Simulation Study of Hydraulic Fracturing Mechanism with Heterogeneous Material

Pore pressure is disregarded in traditional laboratory rock hydraulic fracturing experiments, and the effect of pore pressure is not clear. An integrated experiment for seepage and hydrofracturing was established and used to perform sandstone hydraulic fracturing experiments under an initial evenly distributed pore pressure. The experimental results show that there is a positive correlation between the breakdown pressure and the pore pressure at the initiation stage. The data fitting results show that the breakdown pressure and pore pressure follow a linear growth trend. As the pore pressure increases, the acoustic emission energy at the moment of borehole wall fracturing correspondingly increases. After borehole wall fracturing, the reduced magnitude of the pumping pressure also increases, indicating that the initial rupture range is positively correlated with pore pressure. During fracturing propagation, the propagation range and opening of the fracture increase as the initial pore pressure increases within the same pumping time. During hydraulic fracturing, a pore pressure gradient is generated on both sides of the mineral particles. When the tensile stress or shear stress induced by the pore pressure gradient reaches the ultimate strength of the mineral particle bonding surface, the particle bonding surface breaks and opens. This experimental process is more similar to the actual hydraulic fracturing process of oil and gas reservoirs. These results provide a more comprehensive theoretical basis for resolving technical problems of unconventional oil and gas resource exploitation.

Hydraulic fracturing mechanisms in coal differ in many aspects from those in inorganic rocks. Due to the lower Young's modulus and higher Poisson's ratio, the stress state in coal is higher than in adjacent formations. This leads to wider and shorter fractures than in inorganic rocks. Fractures are affected by the cleat system in coal, so that multiple fractures may follow in tortuous paths the coal cleats. Coal often has a fracture treatment pressure in excess of 22.6 kPa/m (1 psi/ft.). Several hydraulic fracturing mechanisms may account for that, namely fracture blocking by coal chips, fracture tip plugging by coal or the development of a complex multi fracture system. The orientation of the hydraulic fractures is predominantly horizontal at shallow depths ( 600 m. In between these depths, fracture orientation may be either horizontal or vertical. This paper gives a comprehensive overview of the proposed hydraulic fracturing mechanisms in coal. [Received: July 28, 2015; Accepted: January 14, 2016]

An unconventional shale reservoir commonly develops multiple layers and shows strong anisotropy in mechanical properties, which has a great effect on hydraulic fracture propagation geometry. The most common mechanical properties are elastic modulus, Poisson’s ratio, tensile strength, and formation permeability. Therefore, the extended finite element method- (XFEM-) based cohesive zone model (CZM) is applied to analyze the effect of these mechanical properties on both the fracture propagation geometry and breakdown pressure in a layered shale reservoir after verifying the present numerical method by published analytical results. The parametric analysis indicates that the stiff or soft outer layers limit the fracture propagation width, while promoting the fracture propagation length. Higher Poisson’s ratio and formation permeability in outer layers narrow the fracture propagation width in middle layer. Poisson’s ratio contrast between different layers almost has no significant effect on the fracture propagation length and breakdown pressure. The hydraulic fracture propagation geometry presents the trend of “shorter and wider” with the increase of the tensile strength in outer layer. For asymmetric specimen with different mechanical parameters in each layer, hydraulic fracture shows an asymmetric propagation behavior, and hydraulic fracture preferentially propagates to the layer with higher elastic modulus, Poisson’s ratio, formation permeability, and low resistance.

