Experimental Study on the Initiation and Propagation of Multi-Cluster Hydraulic Fractures within One Stage in Horizontal Wells

Competitive propagation of fractures initiated from multiple perforation clusters is universal in hydraulic fracturing of unconventional reservoirs, which largely influences stimulation. However, the propagation mechanism of multi-fractures has not been fully revealed for the lack of a targeted laboratory observation. In this study, a physical simulation experiment system was developed for investigating the initiation and propagation of multi-cluster hydraulic fractures. Different from the traditional hydro-fracking test system, the new one was equipped with a multi-channel shunting module and a strain monitoring system, which could guarantee the full fracture extension at each perforation clusters and measure the internal deformation of specimens, respectively. Several groups of true tri-axial fracturing tests were performed, considering the factors of in situ stress, cluster spacing, pumping rate, and bedding structures. The results showed that initiation of multi-cluster hydraulic fractures within one stage could be simultaneous or successive according to the difference of the breakdown pressure and fracturing fluid injection. For simultaneous initiation, the breakdown pressure of the subsequent fracture was lower than or equal to the value of the previous fracture. Multiple fractures tended to attract and merge. For successive initiation, the breakdown pressures of fractures were gradually increasing. The subsequent fracture tended to intersect with or deviated from the previous fracture. Multiple fractures interaction was aggravated by the decrease of horizontal stress difference, bedding number and cluster spacing, and weakened by the increase of pump rate. The propagation area of multiple fractures increased with the pump rate, decreased with the cluster spacing. The strain response characteristics corresponded with the initiation and propagation of fracture, which was conducive to understanding the process of the fracturing. The test results provide a basis for optimum design of hydraulic fracturing.

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A multi-perforation staged fracturing experimental study on hydraulic fracture initiation and propagation

Energy Exploration & Exploitation ◽

10.1177/0144598720914991 ◽

2020 ◽

Vol 38 (6) ◽

pp. 2466-2484

Author(s):

Jianguang Wei ◽

Saipeng Huang ◽

Guangwei Hao ◽

Jiangtao Li ◽

Xiaofeng Zhou ◽

...

Keyword(s):

Hydraulic Fracturing ◽

Hydraulic Fracture ◽

Oil And Gas ◽

Fracture Initiation ◽

Fracture Network ◽

Hydraulic Fractures ◽

Breakdown Pressure ◽

Tight Sandstone ◽

Heterogeneous Samples ◽

Initiation And Propagation

Hydraulic fracture initiation and propagation are extremely important on deciding the production capacity and are crucial for oil and gas exploration and development. Based on a self-designed system, multi-perforation cluster-staged fracturing in thick tight sandstone reservoir was simulated in the laboratory. Moreover, the technology of staged fracturing during casing completion was achieved by using a preformed perforated wellbore. Three hydraulic fracturing methods, including single-perforation cluster fracturing, multi-perforation cluster conventional fracturing and multi-perforation cluster staged fracturing, were applied and studied, respectively. The results clearly indicate that the hydraulic fractures resulting from single-perforation cluster fracturing are relatively simple, which is difficult to form fracture network. In contrast, multi-perforation cluster-staged fracturing has more probability to produce complex fractures including major fracture and its branched fractures, especially in heterogeneous samples. Furthermore, the propagation direction of hydraulic fractures tends to change in heterogeneous samples, which is more likely to form a multi-directional hydraulic fracture network. The fracture area is greatly increased when the perforation cluster density increases in multi-perforation cluster conventional fracturing and multi-perforation cluster-staged fracturing. Moreover, higher perforation cluster densities and larger stage numbers are beneficial to hydraulic fracture initiation. The breakdown pressure in homogeneous samples is much higher than that in heterogeneous samples during hydraulic fracturing. In addition, the time of first fracture initiation has the trend that the shorter the initiation time is, the higher the breakdown pressure is. The results of this study provide meaningful suggestions for enhancing the production mechanism of multi-perforation cluster staged fracturing.

