Comparison of Hydraulic Fracturing and Deflagration Fracturing Under High-Temperature Conditions in Large-Sized Granite

Abstract

Fracturing is an indispensable technique in geothermal energy development. Large-sized model tests of different fracturing methods are crucial for evaluating the fracturing effect and extrapolating the results to field applications. For common hydraulic and deflagration fracturing methods, 40 × 40 × 40 cm³ granite samples were used to carry out fracturing tests under high-temperature conditions in this paper. Through the analysis of the fracturing parameters and multiscale fracture morphology, a series of key findings were summarized. Deflagration fracturing is more intense, notably unaffected by the principal stress difference, and is capable of generating fracture spaces tens of times larger than those created by hydraulic fracturing. Furthermore, high temperatures tend to produce more fracture zones rather than continuous cracks during hydraulic fracturing. In contrast, deflagration fracturing yields simpler and more regular fractures in granite at high temperatures. Finally, the influence of the borehole number and the quantity of the deflagration agent on the fracturing effect are briefly discussed. These findings provide valuable insights for enhancing reservoir stimulation in geothermal systems.

1. Introduction

In the context of global warming, the development of clean energy is an effective way to reduce CO₂ emissions. And the geothermal energy stored in hot dry rocks (HDRs) [1,2] is not only abundant in reserves but is also more stable as an energy supply. In the exploitation of HDRs, fracturing for enhanced geothermal stimulation [3,4] is a critical process. Currently, several common fracturing technologies mainly include explosive fracturing [5,6,7], deflagration fracturing [8,9,10], hydraulic fracturing [11,12,13], and thermal shock [14,15,16]. For explosive fracturing, due to its excessive intensity and the fact that a large amount of energy is consumed in the crushed zone [17], the fracturing effect is not ideal. When supercritical CO₂ or H₂O is used as the fracturing fluid [18], hydraulic fracturing and thermal shock occur simultaneously during the contact between the relatively low-temperature fracturing fluid and the hot rock mass. Therefore, in comparison, deflagration fracturing and hydraulic fracturing are two more common technologies applied in engineering practice.

Hydraulic fracturing, although proposed as early as 1947, did not enter a stage of large-scale application until 20 years ago [12]. During these two decades, numerous scholars [19,20] have conducted extensive research on hydraulic fracturing. Due to limitations in equipment and experimental conditions, much of this research relies on numerical simulation. The theoretical foundations of these simulations include the mechanisms of crack initiation [21] and coupled models of multi-physical processes [22,23]. Based on these theoretical frameworks, researchers can deeply analyze the influence of factors such as rock mass properties [24] and fracturing parameters [25] on crack propagation. Furthermore, numerical simulations also encompass behaviors such as directional hydraulic fracturing [26] and fracture propagation across discontinuous surfaces [27]. Currently, most laboratory experiments are often limited to using small-sized rock samples [28,29] for fracturing tests, while large-scale fracturing tests under high-temperature conditions are relatively rare.

Deflagration fracturing [30], also known as high-energy gas fracturing, is a technique that utilizes high-pressure gas generated by explosives to fracture rock masses. Similarly to hydraulic fracturing, research on deflagration fracturing also primarily focuses on numerical calculations. In numerical calculations related to deflagration fracturing, special attention needs to be paid to the deflagration characteristics of the powder, the movement patterns of gas within fractures, and the dynamic propagation of cracks. Based on this, Chen Dechun (2011) [31] established a fracture dynamic propagation model for deflagration fracturing and analyzed the changes in the wellbore pressure and crack morphology during the fracturing process. Majid Goodarzi (2015) [32] utilized the extended finite element method to solve for the quasi-static propagation of cracks around holes in deflagration fracturing. LS-DYNA [33,34] has also been commonly used as a tool for solving the dynamic propagation of cracks. Additionally, Huang Bo (2021) [35] analyzed the relationship between wellbore parameters such as the wellbore radius and depth, and peak pressure and production enhancement ratios with the help of numerical simulations. For experiments on deflagration fracturing, cement-poured specimens [36] are typically adopted to simulate the rock-breaking effects of deflagration fracturing.

