Mechanical responses and fracture behaviors of pre-heat-treated carbonate rocks during hydraulic fracturing under different confining-axial pressures

The impact of changes in natural fracture fluid pressure on the creation of fracture networks

Hydraulic fracturing tests on Pocheon granite cylinders at seven different injection rates varying from 1 to 100 mm3/s were conducted. They were compared with sleeve fracturing tests in which borehole was sleeved, and therefore, water infiltration influence was excluded. Hydraulic fracturing behavior of granite is significantly influenced by water infiltration, which is closely related to the preexisting microcracks in granite as well as the cleavage anisotropy. There was a threshold injection rate to fracture the granite specimen under given stress conditions. When the injection rate is below the threshold, water infiltrated granite matrix with slow increment of injection pressure, and the specimen finally reached a full saturation without fracturing. Injection pressure developed nonlinearly with time during water infiltration, while approximately linearly when infiltration was excluded. For both hydraulic and sleeve fracturing tests, breakdown pressure increases with increasing injection rate. The breakdown pressures by sleeve fracturing were more than two times higher than those in hydraulic fracturing. X-ray computed tomography (CT) observations show that induced fractures are along the weaker cleavage parallel to the direction of the vertical stress. The higher breakdown pressure results in a larger aperture of fractures in hydraulic fracturing tests.

Summary Pulse hydraulic fracturing technology can greatly improve the effect of fracture propagation in rock and form complex fracture networks in reservoirs. The interaction mechanism between hydraulic fractures and pre-existing fractures under pulse hydraulic pressure is unclear. The induced laws of pre-existing fractures on the propagation direction of hydraulic fractures under different pulse frequencies and pulse hydraulic pressures are revealed in this work. We have carried out traditional hydraulic fracturing (THF) tests and pulse hydraulic fracturing tests with rock-like specimens. We compared the interaction between hydraulic fractures and pre-existing fractures in the two hydraulic fracturing tests. Acoustic emission (AE) characteristics of the interaction between hydraulic fractures and pre-existing fractures during pulse hydraulic fracturing are analyzed. The results show that pre-existing fractures in the rock-like specimen can induce the direction of propagation of hydraulic fractures. The influence of pre-existing fracture tips on hydraulic fracture propagation is greater with low pulse frequencies than with traditional hydraulic pressures and high pulse frequencies. When the pulse frequency is 1 Hz, hydraulic fractures are easily induced by pre-existing fracture tips. With increasing pulse frequency, the hydraulic fracture propagation direction gradually moves away from the pre-existing fracture tips and extends perpendicularly to the direction of the minimum principal stress. Under pulse hydraulic loading, more hydraulic fractures are generated around the wellbore than under THF and extend to the pre-existing fracture, and more hydraulic fractures around the wellbore are created with low-frequency pulse loading than with high-frequency pulse loading. Compared with traditional hydraulic pressures, hydraulic fracture propagation with low pulse frequencies (1 and 3 Hz) is more complex than hydraulic fracture propagation with traditional hydraulic pressures and high pulse frequencies (5 Hz). Under high pulse hydraulic pressure and pulse frequency, hydraulic fractures easily extend along the direction perpendicular to the direction of the minimum principal stress like propagation under traditional hydraulic pressure. The study of the interaction mechanism between hydraulic fractures and natural fractures under pulsating hydraulic pressure can provide a method for the formation of fracture network systems in large-scale fracturing and may improve the fracturing efficiency.

To reduce near wellbore fracture complexity in continental laminated shales, in-plane perforations completion technology is introduced. The global embedded cohesive elements are capable to simulating arbitrary fracture propagation growth while the laminae effects on fracture propagation path can be considered in established finite element model. Numerical simulations reflect that the existence of weak micro fractures only locally alter the fracture propagation path but laminae can change the fracture propagation path greatly. Furthermore, the in-situ stress anisotropy, pump rate, perforation intersection angle and perforation angle with laminae are critical factors for affecting the fracture propagation path and breakdown pressure. There is a great impact of laminae on the fracture branching at the contact interface and hydraulic fracture tends to cross laminae at high approaching angle but propagate horizontally along laminae at low approaching angle. In most cases, hydraulic fracture tends to propagate along the laminae rather than across the laminae. The adjustment of pump rate, fluid viscosity, perforation intersection angle and perforation angle with laminae can enhance the possibilities of hydraulic fracture across laminae. It was also found that hydraulic fractures induced from different perforations can interact and overlap with each other, resulting in different fracture geometry and pressure behavior for individual fracture. The fracture width of interior fracture is almost close at the near wellbore zone at the end of pumping, and exterior fractures generally deviated away from the perforation tunnel direction because of stress interference of neighboring fractures. Additionally, although a fracture initiated initially from a perforation tunnel to short distance, hydraulic fractures still finally would reorient itself to maximum principle in-situ stress direction, thus increase more chances of creating one large and main fracture along the maximum principle in-situ stress orientation with larger horizontal stress contrast. Therefore, the existence of laminae can enhance the potential of fracture complexity near wellbore but the selection of suitable perforation scenarios in one defining plane can increase the possibility of form a main fracture. The simulation results from this study offer some important insights to hydraulic fracturing design with multiple perforations on interbedded continental shales stimulation.