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Fracture Initiation and Propagation in a Deep Shale Gas Reservoir Subject to an Alternating-Fluid-Injection Hydraulic-Fracturing Treatment

SPE Journal ◽

10.2118/195571-pa ◽

2019 ◽

Vol 24 (04) ◽

pp. 1839-1855 ◽

Cited By ~ 12

Author(s):

Bing Hou ◽

Zhi Chang ◽

Weineng Fu ◽

Yeerfulati Muhadasi ◽

Mian Chen

Keyword(s):

Hydraulic Fracturing ◽

Shale Gas ◽

Fracture Initiation ◽

Fluid Injection ◽

Hydraulic Fractures ◽

Stress Difference ◽

Gas Reservoirs ◽

Complex Fracture ◽

Initiation And Propagation

Summary Deep shale gas reservoirs are characterized by high in-situ stresses, a high horizontal-stress difference (12 MPa), development of bedding seams and natural fractures, and stronger plasticity than shallow shale. All of these factors hinder the extension of hydraulic fractures and the formation of complex fracture networks. Conventional hydraulic-fracturing techniques (that use a single fluid, such as guar fluid or slickwater) do not account for the initiation and propagation of primary fractures and the formation of secondary fractures induced by the primary fractures. For this reason, we proposed an alternating-fluid-injection hydraulic-fracturing treatment. True triaxial hydraulic-fracturing tests were conducted on shale outcrop specimens excavated from the Shallow Silurian Longmaxi Formation to study the initiation and propagation of hydraulic fractures while the specimens were subjected to an alternating fluid injection with guar fluid and slickwater. The initiation and propagation of fractures in the specimens were monitored using an acoustic-emission (AE) system connected to a visual display. The results revealed that the guar fluid and slickwater each played a different role in hydraulic fracturing. At a high in-situ stress difference, the guar fluid tended to open the transverse fractures, whereas the slickwater tended to activate the bedding planes as a result of the temporary blocking effect of the guar fluid. On the basis of the development of fractures around the initiation point, the initiation patterns were classified into three categories: (1) transverse-fracture initiation, (2) bedding-seam initiation, and (3) natural-fracture initiation. Each of these fracture-initiation patterns had a different propagation mode. The alternating-fluid-injection treatment exploited the advantages of the two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs; therefore, we concluded that this method is an efficient way to enhance the stimulated reservoir volume compared with conventional hydraulic-fracturing technologies.

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An Environment Friendly Approach to Reduce the Breakdown Pressure of High Strength Unconventional Rocks by Cyclic Hydraulic Fracturing

Journal of Energy Resources Technology ◽

10.1115/1.4045317 ◽

2019 ◽

Vol 142 (4) ◽

Cited By ~ 4

Author(s):

Zeeshan Tariq ◽

Mohamed Mahmoud ◽

Abdulazeez Abdulraheem ◽

Dhafer Al-Shehri ◽

Mobeen Murtaza

Keyword(s):

Hydraulic Fracturing ◽

High Strength ◽

Progressive Increase ◽

New Method ◽

Hydraulic Fractures ◽

Breakdown Pressure ◽

Environment Friendly ◽

Rock Types ◽

Pressurization Rate ◽

Tight Rocks

Abstract Unconventional hydrocarbon resources mostly found in highly stressed, overpressured, and deep formations, where the rock strength and integrity are very high. When fracturing these kinds of rocks, the hydraulic fracturing operation becomes much more challenging and difficult and in some cases reaches to the maximum pumping capacity limits without generating any fracture. This reduces the operational gap to optimally place the hydraulic fractures. Current stimulation methods to reduce the fracture pressures involvement with adverse environmental effects and high costs due to the entailment of water mixed with huge volumes of chemicals. In this study, a new environment friendly approach to reduce the breakdown pressure of the unconventional rock is presented. The new method incorporates the injection of chemical-free fracturing fluid in a series of cycles with a progressive increase of the pressurization rate in each cycle. This study is carried out on different cement blocks with varying petrophysical and mechanical properties to simulate real rock types. The results showed that the new method of cyclic fracturing can reduce the breakdown pressure to 24.6% in ultra-tight rocks, 19% in tight rocks, and 14.8% in medium- to low-permeability rocks. This reduction in breakdown pressure helped to overcome the operational challenges in the field and makes the fracturing operation much greener.

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Numerical Investigation on the Mechanism of Rock Directional Fracturing Method Controlled by Hydraulic Fracturing in Dense Linear Multiholes

Shock and Vibration ◽

10.1155/2020/6624047 ◽

2020 ◽

Vol 2020 ◽

pp. 1-10

Author(s):

Qingying Cheng ◽

Bingxiang Huang ◽

Xinglong Zhao

Keyword(s):