Therefore, for fracturing in HDRs, whether hydraulic fracturing or deflagration fracturing, there is currently a lack of large-scale rock experiments under high-temperature conditions to support and validate numerous numerical results. Based on a large-scale model test platform, this paper conducts hydraulic fracturing and deflagration fracturing experiments on granite. The damage and crack propagation characteristics of large-sized granites under different experimental conditions are analyzed. And then, the effects of different fracturing methods on the rock-breaking efficiency of granite are explored. This research provides cutting-edge exploration and scientific support for the reservoir stimulation of HDRs in practical engineering.

2. Experimental Preparation

2.1. Large-Scale Model Test Platform

The experimental equipment used is the HDR fracturing model test device, independently developed by the Wuhan Institute of Rock and Soil Mechanics (Wuhan, China). It can be used to conduct fracturing tests on cubic rock samples with a maximum side length of 450 mm. The schematic diagram of its composition is shown in Figure 1a, which mainly consists of three major parts: the temperature and pressure loading system, the fluid injection system (or ignition system), and the control and data acquisition system.

The temperature and pressure loading system encompasses the above HDR fracturing model experimental device and a hand-operated pump (Figure 1c) that provides axial loading. Electrical heating plates are positioned on the bottom and lateral sides of the device, directly contacting the rock sample to heat it. Between the heating plates and the loading plates, rigid insulation boards are installed to maintain the temperature. Vertical (Z-direction) loading on the rock sample is achieved by injecting high-temperature-resistant silicone oil into a stainless-steel bladder-type pressure container via a hand-operated pump. Lateral loading, on the other hand, is accomplished through passive compression by two wedge plates on the side, which can also be considered as imposing fixed displacement boundary conditions.

The fluid injection system can provide a maximum injection pressure of 60 MPa, with an injection flow rate ranging from 10⁻³ to 100 mL/min. When performing deflagration fracturing experiments, the fluid injection system is replaced with an ignition system and the connection to the wellbore changes from fluid pipes to electrical wires. The data acquisition system (Figure 1e) continuously collects and stores data such as the temperatures on the bottom and four sides of the rock sample, the injecting rate, and the fluid pressure within the wellbore of the rock sample.

2.2. Sample Preparation

The granite samples used in this study were from Wenshang County, Shandong Province. The collected raw granites were initially rough-processed into cubes with a side length of 400 mm. Subsequently, the rock sample surfaces were finely polished to ensure that the surface flatness error was less than 0.5 mm and the face parallelism error was less than 1 mm. Finally, a drilling machine was used to drill the borehole at the designed locations on the samples (Figure 2a). The processed samples for testing are as shown in Figure 2b Two types of boreholes were employed: a single borehole which was drilled at the center of the top surface, with a diameter of 25 mm and a depth of 250 mm; and dual boreholes, which were drilled symmetrically on the top surface of the sample, 75 mm away from the center, with the same diameter and depth as the single borehole.

2.3. Test Procedure

In hydraulic fracturing experiments, prior to the start, an epoxy resin high-temperature-resistant sealant is used to fill the gap between the steel wellbore and the wellbore wall, achieving a sealing effect, as shown in Figure 3a. In the case of deflagration fracturing, there is no wellbore, necessitating a different approach for hole treatment. Before the experiment, a specific amount of deflagration agent is wrapped in a waterproof membrane and placed inside a paper cylinder that matches the diameter of the borehole. This paper cylinder is then fixed at the bottom of the borehole, while the upper section of the borehole is sealed with thermally conductive potting adhesive to create an enclosed space inside the paper cylinder. The wires from the igniter are routed through the sealed section.

Then, the rock samples with the wellbores sealed are placed inside the experimental device, and the heating plate, insulation plate, and loading device are placed in sequence. Meanwhile, the fluid injection pipes or the wires for ignition are connected properly. Finally, the top cover is installed and the pressure-bearing nuts are tightened to ensure the overall rigidity of the device.

