Laboratory Hydraulic Fracturing Experiments Research Articles

Microseismic Monitoring of Laboratory Hydraulic Fracturing Experiments in Granitic Rocks for Different Fracture Propagation Regimes

Microseismic Monitoring of Laboratory Hydraulic Fracturing Experiments in Granitic Rocks for Different Fracture Propagation Regimes

Laboratory investigation of hydraulic fracturing in granitic rocks using active and passive seismic monitoring

SUMMARY Knowledge of the fracturing processes can be important for the optimization of pressurized fluid injection operations in the deep underground rock mass. Active and passive seismic monitoring techniques have been used in the field for tracking or mapping the propagating hydraulic fracture. Although both these monitoring techniques provide valuable information about the generated fracture network, it is difficult for either technique to comprehensibly identify the different processes associated with hydraulic fracturing. The combined active and passive monitoring has the potential for better characterization of the complex hydraulic fracturing phenomena. In this study, laboratory hydraulic fracturing experiments with combined active and passive seismic monitoring were conducted on true triaxially loaded Barre granite cubes with different fluid injection rates. The seismic inelastic fracturing was detected by 16 passive acoustic emission sensors, where 3678 and 2370 seismic source events were detected for the high and low injection rate experiments, respectively. For active monitoring, strong variations in the attributes of signals were observed which were transmitted through four source–receiver pairs, placed both perpendicular and parallel to the generated hydraulic fracture. Positive velocity changes were observed for active sensor pairs with ray paths passing through the generated hydraulic fracture indicating fluid permeation, whereas isolated dry deformation was characterized by a slight but permanent velocity decrease. Compared to velocity, the energy of the active signals was 1–2 orders of magnitude more sensitive to different hydraulic fracturing processes. However, the sensitivity and signatures of the active signal attributes were found to be dependent on the frequency range and direction of ray path with respect to the location of the generated fracture network. Using the coupled evaluation of the active and passive signals we were able to systematically identify various hydraulic fracturing processes including: (1) aseismic deformation, (2) fracture initiation and fluid permeation, (3) pressure build-up, (4) fracture propagation and (5) pressure release and leak-off. The results of this study showed that combining the respective advantages of active and passive seismic techniques and using both of them to monitor the failure processes can facilitate a more comprehensive understanding and better control of the hydraulic stimulations in subsurface operations.

Laboratory hydraulic fracturing in layered tight sandstones using acoustic emission monitoring

Understanding the propagating mechanism of complex fracture network is essential for optimizing the hydraulic fracturing design scheme. Laboratory hydraulic fracturing experiments were performed on three tight sandstone specimens with bedding plane perpendicular, oblique and parallel to the axial direction, respectively. Before hydraulic fracturing, three specimens were first loaded close to the critical stress state under 22.5 MPa confining pressure. The entire hydraulic fracturing process was monitored by acoustic emission (AE) and ultrasonic measurement. The experimental results show that the hydraulic fracturing behaviors were significantly influenced by bedding structure and water infiltration. The bedding plane plays an important role on breakdown pressure, distribution characteristics of AE activities and hydraulic fracture networks. The bedding-oblique specimen exhibits lower breakdown pressure than those of the bedding-parallel and bedding-perpendicular specimens. AE activities in the bedding-oblique specimen is characterized by lower b-value, while higher b-value is found in other two specimens. X-ray computed tomography images of the broken specimens show an excellent agreement between AE hypocenters and macroscopic fractures, indicating geometrical morphology of macroscopic fractures is closely related to the bedding plane and well imaged by AE hypocenters. A bedding-parallel shear fault formed in the bedding-oblique specimen. Complex shear fault linking several bedding-parallel echelon arrays or branches formed in the bedding-parallel and bedding-perpendicular specimens. Water infiltration affects the hydraulic fracturing behaviors by inducing AE events initiate at transition region of wet-dry boundary around the central borehole. In view of the formation of fracture network and the risk of induced seismicity, the influence of bedding structure and water infiltration should be taken into consideration under given in-situ stress regime during the hydraulic fracturing design scheme.

