Fracture Initiation Pressure Research Articles

Experimental study on gas adsorption and drainage of gas-bearing coal subjected to tree-type hydraulic fracturing

Exploiting coalbed methane can optimize the energy mix and improve safety in coal mines. Coalbed methane production commonly requires that the coal’s permeability be enhanced. However, conventional permeability enhancement techniques commonly used at present have their own shortcomings, such as a limited range of permeability enhancement or high fracture initiation pressures Tree-type fracturing is one of the new techniques that are expected to eliminate the above-mentioned shortcomings and allow the efficient exploitation of coalbed methane reserves. The unclear of gas migration mechanism in tree-type fracturing limits the development of this technology. For this paper, a fluid–solid–gas coupling experimental system was assembled to study the changes in gas pressure and gas flow in gas-bearing coal after hydraulic fracturing. In the experiments, tree-type branch boreholes can effectively connect hydraulic fractures and surrounding natural cracks, increasing the scope and rate of gas extraction. The degree of connectivity of fractures (cracks and pores) will affect the gas desorption rate, which makes the gas flow trace “U” shaped lines. Tree-type hydraulic fracturing can increase the coal’s permeability by factors of 3.1 to 3.9, increase the effective drainage rate by factors of 1.23 to 1.41, and increase the proportion of gas drained by 3.7% to 5.5%. These results provide a reliable basis for additional research on tree-type fracturing and can aid in its engineering application.

Constraining tectonic components during a geomechanics-aided successful hydrofracturing campaign of tight gas exploration field

The horizontal stress profile plays an important role, extending from wellbore stability analysis to well completion optimization of tight gas reservoirs. When considering exploration fields with planned wells being drilled to 5500-m TVD, it is imperative to quantify tectonic effects at the well location. In addition, accurately predicting stress profile and fracture initiation values in vertical wells is required to identify sweet zones and barriers. This paper presents the details of a pre-fracture geomechanical model using breakouts and advanced acoustic data for post-fracture analysis. The analysis contains a history match of fracture initiation pressure, which consider the effects of filter cake around permeable sand, variation in tensile strength, and quantification of horizontal stress contrast in the different fields. Overall, three reservoirs have been analyzed, each containing more than eight wells with operations history. Core tests were used to calibrate dynamic-to-static rock elastic and mechanical properties, both of which reduced uncertainty in the model. The poroelastic horizontal strain method was used to build a continuous stress profile. Typically, the rock fabric found in the cores, images, and anisotropy data from the three reservoirs is different and required various dynamic-to static conversions. The Aeolian deposits-based reservoir has a wide variation in horizontal stress, and fracture height is typically governed by the stiffness of the layers. The lower permeability zones have relatively higher tensile strength, compared with higher permeability zones leading to relatively higher fracture initiation values. Overall, the ratio of maximum horizontal stress-to-minimum horizontal stress varies between 1.20 and 1.28 based on post-fracture analysis, which correlates well with regional tectonics and structural data. Depending on lithological variation and structure changes, the horizontal strain component varies at the layer level within regional tectonics. Inversion of fracturing data helped to constrain horizontal strain and stress variations in the field.

Experimental Investigation on the Initiation of Hydraulic Fractures from a Simulated Wellbore in Laminated Shale

Abstract Understanding the formation mechanisms of complex fracture networks is vitally important for hydraulic fracturing operations in shale formation. For this purpose, a hydraulic fracturing experiment under a core-plunger scale is conducted to investigate the impact of the bedding plane angle, borehole size, and injection rate on fracture initiation behaviors of laminated shale rock. The results on rock properties demonstrate that the anisotropic characteristics of shale rock are reflected not only in elastic modulus but also in tensile strength. The results of fracturing experiments show that the bedding plane dip angle and borehole size have significant effects on fracture initiation behaviors, in that fracture initiation pressure (FIP) decreases with the increase of those two factors. The impact of injection rate, by contrast, has no obvious variety regulation. The above data is further used to validate our previously proposed fully anisotropic FIP model, which shows better agreement with experimental results than those using other models under various parameter combinations. Finally, a postfracturing analysis is performed to identify the fracture growth patterns and the microstructures on the fracture surfaces. The results show that the hydraulic fractures (HFs) always grow along mechanically favorable directions, and the potential interaction between HFs and bedding planes mainly manifests as fracture arrest. Meanwhile, the roughness of fracture surfaces is physically different from each other, which in turn results in the difficulties of fluid flow and proppant migration. The findings of this study can help for a better understanding of the fracture initiation behavior of laminated shale rock and the corresponding fracture morphology.

