model featuring three-dimensional （3D） Voronoi tessellations （randompoly-crystals） is proposed for simulation of crack damage development in brittle rocks. The grainboundaries in poly-crystal structure produced by Voronoi tessellations can represent flaws in intact rockand allow for numerical replication of crack damage progression through initiation and propagation ofmicro-fractures along grain boundaries. The Voronoi modelling scheme has been used widely in the pastfor brittle fracture simulation of rock materials. However the difficulty of generating 3D Voronoi modelshas limited its application to two-dimensional （2D） codes. The proposed approach is implemented inNeper, an open-source engine for generation of 3D Voronoi grains, to generate block geometry files thatcan be read directly into 3DEC. A series of Unconfined Compressive Strength （UCS） tests are simulated in3DEC to verify the proposed methodology for 3D simulation of brittle fractures and to investigate therelationship between each micro-parameter and the model＇s macro-response. The possibility of numericalreplication of the classical U-shape strength curve for anisotropic rocks is also investigated innumerical UCS tests by using complex-shaped （elongated） grains that are cemented to one another alongtheir adjoining sides. A micro-parameter calibration procedure is established for 3D Voronoi models foraccurate replication of the mechanical behaviour of isotropic and anisotropic （containing a fabric） rocks. 2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

3D morphology and formation mechanism of fractures developed by true triaxial stress

As main part of underground rock mass,the three-dimensional(3D)morphology of natural fractures plays an important role in rock mass stability.Based on previous studies on 3D morphology,this study probes into the law and mechanism regarding the influence of the confining pressure constraints on 3D morphological features of natural fractures.First,fracture surfaces were obtained by true triaxial compression test and 3D laser scanning.Then 3D morphological parameters of fractures were calculated by using Grasselli’s model.The results show that the failure mode of granites developed by true triaxial stress can be categorized into tension failure and shear failure.Based on the spatial position of fractures,they can be divided into tension fracture surface,S-1 shear fracture surface,and S-2 shear fracture surface.Micro-failure of the tension fracture surface is dominated by mainly intergranular fracture;the maximum height of asperities on the fracture surface and the 3D roughness of fracture surfaces are influenced by σ_(3) only and they are greater than those of shear fracture surfaces,a lower overall uniformity than tension fracture surface.S-1 shear fracture surface and S-2 shear fracture surface are dominated by intragranular and intergranular coupling fracture.The maximum height of asperities on the fracture surface and 3D roughness of fracture surface are affected by σ_(1),σ_(2),and σ_(3).With the increase of σ_(2) or σ_(3),the cutting off of asperities on the fracture surface becomes more common,the maximum height of asperities and 3D roughness of fracture surface further decrease,and the overall uniformity gets further improved.The experimental results are favorable for selecting technical parameters of enhanced geothermal development and the safety of underground mine engineering.

Experimental evaluation on well pattern adaptability of ultra-low permeability reservoir using sandstone flat model

As for ultra-low permeability reservoir,the adaptability of common nine-spot well pattern is studied through large-scale flat models made by micro-fractured natural sandstone outcrops.Combined with non-linear porous flow characteristics,the concept of dimensionless pressure sweep efficiency and deliverability index are put forward to evaluate the physical models' well pattern adaptability.Through experiments,the models' pressure distribution is measured and on which basis,the pressure gradient fields are drawn and the porous flow regions of these models are divided into dead oil region,non-linear porous flow region,and quasi-linear porous flow region with the help of twin-core non-linear porous flow curve.The results indicate that rectangular well pattern in fracture reservoirs has the best adaptability,while the worst is inverted nine-spot equilateral well pattern.With the increase of drawdown pressure,dead oil region decreases,pressure sweep efficiency and deliverability index increase; meantime,the deliverability index of rectangular well pattern has much more rational increase.Under the same drawdown pressure,the rectangular well pattern has the largest pressure sweep efficiency.

A new method of building permeability model in low-permeability reservoir numerical simulation

Aiming at solving the problem that big differ-ence exists between logging permeability and true permeability of micro-fractured low-permeability sand reservoir, this paper puts forward a new method to revise logging per-meability by using primiparity data of oil field. This method has been successfully applied to revise logging permeability of micro-fractured low-permeability sand reservoir in Baiyushan area of Jing’An oil field, which shows that the method is reliable because the geological model building through the permeability which has been handled by this method accords with the real reservoir significantly.

Tectonic Stress Wave,Microfracture Wave,and a Modified Elastic-Rebound Model of Earthquakes

Based on a sample of some real earthquakes,we have suggested in previous papers that there is a density-tectonic stress wave with ultra-low frequency which is emitted from the epicenter region for months before earthquakes,and a micro-fracture wave 1 ～ 10 days before earthquakes. The former has been observed by different kinds of measurements and the latter has been observed by a few chance observations which consists of electromagnetic,gravitational and sonic fluctuations. We show real observational results that depict the two waves and they have very different frequencies,which are not difficult to discriminate. The classical elastic-rebound model is one of the most influential theories on earthquakes,and the thermodynamic elastic-rebound model has amended the classical framework. Considering the two waves above,we attempt to further modify the elasticrebound model,and the new framework could be called the "micro-fracture elasticrebound model". We infer that tectonic earthquakes could have three special phases: the accumulation of tectonic stress,micro-fracture,and main-fracture. Accordingly,there would be three waves which come from the epicenter of a tectonic earthquake,i. e. ,the tectonic stress wave with ultra-low frequency a few months before the earthquake,the micro-fracture wave about 1 ～ 10 days before the earthquake and the main-fracture wave (common earthquake wave).

Effect of fault jogs on frictional behavior: An experimental study

Studying the effect of geometrically irregular bodies on the mechanical behavior of fault activity is of significance in understanding the seismic activity along a fault zone. By using rock mechanics ex- periment with medium-scale samples, we have studied the effect of fault jogs, the most common irregularity along fault zones, on frictional behavior. The research indicates that extensional fault jog can be easily fractured because of its low strength and the fractured jog has no obvious resistance to fault sliding, and the micro-fractures occurring in the jog are indicative of stick-slip along the faults. The fault zone containing extensional jogs is characterized by velocity weakening and can be described by rate and state friction law. Compressional fault jog makes fault sliding more difficult because of its high fracturing strength, but the micro-fractures occurring in the tensile areas around fault ends at higher stress level can provide necessary condition for occurrence of stick-slip along the faults before the jog is fractured and thus act as precursors of fault instability. Compression jog can be taken as a stable indicator of fault segmentation until the jog is completely fractured and two faults are linked.

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration（ i. e. ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces）. Growth of micro-crack generally experienced the following stages： cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Microstructure deformation was inhomogeneous in the stage of plastic deformation.

A novel pore-fracture dual network modeling method considering dynamic cracking and its applications

Unconventional reservoirs are normally characterized by dual porous media, which has both multi-scalepore and fracture structures, such as low permeability or tight oil reservoirs. The seepage characteristicsof such reservoirs is mainly determined by micro-fractures, but conventional laboratory experimentalmethods are difficult to measure it, which is attribute to the dynamic cracking of these micro-fractures.The emerging digital core technology in recent years can solve this problem by developing an accuratepore network model and a rational simulation approach. In this study, a novel pore-fracture dualnetwork model was established based on percolation theory. Fluid flow in the pore of two scales, microfracture and matrix pore, were considered, also with the impact of micro-fracture opening and closingduring flow. Some seepage characteristic parameters, such as fluid saturations, capillary pressure, relative permeabilities, displacement efficiency in different flow stage, can be predicted by proposedcalculating method. Through these work, seepage characteristics of dual porous media can be achieved.