Laboratory investigation and evaluation of the hydraulic fracturing of marine shale considering multiple geological and engineering factors

Hydraulic fracturing is widely implemented in the exploration of marine shale gas. Affected by various geological and engineering factors, gas production after stimulation is not always satisfactory. To reveal the influential effect of multiple factors, laboratory hydraulic fracturing experiments are performed on Longmaxi marine shales by considering key parameters (deviatoric stress, confining pressure, pumping rate, fracturing fluid type, and bedding angle). The variation of breakdown pressures and the characteristics of hydraulic fractures are recorded and analyzed. The results show that the breakdown pressure increases with increasing deviatoric stress, confining pressure, pumping rate, and viscosity of the fracturing fluid. As the bedding angle varies from 0° to 90°, the breakdown pressure declines first and increases again. Furthermore, parameter sensitivity analysis indicates that geological factors (confining pressure, bedding angle, and deviatoric stress) would largely determine the breakdown pressure, while engineering factors (pumping rate, fracturing fluid type) could only affect it to a lesser extent. Computed tomography measurements show that natural fractures, originating from tectonic shear failure, could possess greater width than tension-dominated hydraulic and bedding fractures. Statistical analysis shows that the length of the hydraulic fractures alone is only approximately 150 mm. However, the fully activated natural and/or bedding fractures could help substantially increase the total fracture length to 600 mm. Low deviatoric stress, low confining pressure, low viscous slick-water, and high bedding angle are conducive to activating natural and bedding fractures and forming a complex fracture network. The aforementioned findings are valuable for the optimal design of field hydraulic fracturing.

Investigation into Hydraulic Fracture Propagation Behavior during Temporary Plugging and Diverting Fracturing in Coal Seam

Temporary plugging and diverting fracturing (TPDF) is widely used to improve the stimulation effectiveness in coal seam. To study the fracture propagation behavior during TPDF in coal formation, a series of laboratory hydraulic fracturing experiments were performed on natural coal samples. Based on the results of sample splitting and fracture reconstruction, the influences of horizontal stress difference and the size of temporary plugging agent (TPA) as well as the concentration of TPA on hydraulic fracture growth were analyzed. Experimental results show that TPDF is beneficial for improving the fracture complexity even under high stress difference of 8 MPa. When the TPA of small particle size (70/100 mesh) was applied, the primary fracture could not be fully blocked whereas increasing the particle size of TPA to 20/40 mesh tended to cause accumulation and bridging in the wellbore, resulting in an abnormally high fracturing pressure. TPA with particle size of 40/70 mesh tended to be a reasonable choice for the target formation, as it could form effective plugging in primary fractures and promote the generation of new fractures. Meanwhile, optimizing the concentration of TPA was also conducive to improving the plugging effectiveness. Effective temporary plugging can be achieved by using appropriate TPA of proper size and concentration, which varies with different treatment parameters and formations. Laboratory experiments are expected to provide guidance for the parameter optimization for TPDF in coal seam.

Pumping Rate-Dependent Temperature Difference Effect on Hydraulic Fracturing of the Breakdown Pressure in Hot Dry Rock Geothermal Formations

The development of hot dry rock geothermal formations requires hydraulic fracturing to create a thermal extraction area between injection and production wells. A high breakdown pressure increases the operation cost during hydraulic fracturing and possibly causes a large risk of induced seismic activity under certain conditions. This paper conducted laboratory hydraulic fracturing experiments under true triaxial stress and reservoir temperature conditions to investigate the temperature difference effect on the breakdown pressure. An analytical model that considered the temperature difference effect was developed to estimate the breakdown pressure. In hot dry rock geothermal formations, the temperature difference between the injection fluid and the reservoir rock can induce thermal shock which can break rocks; thus, the breakdown pressure can be decreased. In addition, thermal shock is influenced by the pumping rate. This influence is also considered in the analytical model.

Effect of thermal shock on laboratory hydraulic fracturing in Laizhou granite: An experimental study

This paper aims to investigate the effect of thermal shock on hydraulic fracturing in hot dry rock (HDR). To simulate the thermal shock effect caused during the injection of cold water, several natural granite specimens were slowly heated to various target temperatures and then rapidly cooled down by flowing water. Laboratory hydraulic fracturing experiments were performed on granite specimens under triaxial stress condition and room temperature. After that, computed tomography (CT) scanning technique, scanning electron microscope (SEM) observation, and surface profile scanning (SPS) tests were combined to analyze the influence of thermal shock on fracture morphology from micro-scale to macro-scale. Moreover, the characteristics of injection pressure curve were discussed. Experimental results show that a single main hydraulic fracture with large aperture is created and approximately propagates along the direction perpendicular to the minimum principal stress before 300 °C. When the thermal treatment temperature is higher than 300 °C, multiple hydraulic fractures with branches tend to be created due to the existence of thermally-induced damage zone near the well hole. Experimental results of SPS tests indicate that the maximum height difference of the hydraulic fracture surface increases with increasing thermal treatment temperature. The tortuosity of hydraulic fracture significantly increases when the thermal treatment temperature is higher than 300 °C. Additionally, the analysis of injection pressure response shows that apparent non-linear pressurization, and decrease in pressurization rate and breakdown pressure are observed after 300 °C.