Hydraulic fracturing experiments have been performed in the laboratory in pre-fractured material under triaxial states of stress. Tests have been run on naturally fractured blocks of Devonian shale as well as blocks of hydrostone in which the angle of approach of the hydraulic fracture to the pre-fracture was varied in a systematic way. It was found that hydraulic fractures tend to cross pre-existing fractures only under high differential stresses and high angles of approach. In most cases the hydraulic fractures were either diverted or arrested by the pre-existing fractures.

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

As shown in hydraulic fracture monitoring, the hydraulic fracture path in naturally fractured shale reservoirs is complex, and the method for describing the propagation of fractures in homogeneous sandstone cannot achieve sufficient accuracy when used in shale reservoirs. In this study, a discrete fracture network model is proposed by performing a Monte Carlo simulation on the fracture characteristics of downhole shale cores and shale outcrops in the Longmaxi formation in the Sichuan basin of China. Given the fracture fluid pressure drop and intersections between hydraulic and natural fractures, a 2D numerical model is developed to examine the hydraulic fracture propagation in a randomly fractured shale reservoir using the displacement discontinuity method. Numerical simulations show that (1) the density and length of natural fractures are positively correlated with the complexity of natural fractures when the orientation of such fractures is similarly distributed, (2) the influence of small natural fractures on a hydraulic fracture is localized and cannot change the overall direction of the hydraulic fracture, and (3) hydraulic fractures can cross the natural fracture when the intersection angle and minimum horizontal stress are small enough. The rationality of the numerical model results is supported by the hydraulic fracturing tests on fractured shale outcrops with a true triaxial fracturing system and microseismic monitoring from a hydraulic fracture test well in the Longmaxi shale formation.

Hydraulic fracturing is an important and prevalent process both in the natural environment and industrial applications. At the same time, field hydraulic fracturing tests data provide valuable information regarding the mechanical and hydraulic behaviours of the reservoir formation. By history-matching the field bottom-hole-pressure versus time curve from hydraulic fracturing tests, a set of field-validated geomechanical models can be obtained, which is an important asset for any further works on utilizing geomechanics to enhance the injection and production performance. This paper presents a 3-dimensinal finite element model for history matching the complete bottomhole- pressure versus time curve generated during hydraulic fracturing tests considering the injection rate as input. To stimulate the hydraulic fracturing process in unconsolidated sands formation, a poro-elasto-plastic constitutive model together with a strain-induced anisotropic fill permeability model are formulated and implemented into a 3D finite element geomechanical simulator. Unlike the conventional simulation of hydraulic fracturing in hard rock, hydraulic fracture in unconsolidated sands reservoir is stimulated as a large area of dilation zone or a net or micro-cracks, inside which the effective stresses are low and hydraulic conductivities are high. It is shown the proposed numerical model can successfully capture the hydraulic fracture initiation and propagation in unconsolidated sands formation and matches the field pressure versus time curve very well. Introduction Hydraulic fracturing can be broadly defined as a process by which a fracture initiates and propagates due to hydraulic loading (i.e., pressure) applied by a fluid inside the fracture(1). Fractures in the earth's crust are desired for a variety of reasons, including enhanced oil and gas recovery, re-injection of drilling or other environmentally sensitive wastes, measurement of in situ stresses, geothermal energy recovery, and enhanced well water production(2). Although hydraulic fracturing in hard rock has been comprehensively studied both experimentally and numerically, some fundamental mechanisms of hydraulic fracturing in unconsolidated sands have not been well understood. Experimental data clearly show that fracturing in unconsolidated sands is significantly different than those encountered in hard rock. Unconsolidated sands do not exhibit elastic-brittle behavior. In addition, unconsolidated sands have very low tensile and shear strengths at low effective stresses as well as relatively large fluid leak-off(3–5). Based on other researchers' work(3–7) on the fundamental mechanisms of hydraulic fracturing in unconsolidated sands, this paper presents the constitutive modeling and numerical simulation of hydraulic fracturing in unconsolidated sands within the framework of continuum mechanics. Numerical experiments show that this approach has special advantages in numerical modeling of large scale field problems, such as the disposal of waste cutting fluid, and micro/mini fracture tests in unconsolidated sands formation. Even in its most basic form, hydraulic fracturing in unconsolidated sands is a complex process to model, as it involves the coupling of at least three processes:the solid matrix deformation and failure induced by the pore fluid pressure;the flow of fluid within the fracture and solid matrix; andthe fracture initiation and propagation after the failure of formation.