Hydraulic Fracturing (HF) is widely used in PDO in low permeability tight gas formations to enhance production. The application of HF has been expanded to the Oil South as conventional practice in enhancing the recovery and production at lower cost. HF stimulation is used in a number of prospects in the south Oman, targeting sandstone formations such as Gharif, Al Khlata, Karim and Khaleel, most of which have undergone depletion. Fracture dimension are influenced by a combination of operational, well design and subsurface parameters such as injected fluid properties, injection rate, well inclination and azimuth, rock mechanical properties, formation stresses (i.e. fracture pressures) etc. Accurate fracture pressure estimate in HF design and modeling improves reliability of HF placement, which is the key for improved production performance of HF. HF treatments in the studied fields provide large volumes of valuable data. Developing standardized tables and charts can streamline the process to generate input parameters for HF modeling and design in an efficient and consistent manner. Results of the study can assist with developing guidelines and workflow and for HF operations. Field HF data from more than 100 wells in south Oman fields were analyzed to derive the magnitude of breakdown pressure (BP), Fracture Breakdown Pressure (FBP), Instantaneous Shut-In Pressure (ISIP) pressure, and Fracture Closure Pressure (FCP) and develop input correlations for HF design. Estimated initial FCP (in-situ pore pressure conditions) is in the range of 15.6 - 16 kPa/mTVD at reservoir formation pressure gradient of about 10.8 kPa/m TVD bdf. However, most of the fields have undergone variable degree of depletion prior to the HF operation. Horizontal stresses in the reservoir decrease with depletion, it is therefore important to assess the reduction of FCP with reduction in pore pressure (stress depletion). Depletion stress path coefficient (i.e. change on FCP as a fraction of change in pore pressure) was derived based on historic field data and used to predict reduction of FCP as a function of future depletion. Data from this field indicates that the magnitude of decrease in fracture pressure is about 50% of the pore pressure change. Based on the data analysis of available HF data, standardized charts and tables were developed to estimate FCP, FBP, and ISIP values. Ratios of FBP and ISIP to FCP were computed to establish trend with depth to provide inputs to HF planning and design. Results indicate FBP/FCP ratio ranges between 1.24-1.35 and ISIP/FCP ratio ranges between 1.1 to 1.2. Developed workflow and standardized tables, charts and trends provide reliable predictions inputs for HF modeling and design. Incorporating these data can be leveraged to optimize parameters for HF design and modeling for future wells.

The variation of pore water pressure in earthen dams plays an important role in maintaining its stability. The pore water pressure within the dam are altered by the external loading conditions like rapid drawdown of reservoir water, earthquake loading and raise of water table caused by infiltration of rainfall. The seepage through an earthen dam involves saturated and unsaturated flows but to avoid complexity in solving the non-linear partial differential equations, the flow in unsaturated zone is neglected and seepage analysis is carried by constructing the flow net in which the pore water pressures beyond the free surface is taken as zero. In actual conditions negative pore water pressure develops beyond the free surface due to the capillarity which leads development to the matrix suction of the soil. In this paper a comparative study on distribution of pore pressure in a zoned earthen dam under steady state and transient conditions had been carried out considering unsaturated-saturated seepage theory. To solve the non-linear partial differential equations, finite element method has been adopted in the present study. The earthen dam has been modeled in different stages. At each stage a new parameter was added and parametric analysis was carried out. The results indicate that negative pore water pressure developed at the downstream side and the pore pressures at the mid-levels of the core are high. This specifies that, soils with low permeability have higher pore pressure. The pore pressures appeared to be higher in upstream side during rapid drawdown compared to steady state.

A considerable amount of research has been carried out in the past few decades trying to understand the mechanics of hydraulic fracturing. Conventional modeling methods using a fracture mechanics suggest that a tensile crack is generated in hydraulic fracturing, and elongates in the major-principal-stress direction. Whereas, according to the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment, most AE classified as the shear type mechanisms. Thus, the hydraulic fracturing mechanism has not been sufficiently clarified. In this study, the flow-coupled DEM (distinct element method) simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The DEM can directly represent grain-scale microstructural features of rock without complicated constitutive laws. This suggests that the DEM model may be more appropriate for the analysis of rock fracturing than other numerical analysis techniques. The simulation results were in good agreement with the actual experimental results. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously, and the energy emitted from the tensile crack is small compared with that from the shear crack. Such a small AE is easily buried in a noise and hard to be measured in an experiment, the shear type AE with large energy is dominantly observed in AE measurement experiment. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. At this time, fluid pressure becomes very high. Highly pressurized fluid causes large fracture opening, and generates tensile cracks which emit large energy. Due to this reason, both tensile and shear types of AE are observed during the injection of high viscous fluid in an AE monitoring.

Investigation of breakdown pressure in wellbores in complex conditions is of great importance, both in fracture design and in wellbore log interpretation for in situ stress estimation. In this research, using a two-dimensional numerical model, the breakdown pressure is determined in ellipsoidal and breakout wellbores. To find the breakdown pressure, the mixed criterion is used, in which the toughness and the tensile strength criteria must be satisfied concurrently. In breakout boreholes, the breakdown pressure is lower than the circular wellbores; indeed, the ratio of the breakdown pressure of the breakout wellbore to the breakdown pressure in the circular wellbore is between 1 and 0.04, depending on the deviatoric stress and the width and depth of the breakout zone. In breakout wellbores, the fracture initiation position depends on the deviatoric stress. In small deviatoric stresses, the fracture initiation position is aligned with the minimum in situ stress, unlike circular boreholes; and in large deviatoric stresses, the fracture initiates in the direction of the major principal stress. In large wellbores, the breakdown pressure is controlled by the tensile strength of the rock; and in small wellbores, the breakdown pressure is under the control of the energy spent to create new crack surfaces.