Hydraulic Fracturing ◽

Pore Pressure ◽

Water Pressure ◽

Ground Control ◽

Hydraulic Fractures ◽

Seepage Water ◽

Concentration Zone ◽

Propagation Process ◽

Directional Hydraulic Fracturing ◽

Initiation And Propagation

Rock directional fracturing is one of the difficult problems in deep mines. Directional fracturing controlled by hydraulic fracturing in dense linear multiboreholes is a novel directional fracturing technology of rock mass, which has been applied to the ground control in mines. In this paper, a physical model experiment was performed to study the fracture propagation process between multiboreholes. The results show that the intersecting of fractures between boreholes caused the sharp fluctuation of injecting water pressure. A directional fracturing plane was formed along with the direction of boreholes layout, and the surface of the fracturing plane is relatively flat. The dynamic initiation and propagation process of cracks between boreholes during directional hydraulic fracturing were simulated. The evolution of poroelastic stress and pore pressure between multiboreholes was analyzed. The numerical results indicated that a poroelastic stress concentration zone and pore pressure increase zone appeared between boreholes in the direction of boreholes layout. The pore pressure distribution is generally an elliptical seepage water pressure zone with the long axis along the direction of the boreholes layout. After the hydraulic fractures are initiated along the direction of the boreholes layout, the poroelastic stress on both sides of fractures decreases.

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Effect of Joint Geometrical Parameters on Hydraulic Fracture Network Propagation in Naturally Jointed Shale Reservoirs

Geofluids ◽

10.1155/2018/1852604 ◽

2018 ◽

Vol 2018 ◽

pp. 1-23 ◽

Cited By ~ 2

Author(s):

Zhaohui Chong ◽

Qiangling Yao ◽

Xuehua Li

Keyword(s):

Hydraulic Fracturing ◽

Hydraulic Fracture ◽

Fracture Network ◽

Hydraulic Fractures ◽

Geometrical Parameters ◽

Breakdown Pressure ◽

Opening Mode ◽

Hydraulic Fracture Network ◽

Shale Reservoirs ◽

Network Pattern

The presence of a significant amount of discontinuous joints results in the inhomogeneous nature of the shale reservoirs. The geometrical parameters of these joints exert effects on the propagation of a hydraulic fracture network in the hydraulic fracturing process. Therefore, mechanisms of fluid injection-induced fracture initiation and propagation in jointed reservoirs should be well understood to unleash the full potential of hydraulic fracturing. In this paper, a coupled hydromechanical model based on the discrete element method is developed to explore the effect of the geometrical parameters of the joints on the breakdown pressure, the number and proportion of hydraulic fractures, and the hydraulic fracture network pattern generated in shale reservoirs. The microparameters of the matrix and joint used in the shale reservoir model are calibrated through the physical experiment. The hydraulic parameters used in the model are validated through comparing the breakdown pressure derived from numerical modeling against that calculated from the theoretical equation. Sensitivity analysis is performed on the geometrical parameters of the joints. Results demonstrate that the HFN pattern resulting from hydraulic fracturing can be roughly divided into four types, i.e., crossing mode, tip-to-tip mode, step path mode, and opening mode. As β (joint orientation with respect to horizontal principal stress in plane) increases from 0° to 15° or 30°, the hydraulic fracture network pattern changes from tip-to-tip mode to crossing mode, followed by a gradual decrease in the breakdown pressure and the number of cracks. In this case, the hydraulic fracture network pattern is controlled by both γ (joint step angle) and β. When β is 45° or 60°, the crossing mode gains dominance, and the breakdown pressure and the number of cracks reach the lowest level. In this case, the HFN pattern is essentially dependent on β and d (joint spacing). As β reaches 75° or 90°, the step path mode is ubiquitous in all shale reservoirs, and the breakdown pressure and the number of the cracks both increase. In this case, β has a direct effect on the HFN pattern. In shale reservoirs with the same β, either decrease in k (joint persistency) and e (joint aperture) or increase in d leads to the increase in the breakdown pressure and the number of cracks. It is also found that changes in d and e result in the variation in the proportion of different types of hydraulic fractures. The opening mode of the hydraulic fracture network pattern is observed when e increases to 1.2 × 10−2 m.

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Modeling Simultaneous Multiple Fracturing Using the Combined Finite-Discrete Element Method

Geofluids ◽

10.1155/2018/4252904 ◽

2018 ◽

Vol 2018 ◽

pp. 1-20 ◽

Cited By ~ 1

Author(s):

Quansheng Liu ◽

Lei Sun ◽

Pingli Liu ◽

Lei Chen

Keyword(s):