Cyclic Injection to Enhance Hydraulic Fracturing Efficiency: Insights from Laboratory Experiments

In hydraulic fracturing applications, there is substantial interest to reduce the formation breakdown pressure. Previous research results show that the cyclic injection method can be used to reduce that pressure. In this study, we conducted laboratory hydraulic fracturing experiments to apply cyclic injection to reduce the breakdown pressures of very tight and strong sandstones. Experimental results show that using cyclic injection the average breakdown pressure was reduced by 18.9% in very tight sandstones and by 7.18% in normal sandstones. This indicates that the effect of cyclic injection is more significant for stronger and tighter rocks. The experiments also reveal that the rock tensile strength plays a more important role in the formation breakdown pressure with a rock strength factor of 2.85. This suggests that the breakdown pressure is higher than expected. In addition, we empirically related the breakdown pressure reduction and the injection pressure amplitude to the number of injection cycles. The curve fitting results imply that the effect of cyclic injection is more important if the number of cycles or the injection pressure amplitude is increased. Based on the results of this research, the in-situ formation breakdown pressure can be reduced by applying the cyclic injection method, and the breakdown pressure reduction is more significant as the number of cycles increases.

Experimental Study of Fracture Propagation: The Application in Energy Mining

Hydraulic fracturing has been widely used in recent years as a key technology to improve energy mining efficiency in petroleum and geothermal industries. Laboratory hydraulic fracturing experiments recently were completed in six large-scale 300 × 300 × 300 mm rock specimens to better understand this complex process of hydraulic fracturing. When injection flow rate increases from 5 to 30 mL/min. The fracture initiation pressures and breakdown pressures increase, the propagation times and post-fracturing pressures decrease. The fracture geometries are observed and analyzed, mean injection power is proposed and results show that it could be used to roughly estimate the fracture total lengths. Moreover, the fracture permeabilities based on the pressure data are calculated and linearly ascend with the increase of injection flow rates. These results can provide some reasonable advice for implementing hydraulic fracturing reservoir simulations and improving energy production efficiency on application to field-scale operation.

Laboratory hydraulic fracturing of granite: Acoustic emission observations and interpretation

Hydraulic fracturing is routinely used, but the fracturing processes that occur when rocks are hydraulically-fractured are not entirely understood and require further investigation. This study presents the acquisition, analysis and interpretation of acoustic emissions data from a series of laboratory hydraulic fracturing experiments on granite. Specimens with different orientations of two pre-cut flaws were tested under both 0 MPa and 5 MPa of vertical uniaxial stress to understand the effect of the external stress conditions. Acoustic emissions data are related to corresponding visual observations made using high-resolution and high-speed imaging. We find that in general, (1) the AE begin to occur at approximately 80% of peak pressure, (2) the focal mechanisms suggest that 55–60% of the radiation pattern could be explained by a double couple mechanism and is well-explained by simulation results, (3) hypocenter locations tended to agree well with visually observed white patching (process zone) and crack patterns, (4) spatio-temporal analysis revealed points in time at which microcrack coalescence were detected by AE, (5) that the AE could be used to make a non-unique prediction of crack initiation, and lastly (6) that exponential AE energy accumulation in the last 5–10 s prior to crack initiation can be modelled by time-to-failure methods.