Multi-stage hydraulic fracturing in horizontal laterals is a key technology in the development of Unconventional plays around the world wherein many hydraulic fractures (on the order of hundreds) are initiated that can greatly increase the surface area for flow and, correspondingly, well production. Stress profiles, both near-wellbore and far-field, play an important role in the initiation of these hydraulic fractures as well as hydraulic fracture growth. Field measurement of the minimum stress component is often done using DFITS, mini-fracs and leak-off tests, which are very scarce and rarely done on sequenced vertical layers or along horizontal laterals. With the development of dipole sonic tools in the 1970s, log-based, continuous stress profiles (‘Frac logs’) have been used in vertical wells for hydraulic fracture design. Commonly, the minimum horizontal stress, Shmin (often mistakenly called the ‘frac gradient’) has been estimated by assuming no lateral strain (a uniaxial strain assumption) within the earth (sometimes called ‘Eaton's equation’), wherein Shmin is simply a function of Poisson's ratio (from the dipole sonic log), vertical stress, pore pressure, and Biot's constant (also computed from the dipole sonic log). There are fundamental shortcomings to this approach for estimating stresses, essentially related to the change in elastic properties through geological time and plastic deformation, which essentially promotes stress release - not elastic stress buildup. Additionally, tectonic stresses were not originally considered, and Eaton's formulation was extended to include tectonic strain effects as well as the effects of elastic anisotropy (commonly vertical transverse isotropy, VTI). For horizontal wells, the need for accurate stress measurements is even more critical than for vertical wells; however, Poisson's ratio values obtained from dipole sonic tools run in a horizontal lateral cannot be used to generate a profile of stress along the wellbore as done in vertical wells. The focus of the numerical study presented in this paper was on the limitations and inaccuracies of computing log-based stress in horizontal laterals. The results presented in the paper provide important insight into the variation of in-situ stress along horizontal laterals as well as the associated change in breakdown pressure. As estimated breakdown pressure has become a common metric for perforation placement, the results of the presented work will impact common horizontal well completion designs in Unconventional Plays.

Accurate determination of breakdown pressure in the presence of pre-existing fracture (e.g., natural fractures or perforations) can assist engineers better manage expected fracture gradients. The classical models by Hubbert and Willis, and Haimson and Fairhurst did not account for the existence of pre-existing fractures in predicting breakdown pressure. In addition, the available fracture models for the calculation of breakdown pressure do not consider nonlinear internal pressure distribution in the pre-existing fracture. Finally, some of them either ignored near wellbore stress concentrations, or they are limited to specific fracture dimensions. To overcome the limitations of current methodologies, a weight function method is applied to predict breakdown pressure of two general symmetrical radial fractures emanating from a borehole. A weight function parameter table and three weight function parameter correlations are provided for continuous dimensionless crack lengths from 0.001 to 100. For uniform pressure distribution in the pre-existing fracture, the weight function based breakdown pressure is compared against the PSA method [Paris and Sih, 1965; Abou-Sayed et al., 1978], the results show a good agreement. Weight functions are applied to predict breakdown pressure for uniform and nonuniform pressure distribution in a pre-existing fracture, and the results prove that the pressure distribution affects the breakdown pressure. Sensitivity studies are conducted to investigate the influence of pre-existing crack length, orientation, in-situ stress contrast, and fracture toughness on breakdown pressure. It indicates that breakdown pressure (1) does not always increase with increasing dimensionless crack length at different stress contrast, and (2) increase with increasing absolute value of deviation angle and fracture toughness. The weight function based breakdown pressure is further verified against measured breakdown pressure from laboratory hydraulic fracturing experiments by fine-tuning fracture toughness. The results are also in good agreement for selected successful fracturing experiments.