Hydraulic Fracturing ◽

Solid Phase ◽

Horizontal Wells ◽

In Situ Stress ◽

Gas Production ◽

Hydraulic Fractures ◽

Multiple Fractures ◽

Fracture Geometry ◽

Stress Shadow ◽

Oil Gas

Simultaneous multiple fracturing is a key technology to facilitate the production of shale oil/gas. When multiple hydraulic fractures propagate simultaneously, there is an interaction effect among these propagating hydraulic fractures, known as the stress-shadow effect, which has a significant impact on the fracture geometry. Understanding and controlling the propagation of simultaneous multiple hydraulic fractures and the interaction effects between multiple fractures are critical to optimizing oil/gas production. In this paper, the FDEM simulator and a fluid simulator are linked, named FDEM-Fluid, to handle hydromechanical-fracture coupling problems and investigate the simultaneous multiple hydraulic fracturing mechanism. The fractures propagation and the deformation of solid phase are solved by FDEM; meanwhile the fluid flow in the fractures is modeled using the principle of parallel-plate flow model. Several tests are carried out to validate the application of FDEM-Fluid in hydraulic fracturing simulation. Then, this FDEM-Fluid is used to investigate simultaneous multiple fractures treatment. Fractures repel each other when multiple fractures propagate from a single horizontal well, while the nearby fractures in different horizontal wells attract each other when multiple fractures propagate from multiple parallel horizontal wells. The in situ stress also has a significant impact on the fracture geometry.

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Chemically-Induced Pressure Pulse: Fracturing Competent Reservoirs

10.2118/202126-ms ◽

2021 ◽

Author(s):

Ayman R. Al-Nakhli ◽

Zeeshan Tariq ◽

Mohamed Mahmoud ◽

Abdulazeez Abdulraheem

Keyword(s):

Hydraulic Fracturing ◽

Rock Fracture ◽

Applied Pressure ◽

Hydraulic Fractures ◽

Breakdown Pressure ◽

Hydrocarbon Production ◽

Fracture Pressure ◽

Micro Cracks ◽

Chemically Induced ◽

Fracturing Experiments

Abstract Commercial volumes of hydrocarbon production from tight unconventional reservoirs need massive hydraulic fracturing operations. Tight unconventional formations are typically located inside deep and over-pressured formations where the rock fracture pressure with slickwater becomes so high because of huge in situ stresses. Therefore, several lost potentials and failures were recorded because of high pumping pressure requirements and reservoir tightness. In this study, thermochemical fluids are introduced as a replacement for slickwater. These thermochemical fluids are capable of reducing the rock fracture pressure by generating micro-cracks and tiny fractures along with the main hydraulic fractures. Thermochemical upon reaction can generate heat and pressure simultaneously. In this study, several hydraulic fracturing experiments in the laboratory on different synthetic cement samples blocks were carried out. Cement blocks were made up of several combinations of cement and sand ratios to simulate real rock scenarios. Results showed that fracturing with thermochemical fluids can reduce the breakdown pressure of the cement blocks by 30%, while applied pressure was reduced up to 88%, when using thermochemical fluid, compared to slickwater. In basins with excessive tectonic stresses, the current invention can become an enabler to fracture and stimulate well stages which otherwise left untreated. A new methodology is developed to lower the breakdown pressure of such reservoirs, and enable fracturing. Keywords: Unconventional formation; breakdown pressure; thermochemicals; micro fractures.

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Experimental Study on Injection Pressure Response and Fracture Geometry during Temporary Plugging and Diverting Fracturing

SPE Journal ◽

10.2118/199893-pa ◽

2020 ◽

Vol 25 (02) ◽

pp. 573-586 ◽

Cited By ~ 2

Author(s):

Bo Wang ◽

Fujian Zhou ◽

Chen Yang ◽

Daobing Wang ◽

Kai Yang ◽

...

Keyword(s):

Injection Pressure ◽

Fracture Network ◽

Pressure Response ◽

Hydraulic Fractures ◽

Fracturing Fluid ◽

Breakdown Pressure ◽

Multiple Fractures ◽

Differential Stress ◽

Fracture Geometry ◽

Stimulated Reservoir Volume

Summary Temporary plugging and diverting fracturing (TPDF) has become one of the fastest-growing techniques to maximize the stimulated reservoir volume (SRV). During the field operation of TPDF, diverters are injected to redirect the hydraulic fractures into the under-stimulated region of the reservoir, and, thus, to obtain better coverage of the created fracture network. In this study, the commonly used true tri-axial hydraulic fracturing system is modified to investigate the influences of various factors on the injection pressure response and resultant fracture geometry during diversion treatments. The experimental results show the feasibility of creating multiple fractures through TPDF, and more importantly give the following findings: (1) a complex diverted fracture network tends to be created at a small differential stress (2.5 MPa in this case), while diverted fractures tend to grow parallel to the initial fractures at a high differential stress (7.5 MPa in this case); (2) with the same concentration in the fracturing fluid, 40-mesh powder-shaped diverters can plug the created fractures and increase the net pressure more rapidly than 6-mm fiber-shaped diverters; (3) an excess of diverters can lead to a strong injection pressure response, and, thus, enhance the difficulty of creating multiple fractures; (4) when diverters are injected with the fracturing fluid, no obvious breakdown pressure or propagation pressure is shown during the fracture propagation.