Next, the rock sample is subjected to temperature and axial force to ensure that it satisfies the preset test conditions, following the principle of “heating first, then applying force”. Given the relatively large size of the rock sample, in order to ensure effective heating throughout its interior, it is necessary to heat it for at least 8 h. After reaching the target temperature, silicone oil is injected into the stainless steel pressure container by a hand-operated pump to achieve the axial loading. The pressure sensor connected to the container can be transformed into the axial stress value.

Finally, two parallel-connected ISCO pumps are used to continuously inject water into the wellbore at a constant flow rate, achieving hydraulic fracturing of the rock sample. The test is terminated when the fluid pressure within the wellbore drops to a lower value or water flows out of the device. As for the deflagration fracturing, this is achieved through an ignitor. When the switch is pressed, a high-voltage current is transmitted through the wire to the deflagration agent and causes it to undergo a phase change and release a large amount of gas, which creates high pressure inside the wellbore and results in the fracturing of the rock sample. It is important to note that for dual-hole rock samples, the ignitors of the two wellbores are series-connected to ensure synchronized deflagration of both wellbores.

In the test, the axial pressure is kept at 12.5 MPa and other variables such as temperature, the borehole number, and the mass of deflagration agent are set as in

Table 1. Test variable settings for hydraulic fracturing (H) and deflagration fracturing (D).

Sample No.	Temperature	The Borehole Number	The Mass of Deflagration Agent in Each Borehole
H1	20	1	--
H2	80 °C	1	--
H3	150 °C	1	--
H4	40 °C	2	--
D1	20 °C	1	10 g
D2	150 °C	1	10 g
D3	150 °C	2	8 g
D4	150 °C	2	5 g

.

3. Experimental Results

3.1. Hydraulic Fracturing

After the experiment, the red tracer was injected into the wellbore to observe the seepage locations of the tracer on the surface of the rock sample, thereby assessing the fracture morphology. The top surface and four side surfaces of the cubic rock sample are displayed according to the placement shown in Figure 4, and the same is true for the deflagration fracturing. When necessary, a 150× microscope was used to observe the microscopic morphology of the fractures in the fine details of the rock samples.

The four experiments terminated with the observation of water flowing out of the equipment, and the final failure modes of the four specimens are illustrated in Figure 5. Two primary types of specimen failure were observed: cracks and localized damage zones. The cracks represented continuous, interconnected damage with a certain width, and in the case of hydraulic fracturing, the crack widths generally ranged in the order of tens of micrometers (Figure 6a). The localized damage zones, on the other hand, exhibited non-interconnected, cluster-distributed characteristics, which is demonstrated well by the tracer seepage patterns within the damage zones in Figure 6b,c.

For specimens containing a single borehole (Figure 5a–c), the induced damage primarily radiated outward from the borehole center. The damage was predominantly distributed along a “left-top to right-bottom” direction (from a top-view perspective, that is to say, X−-Y+ to X+Y−), and based on the damage locations on the X− and X+ surfaces, it can be inferred that the internal damage essentially penetrated the entire specimen. Under heated conditions, more localized damage zones were observed in the specimens, while only cracks were exposed on the surface of the specimen (a) loaded at room temperature. In the case of the specimen with dual boreholes, the damage was primarily concentrated in the X−Y+ portion of the specimen, and there was no clear evidence indicating that the internal damage had fully penetrated the specimen.

Figure 7 presents the curves of the fluid pressure within the wellbore, the injection rate, and the sample temperatures with time in each experiment. During the injection process, the injection rate was initially set at 15 mL/min and then increased gradually. In Figure 7c,d, the injection rate is shown as 0 due to the limited capacity of the plunger pump, which necessitated refilling the pump with water after depletion. During this time, the valve between the pump and the wellbore was closed to maintain the pressure within the wellbore as consistently as possible.

The comparison reveals that under room temperature conditions, a sudden drop happened for the fluid pressure during the initial stage of the fracturing process ((a)-① and (d)-① in Figure 7), followed by a gradual stabilization. This indicates that once localized brittle fracture occurred within the rock mass, the fluid diffused along the newly formed cracks, leading to continuous crack propagation. In contrast, for the injections under high-temperature conditions depicted in Figure 7b,c, after the fluid pressure reached its maximum, a more gradual stress drop was observed. Moreover, as the injection rate increased, the rate of pressure decline became more pronounced.