Numerical simulation of hydraulic fracturing process with consideration of fluid–solid interaction in shale rock

Experimental and simulation study of the tensile strength of wet carbonate rocks using the Brazilian Test

Currently, the closure stress is often predicted using the conventional tangent method (i.e., G-function) or the variable compliance method. Both methods use several restrictive assumptions such as a single planar fracture. However, the hydraulic fracture often intersects rock fabric features such as bedding planes and/or natural fractures causing the pressure transient behavior to become drastically different compared to that of a single planar hydraulic fracture. Closure of the intersected natural fractures might precede that of the created HF which impacts the interpretation of the pressure derivate plots and also the closure stress. In this paper we present and use an advanced fracture diagnostic model that can help recognize the signatures of rock fabric features and their impact on estimation of the closure stress. An example field data is used to illustrate the potential impact on closure stress. The new DFIT model consists of a fully coupled 3D hydraulic fracture simulator with the ability to handle the opening, propagation, and closure of natural factures so that the pre- and post-closure stress/deformation of both the hydraulic and natural fractures can be captured. Fracture propagation, HF-NF interaction, fracture intersection, and DFIT model are integrated into one simulator to provide a more realistic view of HF propagation and fracture diagnostics in naturally fractured reservoirs. The current model is developed without any major assumptions concerning the fluid flow, fracture deformation, and propagation path. Rock/fracture deformation is calculated using a boundary element formulation whereas the transport processes are solved using finite elements method. Our results indicate that natural fractures affect the pre- and post- shut-in response of the hydraulic fracture in a number of ways. For example, the fracture propagation path, the pumping pressure profile, and interfering with the post shut-in pressure response. These factors, indeed, impact the estimation of the minimum horizontal stress which is a key parameter obtained from DFIT. Moreover, our results show how the normal stiffness of the fracture surface asperities can impact the minimum stress estimation. Closure of natural fractures is reflected in the slope of the pressure derivative and G-function plots so that correct interpretation of these signatures is essential to accurate extraction of the Shmin. Closure of natural fractures is often viewed as a pressure depdendent leakoff mechanism that is reflected on the Gdp/dG curve. The closure behavior of HF-NF sets is, however, not explicitly modeled in the context of pressure transient analysis. Therefore, it is our objective to study the closure behavior of HF-NF sets using a 3D coupled simulator. This novel model is applied to actual field data to illustrate the potential impact on closure stress and to shed light on the subject of fracture diagnostics in naturally fractured reservoirs. Our results indicate that the closure behavior of hydraulic and natural fractures in a HF-NF set differs from that of an isolated fracture due to the effect of stress shadowing. Although the system stiffness method results in distinct signatures on the diagnostic curves, these signatures are not commonly observed in the field data. The absence of stiffness signatures in the field cases could be interpreted in two ways: 1) the closure mechanism assumed in the stiffness/compliance method differs from the actual fracture closure mechanism or 2) the stiffness of the hydraulic fracture is too low to cause any significant changes in the system stiffness after closure.

A fracture propagation model that is intended to describe the geometry of a hydraulic fracture is also supposed to replicate the pressure level and trends during execution. This is often not the case with "abnormally high fracturing pressures" recorded presently. The problem stems from the intrinsic nature of linear elastic fracture mechanics. A relatively new branch of fracture mechanics, Continuum Damage Mechanics (CDM), has been used by the authors to construct a fracture propagation model describing fracturing pressure behavior consistently. The CDM version of the Perkins-Kern/Nordgren model (CDM-PKN) contains a combined parameter responsible for fracture propagation retardation. The parameter is the CDM substitute for the fracture toughness. The new parameter can be identified from step-rate tests and/or treatment pressure diagnostics. Knowing this parameter the CDM-PKN model can be used to predict the net fracturing pressure, fracture length, width and fluid efficiency. In this work several applications of the model are presented. A laboratory experiment, step-rate tests and a minifrac test are evaluated in terms of the new model. A possible range is established for the combined parameter. The large impact of the combined parameter on fracture design is illustrated by a case study.