Investigation of the influence of gas fracturing on fracturing characteristics of coal mass and gas extraction efficiency based on a multi-physical field model

Coal seam gas recovery is a critical way to achieve clean energy development and reduce greenhouse gas emissions. Gas fracturing, as a necessary and advantageous stimulation measure, can effectively achieve gas extraction from deep coal seams and ensure safe production in deep coal mines. In this study, a hydro-mechanical coupling model is established to analyze the damage characteristics of coal seams during gas fracturing. The model focuses on three aspects: (a). the fracture criterion of type-I fracture under the action of multiple stress states in the fracturing process; (b). the characteristics of post-peak strain softening of coal and its influence on coal permeability; (c). the influence of non-uniform stress state on directional compression deformation and directional permeability of coal. The difference between gas fracturing (nitrogen, CO2) and hydraulic fracturing is studied based on this coupling model. The development of fractures in coal seam gas reservoirs after gas fracturing is investigated, and the influences of the coal parameters of the gas extraction process after fracturing are determined. Results show that because of the different compression coefficients, viscosities of water and gas and the slippage effect, there are obvious differences in the fracturing pressure of gas and water. Under the same parameters, the hydraulic fracturing method needs larger fracture initiation pressure than the gas fracturing method (nitrogen, CO2). For two kinds of media (nitrogen, CO2) in gas fracturing, in addition to the parameter difference of gas itself, the adsorption of CO2 is also the main factor affecting the fracturing pressure. The increase of coal permeability and Biot's coefficient will reduce the fracture initiation pressure, and the increase of confining pressure will increase the fracture initiation pressure.

Numerical analysis on fracture initiation from radial micro‐hole in anisotropy formation

AbstractThe radial jet drilling (RJD) technology is typically developed to open multiple lateral micro‐holes from a main wellbore to the formation. The multiple groups of 3D numerical models of radial micro‐holes through a main well are built by incorporating the pore hydro‐mechanical coupling effects. A number of sensitivity analyses were conducted on the effects of the radial jetting azimuth, the stress anisotropy, and the elastic anisotropy on the rock fracture initiation of the micro‐holes. As revealed from the results, the fracture initiation from the micro‐hole exhibited the diversified characteristics with the increase in the jetting azimuth of the micro‐hole. On the whole, the fracture initiation point was concentrated at the maximum principal stress. Thus, the jetting azimuth of the nozzle was recommended to be designed in the interval of 0°‐30°. A higher elastic modulus anisotropy ratio K and a lower Poisson's ratio anisotropy ratio K′ caused the significant rigidity characteristics of the rock, which increased the possibility of fracture initiation in the horizontal direction; a lower elastic modulus anisotropy ratio K and a higher Poisson's ratio anisotropy ratio K′ caused the strong rigidity characteristics, so the rock could not easily fracture in the vertical direction. Several suggestions were given that the rock is easy to be broken by selecting a smaller jetting azimuth angle and a more significant in‐situ stress difference. The numerical simulation results agree well with field experiment results, with an average accuracy of 97.17%. The proposed numerical model has a good performance in predicting the fracture initiation pressure of the radial micro‐hole in anisotropy formation.