The breakdown pressure is a critical parameter for planning well test, surge and swab operations and/or stimulation jobs. To predict breakdown pressure, the fluid injection volume, formation permeability as well as the time dependency of fluid pressure build-up inside the well, are some of the parameters needed to consider. In this paper, the breakdown pressure from a finite-length interval in an inclined wellbore is solved in the framework of the coupled theory of poroelasticity. The in-situ stresses are transformed into a local borehole coordinate system as the far-field boundary conditions. The finite length packed interval is subjected to an injection fluid volume coupled with the applied in-situ stresses. A dual Laplace-Fourier transform technique is used to obtain time-dependent stresses and formation pressure build-up. To more realistically reflect the fluid pumping conditions involved in the field stimulation of hydraulic fracturing, a ramp-type flux loading boundary condition at the borehole wall is adopted, and the stresses and fluid pore pressure at the wellbore wall and in the formation are calculated. Numerical analyses carried out for the most common wellbore setting in fracturing operations, i.e., horizontal wells, show that both "fracturing stress" – in this case, the tangential stress – and pore pressure increase as time progresses and/or injection interval length decreases. It becomes clearly necessary to consider the impacts of the ratio of the packed interval length to wellbore radius, fluid pumping conditions, formation permeability, in adition to the coupled poroelastic effects when estimating in-situ breakdown pressures. The developed solutions can be used to generate the desired engineering charts as a generic, novel, and efficient tool in the interpretation and/or design of well test and hydraulic fracturing jobs for vertical, horizontal and inclined boreholes.

Hydraulic fracturing is the common method for increasing the production rate in reservoirs with low permeability. It is also used to raise the extraction rate of wellbores with low production due to the long-term utilization. The interfaces aligned with direction of the hydraulic fracture propagation, and different physical and mechanical properties of the adjacent layers are the most important factors in the performance of hydraulic fracturing treatments in laminated reservoirs. In this paper, the effects of layer interfaces, inclinations, thicknesses, and physical and mechanical properties on hydro-fracturing breakdown pressure and fracture propagation path were investigated. To achieve this aim, the laboratory-scaled hydraulic fracturing test was performed on thick-walled cylinder samples with different layer thicknesses and inclinations. The effect of the material property contrasts between different layers on the complex behavior of hydraulic fracture propagation in laminated specimens is also studied. The obtained results have indicated that with varying the bedding plane inclination angle from 0 to 45°, the breakdown pressure would be decreased and then increase in breakdown pressure is observed from 45 to 90°. The minimum breakdown pressure would occur at an inclination angle of 45°. During the hydraulic fracture propagation from the hard layer to the soft layer, there is no deviation observed at the layer interface. However, the possible shear slippage along the layer interface and extending the fracture in the interface would occur, when the fracture propagates from the soft layer to the hard layer. The mechanism of fracture on the layer interface is assessed based on fracture energy in fracture tip. High fracture energy would significantly reduce the effect of the inclined interface on the hydraulic fracture propagation path.

The last decade has seen a significantly increased interest in microseismic monitoring by the hydrocarbon industry due to the recent surge in unconventional resources such as shale-gas and heavy-oil plays. Both hydraulic fracturing and steam injection create changes in local pore pressures and in situ stresses and thereby brittle failure in intact rock plus additional slip/shearing in naturally fractured rock. Local rock failure or slip yields an acoustic emission, which is also known as a microseismic event. The microseismic cloud represents thus a volumetric map of the extent of induced fracture shearing, opening and closing. Microseismic monitoring can provide pertinent information on in situ reservoir deformation due to fluid stimulation, thus ultimately facilitating reservoir drainage. This paper reviews some of the current key questions and research in microseismicity, ranging from acquisition, processing to interpretation.

Study on Fracture Mechanism of Hydraulic Fracturing in Sandstone by Acoustic Emission Parameters

Experimental investigation on the breakdown pressure and fracture propagation of radial borehole fracturing

In the assessment of a dam safety with respect to global instability, nominal pore water pressure distributions are often adopted for each stage of the dam life, referring to a hypothetical “expected performance” of the different dam components. The seepage phenomena taking place within the dam may, however, modify substantially the pore water pressure distributions up to make them inconsistent with the nominal ones. With reference to this subject, the present paper represents and interprets the singular pore water pressure distributions measured within an Italian earth dam, pertaining to the typology of zoned earth dam with internal clay core. Pore water pressure interpretation is supported by the representation of the evolution of the measured seepage flows. The work highlights how pore water pressure may differ strongly from the nominal distribution during the first stages of the dam life, when, if nearly undrained conditions take place, pore water pressures are discontinuous as a result of their dependency on total stresses. The work also shows how pore water pressure may assume distributions differing from the expected ones also after some decades of the operational stages, this time as a result of suffusion phenomena induced by the seepage processes within the dam embankment. Measured pore water pressure distribution, suitably interpreted as piezometric heads contours, clearly show how part of the downstream shell contributes to the embankment watertightness.