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Influence of Finite Hydraulic-Fracture Conductivity on Unconventional Hydrocarbon Recovery With Horizontal Wells

SPE Journal ◽

10.2118/187947-pa ◽

2017 ◽

Vol 22 (06) ◽

pp. 1790-1807 ◽

Cited By ~ 1

Author(s):

Deming Mao ◽

David S. Miller ◽

John M. Karanikas ◽

Ed A. Lake ◽

Phillip S. Fair ◽

...

Keyword(s):

Hydraulic Fracturing ◽

Horizontal Wells ◽

Normal Derivative ◽

Unconventional Reservoirs ◽

Analytical Models ◽

Hydraulic Fractures ◽

Multiple Fractures ◽

Hydraulic Stimulation ◽

Fracture Conductivity ◽

Hydrocarbon Recovery

Summary The classic plots of dimensionless fracture conductivity (CfD) vs. equivalent wellbore radius or equivalent negative skin are useful for evaluating the performance of hydraulic fractures (HFs) in vertical wells targeting conventional reservoirs (Prats 1961; Cinco-Ley and Samaniego-V. 1981). The increase in well productivity after hydraulic stimulation can be estimated from the “after fracturing” effective wellbore radius or from the “after fracturing” equivalent negative skin. However, this earlier work does not apply to the case of horizontal wells with multiple fractures. A revision of the diagnostic plots is needed to account for the combination of the resulting radial-flow regime and the transient effect in unconventional reservoirs with ultralow permeability. This paper reviews and extends this earlier work with the objective of making it applicable in the case of horizontal wells with multiple fractures. It also demonstrates practical application of this new technique for fracture-design optimization for horizontal wells. The influence of finite fracture conductivity (FC) on the HF flow efficiency is evaluated through analytical models, and it is confirmed by a 3D transient numerical-reservoir simulation. This work demonstrates that a redefined dimensionless fracture conductivity for horizontal wells CfD,h = 4 is found to be optimal by use of the maximum of log-normal derivative (subject to economics) for HFs in horizontal wells, and this value of CfD,h can provide 50% of the fracture-flow efficiency and 90% of the estimated ultimate recovery (EUR) that would have been obtained from an infinitely conductive fracture for the same production period. This new master plot can provide guidance for hydraulic-fracturing design and its optimization for hydrocarbon recovery in unconventional reservoirs through hydraulic fracturing in horizontal wells.

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A New Method to Improve the Fracturing Effect of Coal Seams by Using Preset Slots and Induced Stress Shadows

Geofluids ◽

10.1155/2021/5564572 ◽

2021 ◽

Vol 2021 ◽

pp. 1-17

Author(s):

Li Li ◽

Lei Zhou ◽

Honglian Li ◽

Binwei Xia ◽

Junping Zhou

Keyword(s):

Coal Seam ◽

Hydraulic Fracturing ◽

Fracture Network ◽

Coal Bed ◽

Hydraulic Fractures ◽

Large Area ◽

Multiple Fractures ◽

Complex Fracture ◽

Induced Stress ◽

Complex Fracture Network

Efficient extraction of coal bed methane before coal mining is essential to eliminate the risk of coal-gas outbursts. However, stimulation technologies should be implemented to enhance the conductivity of the coal seam. In this study, we propose a novel method to create a complex fracture network in underground coal mines with the integration of multiple hydraulic slotting and hydraulic fracturing. In this method, hydraulic slots are used to direct hydraulic fractures and initialize branch fractures, while hydraulic fracturing is used to extend the fractures. Given the mutually exclusive and attractive propagation of multiple fractures, a relatively evenly distributed fracture network can be generated. The results show that (1) the dynamically induced stress shadows of hydraulic fractures can cause exclusive and attractive propagation of multiple hydraulic fractures; (2) a preset slot that deviates from the principal stress can direct hydraulic fractures to a certain extent and generate branch fractures; and (3) with a staggered distribution of preset slots, a relatively large volume of the coal seam in both the minimum and maximum horizontal stress directions can be stimulated, creating a complex fracture network including many vertical branch fractures and a large area of horizontally layered directional fractures.

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