Initiation mechanisms of radial drilling‐fracturing considering shale hydration and reservoir dip

AbstractRadial drilling‐fracturing is an innovative fracturing technology that achieves superior stimulation effects. This paper develops a model for radial drilling‐fracturing in shale reservoirs, which can predict fracture initiation pressure and failure mode of shale. Compared with published models, this model additionally considers shale hydration and inclinations of boreholes and reservoir. Then, the influences of 7 factors are studied and main conclusions are as follows. First, with the angle between radial borehole and formation strike increasing, matrix failure pressure declines only around 90° and 270°. Bedding tensile failure pressure ascends and then falls back. Bedding shear failure pressure rapidly descends, then reaches a plateau, and finally rebounds. A small or large angle is favorable for bedding tensile failure. Shale tends to crack with bedding shear failure under a moderate angle and with matrix failure only at 90° and 270°. Second, with the enlargement of azimuth difference between the radial borehole and main wellbore, matrix failure pressure periodically fluctuates, forming an inverted W‐shape curve. Bedding tensile failure pressure and bedding shear failure pressure both ascend, then slide downward, and finally rise again. Besides, a small azimuth difference inclines shale to bedding shear failure, and the other failure modes tend to occur when azimuth difference is around 90°. Third, pressures required for all failure modes descend as the dip of shale formation increases. However, the difference in descending rate leads to that the increase of dip angle can incline shale to bedding shear failure. Besides, pressures required for bedding tensile failure and matrix failure decline linearly with the increasing reservoir temperature. Temperature has no impact on bedding shear failure. Hence, matrix failure and bedding tensile failure are more prone to occur in the high‐temperature reservoir. The research provides a reference for field applications of radial drilling‐fracturing in the shale reservoir.

An Analytical Model for Fracture Initiation of Radial Drilling-Fracturing in Shale Formations

Abstract Radial drilling-fracturing, the combination of radial drilling and hydraulic fracturing, can guide fractures toward the target area and effectively enhance the recovery of the low permeable reservoir. In this paper, based on the stress superposition principle, we establish an analytical model to predict fracture initiation pressure (FIP) and the shale failure mode for radial drilling-fracturing applied in shale formations. In contrast with the former studies, this model can additionally consider the failure from shale beddings and is more applicable in the shale reservoir. The model classifies the shale failure into three modes and, respectively, gives the criterion for each failure mode. Then, a series of sensitivity analyses is conducted by examining effects of various parameters. By analyzing the variation characteristic of the initiation pressures required for three failure modes, the main conclusions are as follows. Firstly, matrix failure and shear failure along bedding tend to take place when the azimuth of radial borehole is moderate. Small and large azimuths are favorable for the occurrence of tensile failure along bedding. Secondly, a high ratio of horizontal in situ stress predisposes shale to generate matrix failure, and bedding tensile failure and bedding shear failure are apt to occur when the ratio of horizontal in situ stress is low. Thirdly, with the increasing intersection angle of the radial borehole wall and bedding plane, the failure mode apt to occur changes from bedding tensile failure to bedding shear failure and then to matrix failure. Fourthly, shale prefers to yield bedding shear failure under a small Biot coefficient and generate the other two failure modes when Biot coefficient is large. Fifthly, permeability coefficient virtually has no influence on the failure mode of shale. The research clarifies the fracture initiation characteristics of radial drilling-fracturing in shale formations and provides a reference for the field application of radial drilling-fracturing.

A holistic review on hydraulic fracturing stimulation laboratory experiments and their transition to enhanced geothermal system field research and operations

This paper presents a thorough overview of hydraulic stimulation techniques, conducted in laboratories. It further analyses field experiments and ongoing projects for geothermal energy production to investigate if the findings from the lab can be practically applied to the field. Stimulation techniques have been long used in the oil and gas industry as a means to increasing the rock permeability and consequently the reservoir’s fluid production rate. Among the different stimulation methods, hydraulic fracturing is known to be the most successful in creating new passageways in the formation. Nevertheless, the benefits of fracturing have been hindered by the handful of events in which poor planning had led to severe seismic activities. Therefore, across the globe, many efforts were dedicated to characterizing fracture creation and propagation in different rocks, not only to provide know-how for further and safer developments in the oil and gas front but also to adapt such findings to the ever-emerging field of geothermal energy recovery. In the course of this work, over 100 papers were studied. The papers included laboratory experiments on various rock types encountered in reservoirs, where parameters such as stress regime, fracture initiation pressure, formation breakdown pressure, volume, and types of fluid injected were monitored. To investigate whether or not such practices had been previously applied in geothermal energy production, a thorough study was also conducted on large-scale experimental setups constructed in the field as well as hydraulic fracturing procedures performed in operational projects, going back as far as a decade. The results show an agreement between laboratory experiments and field operations, yet naturally including individual results from cases where either the lab parameters or field characteristics were extraordinarily unique. Multiple cross-correlations were also performed between different key parameters that play a role in a fracturing process, providing trends that could be intra- or extrapolated for further research and planning. The novelty of this work is the comprehensive analysis of numerous research projects done around the world. As a result, this paper will not only be an informative and yet compacted source of information concerning previous projects, but it also points out the main factors and their relationships which need to be understood to guide a future project to success.