Due to the limited monitoring means employed in this experimental study, the internal damage evolution within the sample during the fracturing process remains unknown. Therefore, the 3DEC (3-Dimensional Distinct Element Code) software (Version 7.00) [20] was utilized to simulate the fluid diffusion along the joined fractures, aiming to qualitatively illustrate the correspondence between the fluid pressure variation curve and the damage behavior of the sample. The proposed model has dimensions of 40 × 40 × 40 cm³. Considering its symmetry, half of it is taken as shown in the three-dimensional model on the left side of Figure 8. The injection wellbore is located at the center of the model, which is the center on the right side of the half-model. A compressive stress of 12.5 MPa was applied to the top of the model, while fixed displacement boundary conditions were assigned to the other five surfaces. For the 3DEC simulation of the fracturing process, it is necessary to predefine the location of the fracture planes. Specifically, the y-z plane passing through the midpoint of the x-axis is designated as the predefined fracture plane, also known as the flowplane in 3DEC. To enhance the visualization of fluid boundary conditions, the flowplane is translated and displayed separately on the right side of Figure 8. The three peripheral edges of the flowplane are configured as free fluid boundaries. Additionally, the right-side partial edge of the flowplane is designated for constant flow injection, with a specified flow rate of 7.5 mL/min. Initially, the predefined fracture plane was joined, but it was allowed to open under hydraulic action. The rock matrix is assumed to be elastic, with an elastic modulus of 80 GPa, and a Poisson’s ratio of 0.23, and its permeability is neglected. The parameters of the fracture plane and fluid are detailed in

Table 2. The parameters of the fracture plane and the fluid.

Parameter Name	Parameter Value	Parameter Name	Parameter Value
Normal stiffness (N/m)	10 × 109	Initial/max aperture (m)	1 × 10−5/1 × 10−4
Shear stiffness (N/m)	10 × 109	Fluid bulk modulus (Pa)	2 × 105
Cohesion (Pa)	1 × 109	Fluid density (kg/m3)	1000
Tensile strength (Pa)	1 × 104	Viscosity (pa·s)	1 × 10−3

.

From the moment fluid injection commences until it fully penetrates the fracture, the fluid injection pressure at different time points within the wellbore is illustrated in Figure 9. When the fracture initiation begins, there is a notable drop in fluid pressure. As the fracture propagates stably, the fluid pressure gradually becomes stable. Finally, when the fluid fully traverses the fracture to reach a free boundary, the fluid pressure accelerates its decline once again, subsequently stabilizing at a lower level.

Based on the simulation results, it can be inferred that for sample H1, cracks initiated at point ①, followed by continuous crack propagation, with the fluid pressure gradually stabilizing. When the injection rate increased to 20 mL/min, cracks continued to propagate stably. As the injection rate further increased, the decrease in the fluid pressure accelerated suddenly, indicating that the cracks had permeated the entire sample. For sample H2, cracks also initiated at point ①. The sudden drop in the fluid pressure at point ② suggested significant internal damage coalescence at this stage. The abrupt decrease in temperature at point ③ indicated that the cracks had fully run through the sample, allowing fluid to flow out from the sample. Sample H3 exhibited similar behavior to H2, with cracks initiating at point ① and significant propagation at point ②, followed by an accelerated decrease in fluid pressure, indicating that cracks penetrated the sample. For sample H4, two major crack propagations occurred at points ① and ②, and subsequently, at point ③, the fluid diffused along the crack to the sample surface.

3.2. Deflagration Fracturing

The experimental failure results of deflagration fracturing for different cases of D1-D4 are illustrated in Figure 10. For single-borehole specimens, the one under the room temperature condition exhibited a greater number of fractures. In contrast, the granite sample heated to 150 °C displayed only two prominent main fractures, approximately aligned with the X-axis and Y-axis. Similarly, the fractures induced by deflagration in the double-borehole under high-temperature conditions were also more regular. Besides the fracture traversing both boreholes along the X-axis, the deflagration with less deflagration agent per borehole unexpectedly generated two fractures parallel to the Y-axis, which penetrated the entire specimen. Compared to the cracks created by hydraulic fracturing, the fractures resulting from deflagration exhibited significantly larger fracture widths, commonly on the order of millimeters.