Experimental investigation of in situ and injection fluid effect on hydraulic fracture mechanism using acoustic emission in Tennessee sandstone

Hydraulic fracture propagation and associated rock failure mechanism is dependent on multiple factors including the reservoir pore fluid and its interaction with the rock and the fracturing fluid. This dependence of rock failure on in situ pore fluid could affect the stimulated volume and could manifest itself in the hydraulic fracturing diagnostics such as treatment pressure and microseismic. Therefore, it would be beneficial to an engineer designing, planning or interpreting a fracturing treatment to be aware of expected differences in hydraulic fracture responses in formations having different reservoir fluids, i.e. fracturing a gas or oil bearing formation.We investigate this effect by way of laboratory hydraulic fracturing experiments on triaxially stressed samples of Tennessee sandstone. The acoustic response during rock failure was monitored using 16 P-wave transducers and the waveforms were acquired using a continuous acoustic monitoring system. The effects of pore pressure, water weakening and viscosity of the injection fluids on hydraulic fracture propagation and associated damage are investigated using six experiments on dry, brine saturated and dodecane saturated Tennessee sandstone, fractured with either water (1 cP) or oil (50 cP). Significant rock weakening was observed in fluid saturated samples (breakdown pressures exhibited a reduction of 15–40% from dry state breakdown pressures); the presence of more polar fluid (water) in the pore space resulted in more weakening (800–900 psi lower breakdown pressures when water saturated vs. dodecane saturated). The effect of injection fluid viscosity is observed to be suppressed for fluid saturated samples as breakdown pressures for samples which were saturated with either fluid (brine and dodecane) were observed to be almost same. Acoustic event distributions, continuous stream monitoring, failure mechanisms and permeability measurements on fractured cores all seem to suggest a more complex fracture network in dry rock and more conventional tensile type failure in brine saturated rock. Continuous acoustic stream energy variation with time offers insight into the induced fracture complexity and should be used along with microseismic cloud distributions to assess the stimulated reservoir volume.

Evaluation of stimulated reservoir volume in laboratory hydraulic fracturing with oil, water and liquid carbon dioxide under microscopy using the fluorescence method

In shale gas industry, it is desired to develop new reservoir fracturing and enhanced gas recovery technologies to replace the conventional hydraulic fracturing (HF), in order to reduce water usage to guarantee the environmental sustainability and boost individual well production. As the goal of HF is to create high conductivity fracturing networks as flow paths for gas, it is necessary for HF to activate and connect existing natural fractures to generate large fractures network. The success or failure of HF often depends on the stimulated reservoir volume (SRV) which is characterized by the quantity and the quality of the fractures network resulted. This study investigates the micro-fractures network resulted in laboratory HF experiments in 2-D thin polished section by using a fluorescent method supported by advanced computerized image analysis. To evaluate difference of resulted SRV due to the difference of fracturing fluid, using three cylindrical shale cores and three granite cubes having fractures induced by HF using three fluids having different viscosity; oil, water and liquid carbon dioxide (L-CO2). The observation and statistical analysis of fractures induced in HF by the three different fluid viscosities using the fluorescent method showed ability of L-CO2 injection to achieve effective stimulation. The results suggest that employing a low viscosity fluid in HF of shale reservoirs can achieve more productive network with better SRV. In addition, the observation seems to be consistent with the tendency observed in the previous researches.

Numerical Modelling of Hydraulic Fracturing in Cohesionless Sand: Validation Against Laboratory Experiments

Summary In this paper, a new hydraulic-fracturing model is introduced for cohesionless sand, which is also applicable to weak sandstone formations with high permeability and low shear strength. Phenomena such as shear-band development and shear-enhanced permeability are of paramount importance during hydraulic fracturing of cohesionless sand or weak sandstones, which make the fracturing response quite different from what it is conventionally believed to be in competent rocks. The smeared approach in simulating hydraulic fracturing has been implemented in the proposed model within the continuum mechanics framework. Both matrix and fracture flow have been considered in this model. Tensile- and shear-fracture development and their fluid flow were simulated. The cubic law and Touhidi-Baghini’s shear-permeability model (Touhidi-Baghini 1998) were used to capture the permeability evolution and to model flow in tensile and shear fractures, respectively. Shear fracturing of geomaterials involves intense localization of deformation and strain softening, which is a discontinuous phenomenon, resulting in mesh dependency of the results in the continuum model. The fracture-energy-regularization method was used in this model to reduce the mesh-size dependency of the energy dissipated during fracture propagation. The smeared-fracture approach has been validated against laboratory hydraulic-fracturing experiments with reasonable agreement. Consistent with the experiments, the results of the numerical model indicate that tensile fractures are formed in a very small area around the injection point despite the application of high injection pressure compared with the minimum boundary stress. It is found that shear fracturing and shear-permeability evolution are the most important mechanisms that influence and control the fracturing response. The dominant fracturing mechanism is found to be governed by the high permeability and low shear strength of the material.