The authors should be commended for their long-term extensive research effort in hydraulic fracturing (Murdoch 1993a, 1993b, 1993c). In the investigation under discussion, four sand-filled hydraulic fracture tests were carried out to better understand the forms of hydraulic fractures created in shallow saprolite and to compare them with those in other geologic materials under similar conditions. It was observed by the authors in their field experiments that the fractures were shaped like slightly asymmetric saucers that were roughly flat-lying in their centers and slightly curve upwards. In fact, similar fracture pattern was observed in laboratory experiments on hydraulic fracturing conducted by injecting cement bentonite and epoxy into clay specimens with overconsolidation ratio of 5 under similar free surface boundary condition (Chin 1996; Au 2001; Au et al. 2003, 2005; Soga et al. 2003). It has also been demonstrated experimentally in the laboratory that hydraulic fracture may be initiated by injecting a small volume of fluid into highly overconsolidated clay. The fracturing fluid then propagates along the initiated cracks, perpendicular to the direction of major principal stress, and develops them into large fractures afterwards. A theoretical model of hydraulic fracture propagation has been developed by the authors by adapting the software Fran2d under the framework of linear elastic fracture mechanics to explain the observed fracture forms. The discussers agree with the authors wholeheartedly that understanding of the controls on fracture form has useful applications. However, there are some points presented in the paper that are worth discussing and clarifying. It is unclear to the discussers why the hydraulic fractures were created in the B horizon above the saprolite while ‘‘the primary objective of the investigation is to describe the forms of hydraulic fractures in saprolite and compare them with the forms created in other geologic materials.’’ It is evident from the authors’ description that the material properties of B horizon are quite different from those of saprolite. As the degree of saturation of the specimens is well below unity, the accuracy of the results of unconsolidated– undrained (UU) triaxial tests in describing the stress–strain characteristics and quantifying the shear strength parameters is doubtful. Increase in effective stress within the specimens can still occur with the increase in confining stress even in the UU test. The argument is substantiated by the fact that the friction angle of the soil measured by the authors decreases when the confining stress increases. The modulus of elasticity of soil, determined by the UU test, may not be appropriate for the numerical simulation of the tensile failure fracture. The modulus of elasticity of soil is very dependent on its strain. It is well known that the modulus of elasticity of soil determined by the triaxial test is considerably lower than its actual value in the field, as the strain is considerably higher. The Ko value determined by Malin (2005) in his hydraulic fracture experiment is quite different from that obtained by the authors using flat-blade dilatometer measurements in the same project. The authors may have to elaborate on their choice of the Ko value used in their numerical simulations. The fracture toughness (KIC) was introduced as a parameter used to estimate the fracturing pressure by Murdoch (1993b). The measured fracturing pressure of approximately 300 kPa is approximately 9 times the undrained shear strength, i.e., 34 kPa. The result is consistent with the theoretical ultimate cavity expansion pressure in soil (Au et al. 2006). However, the magnitude of fracturing pressure is dependent on injection rate and overconsolidation ratio of the soil (Mori and Tamura 1987; Au 2001; Soga et al. 2003), as shown in Fig. D1, where Pf is the fracturing pressure and 0 v is the effective overburden pressure. However, both the injection rate for the experiments and the overconsolidation ratio of the soil were not given in the paper. On the basis of the given Ko value, the discussers envisage that the soil is highly overconsolidated. As a result, the soil may behave elastically. Therefore, the use of linear elastic fracture mechanics to simulate the fracture propagation process may be appropriate. Otherwise, the applicability of elastic analysis in hydraulic fracturing should be carefully considered. However, the adjusted intrinsic soil parameters obtained by the authors assuming the validity of the approach is worth discussing. It was concluded by Murdoch (1993b) that ‘‘When soil is completely saturated, KIC is essentially zero. Nevertheless, well-developed hydraulic fractures were created in soil of negligible toughness.’’ The exReceived 9 March 2007. Accepted 15 October 2007. Published on the NRC Research Press Web site at cgj.nrc.ca on 1 February 2008.

Numerical simulation of hydraulic fracture propagation in conglomerate reservoirs

Modeling multisegmented hydraulic fracture in two extreme cases: No leakoff and dominating leakoff