4. Discussion

4.1. Comparison of Fracturing Methods

For hydraulic fracturing, the resulting cracks are very narrow and accompanied by numerous localized damage zones, a feature absent in deflagration fracturing, which tends to completely separate the rock blocks. For the purpose of reservoir enhancement, deflagration fracturing undoubtedly produces larger-scale fractures and achieves superior results. To further compare the fracture spaces created by the two fracturing methods, rough modeling of the post-fracturing samples was conducted based on the exposed fracture traces and the morphologies of some fracture surfaces (Figure 11). The resulting fracture surface areas obtained from the models are presented in

Table 3. A rough statistical estimation of fracture area and space generated by different fracturing methods.

Fracturing Methods	a/cm2	b/cm2	c/cm2	d/cm2	Average Area/cm2	Average Width of Crack/mm	Total Fracture Space/cm3
Hydraulic fracturing	5620	2936	1677	4989	3805.5	1.5	570.8
Deflagration fracturing	630	207	1061	973	717.75	0.05	3.6

. Based on Figure 5, Figure 6 and Figure 10, if an average fracture width of 0.05 mm is assumed for hydraulic fracturing and 1.5 mm for deflagration fracturing, it can be observed that the fracture space generated by deflagration fracturing is hundreds of times larger than that of hydraulic fracturing. Additionally, based on the exposure of cracks and damage zones, it can be seen that nearly all the damage caused by hydraulic fracturing is distributed along a vertical plane oriented in the “upper left–lower right” direction, which is evidently independent of temperature and the number of boreholes. Experiments on hydraulic fracturing conducted by Li Ning (2017) [37], Xu Changzhuo (2024) [38], and others on glutenite have demonstrated that geostress difference is the primary controlling factor for crack propagation. This is further corroborated by the numerical simulation results of Li Lianchong (2013) [39], who found that the main crack propagates primarily along the direction of maximum principal stress. Therefore, it can be inferred that under the loading conditions of this study, the direction of maximum principal stress is approximately along the “upper left–lower right” direction, and it significantly controls the crack propagation and damage distribution in the four experiments. In contrast, during the deflagration fracturing, the crack propagation seems unaffected by the geostress difference caused by external loads.

4.2. Influence of Temperature on Fracturing Effect

For hydraulic fracturing of high-temperature granite, the role of temperature primarily lies in changing the mechanical properties of the rock. Elevated temperatures induce thermal cracks within the rock [40], subsequently reducing the tensile strength of the rock samples. Both laboratory experiments [16,41] and numerical simulations [42] have demonstrated that as the temperature increases up to 600 °C, the tensile strength of the rock decreases significantly. Using the previously mentioned 3DEC numerical model for recalculation but with the tensile strength of the fracture reduced to 1 × 10³ Pa, the fluid pressure variation during the water injection process is illustrated in Figure 12. It can be observed that crack propagation initiates before the fluid pressure reaches its maximum value. Following the peak, the fluid pressure no longer experiences a sharp drop but rather a more gradual decline, which aligns well with the experimental results for H2 and H3 shown in Figure 7. This phenomenon is also referred to as a reduction in the brittleness of granite [40,43]. Furthermore, the growth of microcracks at high temperatures leads to the formation of more localized damage zones within the sample, known as “cloud-like fractures” [28]. Therefore, based on the impact of high temperatures on the tensile strength of rocks, the injection parameters of fracturing fluid can be adjusted to accommodate changes in the rock mechanical properties in high-temperature environments and enhance the fracturing effectiveness. For instance, adopting a low-temperature injection method can promote crack propagation through increasing the thermal stress generated from the temperature difference, thereby creating larger fracture spaces.

In the process of deflagration fracturing, the fracture morphology is directly correlated with the peak pressure and duration of its action within the wellbore, which, in turn, are influenced by factors such as explosive parameters [44] and wellbore structure [35]. Under high-temperature conditions, the increase in microcracks within the rock reduces the pressure and the duration of the high-energy gas generated inside the wellbore, thereby resulting in a paradoxical decrease in the number of cracks generated during fracturing.