Fracture Initiation Mechanisms of Multibranched Radial-Drilling Fracturing

Abstract Compared with conventional hydraulic fracturing, radial-drilling fracturing presents remarkable advantages and can effectively develop low-permeability reservoirs. The radial borehole can reduce formation fracture pressure and guide the fracture initiation and propagation. Due to the large radial borehole azimuth or the strong anisotropy of the reservoir, the single radial borehole may not efficiently guide the fracture propagation. The researchers proposed multibranched radial-drilling fracturing. However, the research on fracture initiation of multibranched radial-drilling fracturing is inadequate. Radial boreholes usually need certain dip angles to avoid penetrating the interlayer, but the effect of dip angle on the stress field has never been considered before. In this paper, an analytical model for predicting stress distribution around the main wellbore with multiradial boreholes of arbitrary dip angle, azimuth angle, and phase angle is established for the first time, taking full account of the influences of in situ stress, internal pressure, and fracture fluid infiltration on the stress field. The model is utilized to calculate the fracture initiation pressure (FIP) and point out the specific fracture initiation location (FIL). The influences of azimuth angle, dip angle, phase angle, depth difference, and the stress profile radius on fracture initiation pressure, fracture initiation location, and maximum tensile stress distribution are investigated, and a series of sensitivity analyses are carried out. The results show that the areas between the radial boreholes and closer to the walls of radial boreholes are more prone to tensile failure, which provides a theoretical basis for radial boreholes guiding fracture initiation. The reduction of phase angle and depth difference enhances the interference between radial wells, which is conductive to the formation of hydraulic fracture networks between them. As the dip angle increases, the stress becomes increasingly concentrated, and the preferential rock tensile failure becomes increasingly easy. The farther the stress profile is from the main wellbore axis, the smaller it will be influenced by the main wellbore. When the distance exceeds 2R, the maximum tensile stress distribution on the profile remains constant. The research enriches the fracture initiation mechanism of multibranched radial-drilling fracturing and provides guidance for optimizing radial borehole layout parameters of hydraulic fracturing directed by multiradial boreholes.

Reservoir Wellbore Stability Analysis and Weak Zones Identification Using the 1D MEM, Swelling Tests and UCS: A Case Study From Mumbai Offshore, India

Reservoir geomechanical studies play a decisive role in identifying wellbore instability and weak zones in offshore petroleum reservoirs. In this study, an integrated workflow developed with 1D Mechanical Earth Models (1D MEM), uniaxial compressive strength and swelling tests using well logs, drilling, and core data of production and exploratory wells from the North-Heera field, Mumbai offshore, India. Lithology-based correlations are utilized for calculating continuous profiles of rock mechanical properties by taking well logs as a primary source of input. Insights of fracture gradients and stresses are generated by interpretation of the pore pressure. UCS created from the well logs shows the low value of 15 MPa and high Poison's ratio of 0.38–0.45. Fracture pressure analysis of the wells specifies the lower fracture initiation pressures at 300 m are 5ppg–15ppg, which are shale and clay formation. Shale swelling tests are performed to know the hydration properties of shales present at shallow depths. The core samples' loading tests' results are very close to the UCS models constructed from the well logs. The combined analysis examines fracture pressures around wellbores, hydration of shales, weak zones identification, and wellbore instability.