The impact of cleats on hydraulic fracture initiation and propagation in coal seams

Cleats are systematic, natural fractures in coal seams. They account for most of the permeability and much of the porosity of coalbed methane reservoirs and can have a significant effect on the success of hydraulic fracturing stimulation. Laboratory hydraulic fracturing experiments were conducted on coal blocks under true tri-axial stress to simulate fracturing stimulation of coal seams. Fractures were initiated by injecting a water gel with luminous yellow fluorescent dye into an open hole section of a wellbore. The impact of cleats on initiation and propagation of hydraulic fractures in coal seams is discussed. Three types of hydraulic fracture initiation and propagation pattern were observed in this study: 1) The hydraulic fracture initiated and then grew along the cleat. 2) The hydraulic fracture initiated along a butt cleat or a fracture (natural or induced by drilling) oriented roughly in the minimum horizontal stress direction, then turned to propagate along the first face cleat that it encountered or gradually turned towards the maximum horizontal stress direction. 3) The hydraulic fracture initiated perpendicular to the minimum stress and, when it encountered a face cleat, tended to propagate along the cleats if the extension direction does not deviate greatly (<20° as determined in this paper) from the maximum horizontal stress direction. When a coal seam is hydraulically fractured, the resulting fracture network is controlled by the combined effect of several factors: cleats determine the initiation and extension path of the fracture, the in-situ stress state dominates the main direction of the fracture zone and bedding planes impede fracture height growth.

Laboratory Hydraulic Fracturing Experiments for Determining Reopening and Closing Pressures of Fractures

We carried out laboratory hydraulic fracturing tests, which revealed that the pressure at the reopening and closing of fractures during hydraulic fracturing can be detected by monitoring the strains in the borehole wall and the triggering of acoustic emissions.In the experiments, we measured the pressures during the reopening and closing of a fracture caused by elastic restitution in a block of rock not subjected to compressive load. The pressures are used for correcting those obtained in blocks of rock when biaxial compressive load is applied. The reopening pressure (Pr)is smaller than that obtained by the conventional method. This result expresses the following theoretical relationship.Pr=(3Sh-SH)/2where SH is the maximum horizontal stress, and Sh is the minimum horizontal stress.The corrected reopening pressure of fracture agrees to within 0.7 MPa with that obtained from the above equation.

A Fully Implicit Single Phase T-H-M Fracture Model for Modelling Hydraulic Fracturing in Oil Sands

Abstract Enhancing oil extraction from oil sands with a hydraulic fracturing technique has been widely used in practice. Due to the complexity of the actual process, modelling of hydraulic fracturing is far behind its application. Reproducing the effects of high pore pressure and high temperature, combined with complex stress changes in the oil sand reservoir, requires a comprehensive numerical model which is capable of simulating the fracturing phenomenon. To capture all of these aspects in the problem, three partial differential equations, i.e., equilibrium, flow, and heat transfer, should be solved simultaneously in a fully implicit (coupled) manner. A fully coupled thermo-hydro-mechanical fracture finite element model is developed to incorporate all of the above features. The model is capable of analyzing hydraulic fracture problems in axisymmetric or plane strain conditions with any desired boundary conditions, e.g., constant rate of fluid injection, pressure, temperature, and fluid flow/thermal flux. Fractures can be initiated either by excessive tensile stress or shear stress. The fracture process is simulated using a node-splitting technique. Once a fracture is formed, special fracture elements are introduced to provide in-plane transmissivity of fluid. Effectiveness of the model is evaluated by solving several examples and comparing the numerical results with analytical solutions. The model is also used to simulate large-scale laboratory hydraulic fracturing experiments. Introduction Hydraulic fracturing technique has been a fast growing technology since its first application in 1947. By 1988, more than one million hydraulic fracturing treatments had been performed(1), and today this technique is one of the most important methods in enhancing oil extraction from wells. Hydraulic fracturing in oil sand reservoirs plays an even more important role. Due to low temperature and low permeability of oil sand deposits and high viscosity of bitumen, oil is virtually immobile(2). Hence, any attempt for in situ oil extraction should employ one of the following techniques: cyclic steam stimulation, in situ combustion, or hydraulic fracturing. Despite the fact that hydraulic fracturing technology has advanced significantly over the past fifty years, our ability to model the process has not changed as rapidly. As a matter of fact, this technique has been so successful that in the past, designingthe treatment with a high degree of precision was not of any interest. But as the industry moved towards applications of very high volume/rate, and highly engineered and sophisticated hydraulic fracturing treatments, the demand for more rigorous designs in order to optimize the procedure have become more important. On the other hand, without a thorough understanding of the physical process and the factors that are involved, our ability for an optimal design is limited. Modelling fluid flow combined with heat transfer in the reservoir has been used by the industry for a long time, and the fracturing process was often designed based on twodimensional closed-form solutions, such as Geertsma-de Klerk(3), or GdK in brief, and Perkins-Kern(4) and Nordgren(5), or PKN. Most of the flow and heat transfer models are based on the finite difference method, and effects of stresses and deformations in the ground, if not totally ignored, are solved in a decoupled or partially coupled manner with other elements.