4.3. Influence of the Borehole Number on Fracturing Effect

From the fracture traces exposed in Figure 5, it can be observed that compared to scenarios H1 and H2, which have similar temperatures, dual-wellbore hydraulic fracturing has produced slightly more cracks. This is similar to scenario H3 under high-temperature conditions. Table 3 also supports this point from a quantitative perspective. According to the analysis in Section 4.1, hydraulic fracturing is greatly influenced by the stress difference arising from external loads. At the scale of the samples in this study, the fact that the dual wells are located closer to the boundaries of the sample will further complicate the stress field within the rock sample, leading to the generation of a few cracks and localized damage zones in the X−Y+ region. Because of the serial connection of the two wellbores, the pressure reduction in the left wellbore after fracture initiation prevents the formation of cracks in other regions of the sample.

From Figure 10, it can be initially observed that in the case of deflagration fracturing, the increase in wellbores does not significantly enhance the fracturing effect. With the aid of Table 3, it can be further noted that compared to the two single-wellbore configurations (H1 and H2), the crack areas produced under the two dual-wellbore conditions (H3 and H4) are actually slightly smaller. The presence of dual wellbores makes the XZ plane passing through the center of the sample the dominant failure plane, which is more conducive to achieving directional fracturing. The rapid penetration and severe damage of this failure plane also allow the high-energy gas within the wellbore to escape swiftly, which may explain why only a single fracture is observed in sample D3. For sample D3, a larger powder charge makes the wellbore pressure rapidly increase to its peak value, followed by a swift decline in pressure upon the formation of the main fracture. Although the peak pressure during this process is high, its duration is relatively short [45]. In contrast, a smaller powder charge in the sample D4 maintains a relatively lower wellbore pressure for a longer duration, while still remaining significantly above the rock’s fracture initiation stress, thereby resulting in the generation of more cracks.

In general, for hydraulic fracturing, dual wellbores can generate more cracks; meanwhile, for deflagration fracturing, the configuration of dual wellbores primarily aids in achieving directional fracturing. Merely increasing the number of wellbores does not necessarily lead to better fracturing effects. Instead, it is crucial to consider the wellbore locations and other fracturing parameters in combination to generate more cracks in the rocks.

5. Conclusions

To evaluate the rock-breaking effect of various fracturing methods on granite, this study employs a self-developed high-temperature and high-pressure model testing platform to conduct hydraulic fracturing and deflagration fracturing experiments on large-sized 40 × 40 × 40 cm³ granite specimens under different conditions. Based on an analysis of the fracture morphology, fluid pressure, and other parameters, and supplemented by the numerical simulation results from the fracturing process, this paper draws the following conclusions.

• From the perspective of increasing the reservoir capacity, the rock-breaking effect of deflagration fracturing is significantly better than that of hydraulic fracturing. Moreover, the crack propagation in deflagration fracturing is almost unaffected by the geostress difference resulting from external loads. In contrast, the damage caused by hydraulic fracturing is mainly distributed along the direction of the maximum principal stress.
• Temperature has a significant impact on the fracturing process of granite. Essentially, the microcracks generated within high-temperature rocks change their own mechanical properties, thereby influencing the fracturing process. During hydraulic fracturing, more localized damage zones will emerge in high-temperature rocks, and high temperatures tend to make the cracks produced by deflagration fracturing more regular.
• Compared to a single wellbore, dual wellbores in hydraulic fracturing result in greater internal damage to the specimen. However, for deflagration fracturing, the configuration of dual wellbores exerts minimal influence on the overall rock-breaking effect, beyond enabling directional fracture propagation.
• In summary, within EGS projects, the characteristics of both deflagration and hydraulic fracturing can be combined. Initially, deflagration fracturing can be utilized to create directional main fractures in the fractured section of the reservoir, addressing the issue of hydraulic fracturing, which tends to form main fractures along the direction of maximum principal stress. Subsequently, established and cost-effective hydraulic fracturing techniques can be employed to create more secondary fractures and fracture zones, thereby achieving the purpose of increasing the reservoir storage capacity.