Effect of the Triaxial State of Stress in the Hydraulic Fracturing Processes of Granite: Part 1—Visual Observations and Interpretation

A set of hydraulic fracturing tests were conducted in prismatic granite specimens to investigate the influence of the triaxial state of stress on the cracking processes. The tests considered three states of stress, in which the horizontal and out-of-plane loads were 2 MPa, while the vertical loads were either 1 MPa, 2 MPa, or 4 MPa. A hydraulic pressure inside pre-fabricated flaws was increased at a constant injection rate until hydraulic fractures initiated and propagated. In order to materialize this state of stress, a test setup was developed to independently apply and control three orthogonal stresses in the prismatic granite specimens while simultaneously applying a hydraulic pressure inside pre-fabricated flaws. More importantly, the setup also allowed one to observe the damage processes visually and through acoustic emission monitoring. This paper (the first of two) looks into the visual (macroscopic) observations, whereas the companion paper focuses on the acoustic emission monitoring. In particular, the present paper looks into the white patching (micro-damage) development, crack initiation, propagation and coalescence, and relates these with fracture initiation and breakdown pressures. From the tests, it was observed that the initiation of visible cracks is, generally, preceded by white patching and occurs at or near the pre-fabricated flaws. The visual analysis showed that the induced fractures were primarily intragranular when the triaxial stresses were same in the three orthogonal directions and intergranular otherwise. Additionally, micro-fracture initiation pressures were obtained based on white patch initiation pressures (visually) and using a graphical method which relates the changes in pressurization rate to fracture initiation. It was observed that the graphical method identifies when the initial damage occurred significantly earlier than the visual method, which indicates that important micro-damage processes may occur without macroscopic evidence. Most importantly, the coalescence patterns and the ratio between the breakdown pressure and the vertical stress are stress-state dependent. In fact, for lower vertical loads (VL = 1 MPa), the ratio between the breakdown pressure and vertical load is approximately 10 and for higher vertical loads (VL = 4 MPa) approaches 1.0, which is in agreement with data observed in the field.

An Analytical Model for Fracture Initiation from a Particular Radial Borehole in Hydraulic Fracturing Guided by Multiradial Boreholes

Radial drilling-fracturing, the combination of the hydraulic fracturing and radial borehole, is a technology that can guide the hydraulic fractures to directionally propagate and efficiently develop low permeability reservoir. In this paper, an analytical model of two radial boreholes (a basic research unit) is established to predict fracture initiation pressure (FIP) from one particular radial borehole and the interference between radial boreholes when the hydraulic fracturing is guided by multi-radial boreholes. The model is based on the stress superposition principle and the maximum tensile stress criterion. The effects of in situ stress, wellbore pressure, and fracturing fluid percolation are considered. Then, sensitivity analysis is performed by examining the impact of the intersection angle between radial boreholes, the depth difference between radial boreholes, the radius of radial boreholes, Biot coefficient, and the number of radial boreholes. The results show that FIP declines with the increase of radial boreholes number and the decrease of intersection angle and depth difference between radial boreholes. Meanwhile, the increase of radial borehole number and the reduction of intersection angle and depth difference strengthen the interference between radial boreholes, which conduce to the formation of the fracture network connecting radial boreholes. Besides, FIP declines with the increase of radial borehole radius and the decrease of Biot coefficient. Large radius and low Biot coefficient can enlarge the influence range of additional stress field produced by radial boreholes, enhance the mutual interference between radial boreholes, and guide fracture growth between radial boreholes. In hydraulic fracturing design, in order to reduce FIP and strengthen the interference between radial boreholes, the optimization design can be carried out by lowering intersection angle, increasing radius and number of boreholes, and reducing the depth difference between boreholes when the conditions permit. The research clarifies the interference between radial boreholes and provides the theoretical basis for optimizing radial boreholes layout in hydraulic fracturing guided by multi-radial boreholes.