The mechanics of diamond core drilling of rocks

This paper presents observations and modeling results on the effects packers have on fracture initiation and near wellbore fracture geometry for a set of laboratory hydraulic fracturing experiments. An interesting observation is that significant discrepancies in fracture initiation pressures between the experimental results and theoretical predictions, which was ascribed to packer effects, occurred only for highly deviated boreholes. 3-D FE simulations of the laboratory tests, with emphasis on the packer structure and its response to the pressurization of the sealed-off interval, were subsequently conducted in trying to explain the mechanisms associated with the observed phenomena. The packer design was found to be the major cause of the discrepancy. Based on the observations and analyses conducted, significant effects of packer on hydraulic fractures initiated from highly inclined and horizontal boreholes were delineated, and conditions that may favor/impair such effects are discussed.

Effects of packer on hydraulic fractures initiated from highly-deviated and horizontal boreholes

This paper presents observations and modeling results on the effects packers have on fracture initiation and near wellbore fracture geometry for a set of laboratory hydraulic fracturing experiments. An interesting observation is that significant discrepancies in fracture initiation pressures between the experimental results and theoretical predictions, which was ascribed to packer effects, occurred only for highly deviated boreholes. 3-D FE simulations of the laboratory tests, with emphasis on the packer structure and its response to the pressurization of the sealed-off interval, were subsequently conducted in trying to explain the mechanisms associated with the observed phenomena. The packer design was found to be the major cause of the discrepancy. Based on the observations and analyses conducted, significant effects of packer on hydraulic fractures initiated from highly inclined and horizontal boreholes were delineated, and conditions that may favor/impair such effects are discussed.

Relation between Reopening Pressure and Tectonic Stresses for Hydraulic Fracturing Tectonic Stress Measurements.

In order to establish the methodology of hydraulic fracturing tectonic stress measurements, the relation between the so-called reopening pressure of longitudinal cracks and tectonic stresses is studied. To this end, the coupled problem of fluid flow in the crack and the crack reopening behavior in an impermeable rock is analyzed based on the linear theory of elasticity and also on the linear fracture mechanics, where it is taken into account that the crack does not reclose perfectly upon deflation due to crack surface damage and some minor shear displacement. The results show that, contrary to the conventional theory, the apparent reopening pressure, i. e., the borehole pressure at which the pressure-time history apparently deviates from a linear behavior, is hardly affected by the maximum horizontal tectonic stress acting in the direction parallel to the crack line. It decreases and approaches the minimum horizontal tectonic stress normal to the crack plane with decreasing injection flow rate. Moreover, it is shown that the results of laboratory hydraulic fracturing experiments conducted in the present work support those conclusions.

Physical background to the breakdown pressure in hydraulic fracturing tectonic stress measurements

A new theory is presented to interpret the breakdown pressure which is the wellbore pressure at fracture initiation in hydraulic fracturing tectonic stress measurements. The theory is able to explain the effects of wellbore diameter and pressurization rate on the breakdown pressure. It was shown that these effects were explainable in a simple way by taking into account the effective stress distribution beyond a wellbore surface and by applying a newly-constructed fracture criterion. The criterion assumes the fraction initiation occurs when the maximum tensile effective stress reaches the tensile strength of a rock at a point that is not on the wellbore surface but is inside the rock. The distance between that point and the point of stress concentration is a material constant. In order to verify the present theory, laboratory hydraulic fracturing experiments were performed on cubical rock specimens under uniaxial compression. Results show that experimental breakdown pressures decrease with the increasing wellbore diameter and increase with the pressurization rate. In all cases, the experimental results agree perfectly well with the prediction based on the present theory contrary to the prediction based on the conventional theory.