Shear mechanism of fracture initiation from a horizontal well in layered shale

Prediction of fracture initiation pressure (FIP) is an essential issue for safety drilling, fracturing evaluation and efficient production in shale gas exploitation. To quickly and accurately evaluate the FIP of a horizontal borehole drilled in anisotropic rocks, an analytical solution for borehole stress was first deduced based on the complex potential method and superposition principle. In combination with anisotropic tensile and shear fracturing criteria for the bedding planes and shale matrix, the FIP for a particular case was then evaluated using the analytical solution, and the initiation mechanism was also revealed. Finally, parametric studies were conducted to explore the main factors influencing the initiation pressure and fracture mode. The results showed that the elastic anisotropy significantly alters the borehole stress and thus influences the fracture pressure and initiation orientation. The fluid pressure needed for fracture initiating in shear mode is much lower than that in tensile mode. This is mainly resulted from the weak shear strength of the bedding planes. Fracture will probably initiate from the bedding plane in shear mode under certain in situ stress conditions, regardless of the bedding inclination. A lower in situ stress difference appears to induce fracture initiating in shear mode; a larger one, however, is more likely to produce tensile failure. By incorporating the elastic anisotropy, fracture all initiates in shear mode with the increase in elastic anisotropy. However, when the elastic anisotropy degree is larger than 2, fracture will be produced from the rock matrix due to shear failure. The weaker the bedding's shear strength, the easier the fracture will initiate from the bedding planes due to shear failure. The analytical solution developed in this study is extremely useful and convenient to assess the influence of mechanical anisotropy on borehole stress, borehole collapse, wellbore stability and fracture generation of horizontal wells in anisotropic rocks.

A comparative study of oriented perforating and fracture initiation in seven shale gas plays

Oriented perforating strategies have been proposed as a method of suppressing fracture initiation pressures and optimizing the orientation of the initiated fractures, including a semi-empirical approach for predicting the fracture initiation pressure for given in-situ stress conditions and the orientation of fracture initiation (longitudinal or transverse-to-the-wellbore). In this study, a comparative analysis is made on the fracture initiation in stress conditions found in seven prolific shale gas plays, Barnett, Bakken, Fayetteville, Haynesville, Niobrara, Marcellus and Vaca Muerta, for cases of horizontal wells drilled parallel to the least horizontal principal stress. The developed semi-empirical approach is employed to determine the optimal perforation direction for each play, targeting transverse fracture initiation and fracture initiation pressure minimization. This involves a numerical model used to derive correction factors for closed-form analytical approximations from the literature, which are used to evaluate the stresses acting on the perforation base. While the leak-off and breakdown pressures can easily be pointed out from pressure monitoring, the fracture initiation pressures are not easily distinguishable. Reliable fracture initiation pressure estimations are of importance to stimulation engineers planning a treatment, as it controls the number and capacity of the injection pumps needed as well as being a major input for computational modeling. For normal faulting stress states, transverse fracture initiation is best promoted by perforations on the side of the hole, while for strike-slip faulting stress states, by perforations on the top of the hole. When only longitudinal fracture initiation is possible, the fracture initiation pressure is minimized from perforations on top of the borehole in normal faulting stress states and from the side of the borehole for strike-slip faulting stress states. Fracturing fluid leakage around the wellbore makes transverse fracture initiation harder and can also impact the fracture initiation pressures.

A Semianalytical Modeling Approach for Hydraulic Fracture Initiation and Orientation from Perforated Wells

Summary Accurate prediction of fracture initiation pressure and orientation is paramount to the design of a hydraulic fracture stimulation treatment and is a major factor in the treatment's eventual success. In this study, closed-form analytical approximations of the fracturing stresses are used to develop orientation criteria for relative-to-the-wellbore (longitudinal or transverse) fracture initiation from perforated wells. These criteria were assessed numerically and found to overestimate the occurrence of transverse fracture initiation, which only takes place under a narrow range of conditions in which the tensile strength of the rock formation is lower than a critical value, and the breakdown pressure falls within a “window.” For a case study performed on the Barnett Shale, transverse fracture initiation is shown to take place for breakdown pressures below 4,762 psi, provided that the formation's tensile strength is below 2,482 psi. A robust 3D finite volume numerical model is used to evaluate solutions for the longitudinal and transverse fracturing stresses for a variable wellbore pressure, hence developing correction factors for the existing closed-form approximations. Geomechanical inputs from the Barnett Shale are considered for a horizontal well aligned parallel to the direction of the least compressive horizontal principal stress. The corrected numerically derived expressions can predict initiation pressures for a specific orientation of fracture initiation. Similarly, at known breakdown pressures, the corrected expressions are used to predict the orientation of fracture initiation. Besides wellbore trajectory, the results depend on the perforation direction. For the Barnett Shale case study, which is under a normal faulting stress regime, the perforations on the side of the borehole yield a wider breakdown pressure window by 71% and higher critical tensile strength by 32.5%, compared to perforations on top of the borehole, implying better promotion of transverse fracture initiation. Leakage of fracturing fluid around the wellbore, between the cemented casing and the surrounding rock, reduces the breakdown pressure window by 11% and the critical tensile strength by 65%. Dimensionless plots are employed to present the range of in-situ stress states in which longitudinal or transverse hydraulic fracture initiation is promoted. This is useful for completion engineers; when targeting low permeability formations such as shale reservoirs, multiple transverse fractures must be induced from the horizontal wells, as opposed to longitudinal fracture initiation, which is desired in higher permeability reservoirs or “frac-and-pack” operations.

Research on Fracture Initiation Pressure in Deviated Well of WCH9 Block

Highly-deviated well are applied for effectively developing WCH9 gas field with deep buried and low permeability, and the payzone are candidates for hydraulic fracturing by evaluation of gas reservoir. Therefore, fracture initiation pressure is a key parameter for design of hydraulic fracturing treatment. Firstly, a series of experiments were completed by core sample from target formation, Young's modulus is about 13GPa and Poisson's ratio is 0.286 at a confining pressure of 40MPa, horizontal principal stress is 78 MPa and 63 MPa, and in-situ stress profiles were interpreted by logging data. Then, the formation rock is regarded as isotropic linear elastic material, thus total stresses distribution on the deviated wellbore wall was determined by stress superposition principle, in which in-situ stress redistribution around the deviated wellbore, fluid pressure acted on the borehole wall and filtration stress are taken into account when fracturing fluid was injected into wellbore. Further, prediction model of fracture initiation pressure was established by applying criterion of maximum tensile stress and effective stress transformation. Lastly, according to the borehole trajectory, in-situ stress and other parameters from payzone of WCH9 block, fracturing initiation pressure varied with <i>Deviation angle and azimuth angle</i> were computed by numerical simulation method, these results provide a basis for optimization design of hydraulic fracturing technology parameters.

Fracture toughness interpretation from breakdown pressure

One of the tests used to assess the possible dependence of the fracture toughness on stress relies on injecting fluid into a slotted borehole within a cylindrical rock sample subject to confining pressure and interpreting the toughness from the observed breakdown pressure. There is a wealth of experimental evidence that the so-measured toughness indeed increases with the confining stress. However, the interpretation of the observed breakdown pressure relies critically on assuming that the fluid pressure in the slots and the cracks ahead is uniform and on identifying the peak (breakdown) pressure with the fracture initiation pressure. The model described in this paper challenges these assumptions by considering the existence of a fluid lag at the tip of the hydraulic fracture and by incorporating the hydraulic compliance of the injection system. Numerical simulations indicate that there is an episode of stable crack growth, which is reflected by a continuous increase of the injection pressure after fracture initiation until the fracture becomes unstable. This model suggests that the actual toughness is generally overestimated, when it is interpreted from the breakdown pressure under the assumption of a uniform fluid pressure in the crack. Furthermore, this interpretation results in an artificial dependence of the toughness on the confining pressure. The overestimation depends essentially on two numbers, a scaled toughness and a scaled confining stress.

The alterations of critical pore water pressure and micro-cracking morphology with near-wellbore fractures in hydraulic fracturing of shale reservoirs

The crack initiation and propagation in a laminated shale formation have been widely investigated, but the alterations of critical pore water pressure (including fracture initiation pressure and breakdown pressure) and the micro-cracking morphology with near-wellbore fracture geometry are still unclear. This paper numerically investigates the alterations of critical pore water pressure and the morphology of the hydraulic fractures in shale formation with different near-wellbore fractures. First, a hydro-fracturing model is developed for a laminated shale formation within the framework of PFC2D. Then, this model is validated with the Blanton’s criterion, pressure history and fracture geometry. Finally, the effects of length, shape, and symmetry properties of near-wellbore fractures on the critical pore water pressure and micro-cracking morphology are comparatively investigated. The critical pore water pressure, micro-cracking morphology, aperture and permeability, and the fractal dimension of hydraulic fractures are quantitatively analyzed at three initial near-wellbore fractures. Simulation results indicate that the length and the distribution of near-wellbore fracture branches play a crucial role in pore water pressure variation and micro-crack propagation during hydraulic fracturing. The critical pore water pressure is sensitive to the initial near-wellbore fracture length especially when the initial fracture length exceeds a critical fracture length. The shape of initial near-wellbore fracture could change the distribution of micro cracks and induce large pore water pressure. The symmetrical radial fracture (θ=0°) more likely causes a hydraulic fracture with high permeability while the X-shape fracture (θ=30°) contributes greatly to the complexity of the fracture network and stimulated reservoir volume. These findings on the pore water pressure variation and the micro-cracking morphology are helpful in the optimization design of hydraulic fracturing treatment.

Prediction of fracture initiation pressure in multiple failure hydraulic fracturing modes: Three-dimensional stress model considering borehole deformation

Hydraulic fracturing technology has been widely used as the main method of exploiting coalbed methane (CBM). The core of this technology is the accurate prediction of the fracture initiation pressure. As the exploitation depth increases, the influence of the in-situ stress obviously increases. Because the existing model of calculating the fracture initiation pressure ignores the influence of in-situ stress on the extraction borehole (CBM well) deformation, and the fracture initiation failure mode is single, the fracture initiation pressure of hydraulic fracturing cannot be accurately predicted in the field. In this study, a new model for predicting the fracture initiation pressure in hydraulic fracturing under a three-dimensional stress state was established. For the first time, the deformation of a circular borehole is related to the in-situ stress, and the size of the elliptical borehole can be accurately obtained after deformation. Eleven fracture initiation failure modes for the hydraulic fracturing of coal seams are proposed. The fracture initiation pressure of each failure mode was calculated in MATLAB, and the fracture initiation pressure of the shear failure of the ρz plane (a) was the smallest. The effects of the horizontal maximum principal stress, horizontal minimum principal stress, vertical intermediate principal stress, coal seam dip angle, horizontal azimuth of borehole, and gas pressure on the fracturing pressure were investigated using the proposed model. The coal seam dip angle exerted the greatest influence, and a new hydraulic fracturing construction procedure is proposed: the pump pressure should be gradually increased from the minimum initiation pressure according to the fracture initiation pressure gradient required by each failure mode. A field hydraulic fracturing test was conducted, and the experimental results are in good agreement with the theoretical calculation results. The maximum error was 5.5%, which validates the proposed theoretical model. The accuracy of the proposed theoretical model in the calculation of the fracture initiation pressure increased by 364.4% compared with the classical hydraulic fracturing theory. The results obtained by this study can provide more precise guidance to hydraulic fracturing projects.

Application of particle swarm optimization and simple intelligent algorithms in building nonlinear models for predicting formation fracture initiation pressure

ABSTRACT The main objective of this study is to estimate the FIP on Sarvak and Ilam formations using neural networks and intelligent algorithms. For this purpose, we first calculate the geomechanical parameters and the formation FIP of three wells, using well log data and experimental relationships. Secondly, we employ neural networks to estimate the FIP in three wells which randomly selected. Thirdly, we obtain a nonlinear mathematical model to calculate the formation FIP using simple genetic algorithms and particle swarm optimization (PSO). Based on our experiments, the PSO performs better than the genetic algorithm. Using the two new nonlinear models we estimated the pore pressure to be between 34.47 and 48.26 MPa, vertical stress between 75.84 and 131 MPa. Maximum and minimum horizontal stress between 62.05 and 87.7 MPa, FIP between 87.7 and 110.3 MPa.