Radial Boreholes Research Articles

Laboratory investigation on fracture initiation and propagation behaviors of hot dry rock by radial borehole fracturing

Creating complex and interconnected fracture networks between injection and production wells is crucial for exploiting hot dry rock (HDR) geothermal energy. However, the simple planar fractures created by conventional hydraulic fracturing, primarily controlled by in situ stress, fail to connect directionally with the target well. This study proposes a novel stimulation method, i.e. radial borehole fracturing, which shows great potential for guiding the directional propagation of fractures. The fracture initiation and propagation behaviors of high-temperature granite under radial borehole fracturing are investigated and compared with those of conventional fracturing. Three-dimensional morphological scanning and reinjection tests are used to quantitatively evaluate fracturing performance. Additionally, the influences of key parameters, including rock temperature, in situ stress, injection rate, fluid viscosity, azimuth of the radial borehole, and the number of radial boreholes on the fracture morphology and breakdown pressure are investigated. The results show that radial borehole fracturing can effectively guide the initiation and propagation of fractures along the radial borehole. The breakdown pressure of radial borehole fracturing can be reduced by 14.1%-43.7% compared to conventional fracturing. A higher fluid-rock temperature difference reduces the directional propagation range of fractures guided by the radial borehole. Increases in the vertical density of radial boreholes, injection rate, and fluid viscosity enhance the guiding ability of radial boreholes. Furthermore, there is a competitive relationship between in situ stress and the azimuth of radial boreholes in controlling fracture propagation. This research provides a viable alternative for the directional connection of injection-production wells in HDR reservoirs.

Enhancing Deposit Exploitation Efficiency Utilizing Small-Diameter Radial Boreholes via Hydraulic Drilling Nozzles for Optimal Resource Recovery

The exploration and development of new hydrocarbon deposits is facing increasing challenges as the global shift to renewable energy sources, such as shallow geothermal deposits, wind farms, and photovoltaics, reduces the dependence on hydrocarbons. To navigate this evolving landscape, it becomes crucial to find solutions that optimize the energy extraction efficiency while maximizing the use of hydrocarbon deposits. This requires exploring opportunities in existing fields and wells, including those slated for decommissioning. This article discusses the potential for extracting resources from seemingly depleted fields, where some 60–70% of the resources remain unrecoverable due to low reservoir energy. Meeting this challenge requires the implementation of secondary and tertiary EOR methods that involve the introduction of external energy to increase reservoir pressure and enhance resource recovery. One of the proposed innovative tertiary methods involves reaming the reservoir using multiple small-diameter radial boreholes generated by a hydraulic drilling nozzle. This strategy is designed to intensify the contact between the production hole and the reservoir layer, resulting in increased or commenced production in certain cases. The described method proves to be a practical application in hydrocarbon deposits, offering the dual benefits of mitigating environmental pollution by eliminating the need for drilling new boreholes and providing a cost-effective means of accessing resources in decommissioned deposits with insufficient reservoir energy for self-exploitation. Another article points out the design variation of a hydraulic drilling nozzle tailored specifically for reaming a reservoir layer. Taking the above into account, this article provides very practical information for future projects in which paths should be sought for the design and development of hydraulic wellheads, among other things, in order to intensify the production from hydrocarbon deposits.

Multi-step Fracturing Radial Boreholes for Efficient Development of Unconventional Gas Reservoirs

Multi-step Fracturing Radial Boreholes for Efficient Development of Unconventional Gas Reservoirs

CO2-Enhanced Radial Borehole Development of Shale Oil: Production Simulation and Parameter Analysis

Shale oil resources, noted for their broad distribution and significant reserves, are increasingly recognized as vital supplements to traditional oil resources. In response to the high fracturing costs and swift decline in productivity associated with shale oil horizontal wells, this research introduces a novel approach utilizing CO2 for enhanced shale oil recovery in radial boreholes. A compositional numerical simulation method is built accounted for component diffusion, adsorption, and non-Darcy flow, to explore the viability of this technique. The study examines how different factors—such as initial reservoir pressure, permeability, numbers of radial boreholes, and their branching patterns—influence oil production and CO2 storage. Our principal conclusions indicate that with a constant CO2 injection rate, lower initial reservoir pressures predominantly lead to immiscible oil displacement, hastening the occurrence of CO2 gas channeling. Therefore, maintaining higher initial or injection pressures is critical for effective miscible displacement in CO2-enhanced recovery using radial boreholes. Notably, the adsorption of CO2 in shale oil results in the displacement of lighter hydrocarbons, an effect amplified by competitive adsorption. While CO2 diffusion tends to prompt earlier gas channeling, its migration towards areas of lower concentration within the reservoir reduces the extent of channeling CO2. Nonetheless, when reservoir permeability falls below 0.01 mD, the yield from CO2-enhanced recovery using radial boreholes is markedly low. Hence, selecting high-permeability “sweet spot” regions within shale oil reservoirs for the deployment of this method is advisable. To boost oil production, utilizing longer and broader radial boreholes, increasing the number of boreholes, or setting the phase angle to 0° are effective strategies. Finally, by comparing the production of shale oil enhanced by CO2 with that of a dual horizontal well fracturing system enhanced by CO2, it was found that although the former’s oil production is only 50.6% of the latter, its cost is merely 11.1%, thereby proving its economic viability. These findings present a new perspective for the economically efficient extraction of shale oil, offering potential guidance for industrial practices.

Example of thermal retrofitting for the Piedicastello tunnel

Over the last decades, global warming has become one of the major issues to cope with: as it stands, human activities are responsible for a global surface temperature increase of approximately 1.1°C since the pre-industrial age. Climate change is directly linked to the level of global warming, thus affecting different kind of regions around the world in multiple ways, leading not only to increments of heat extremes, but also to changes in rainfall patterns, sea level rise, amplification of the permafrost thawing, ocean acidification and much more [1].
 In this context, carbon dioxide emissions due to heating systems are one of the main enemies in the pathway for reaching the 1.5°C Paris climate goal and energy geostructures could play a relevant role. Among them, the thermal activation of tunnels has raised interest [2, 3, 4] and recent full scale projects have demonstrated the feasibility of such technology [5, 6]. However, so far applications have been related to new tunneling projects, whereas possibilities of implementation in the existing heritage of tunnels have not yet been investigated.
 The Authors are developing investigations for the thermal retrofitting feasibility of existing tunnels. In this abstract the attention is posed on the case study of the Piedicastello tunnel, located in the city of Trento, crossing the city centre through a 100 m high spur of limestone, called Doss Trento. This tunnel used to be part of the Italian roadway network but, following the construction of a new tunnel in 2007, it fell into disuse, being partly transformed into a museum one year later, i.e. the tunnel is now a closed environment.
 One of the design hypotheses identified for thermal retrofitting of the Piedicastello tunnel consists in taking advantage of its 100 m overburden by drilling radial borehole heat exchangers (rBHEs), arranged along several tunnel cross sections. In Figure 1a, a 3D view of the rBHEs solution is shown. This solution leaves the lining intrados visible for future inspection and is essentially unaffected by the internal aerothermal condition of the tunnel.
 In order to assess its thermal performance, some 3D numerical models were built with the Finite Element software Feflow [7]. The above-mentioned models are 200.0 m high and wide, with a thickness that varies as a function of the rBHEs cross section interaxis distance (Figure 1-b). The two horse-shoe shaped tubes of the Piedicastello tunnel have an external equivalent diameter of 12.7 m, with an 80 ÷ 130 cm thick concrete lining. Heat exchanger pipes, instead, were modelled through one-dimensional elements (“discrete features”), with a cross section of 201 mm2, corresponding to an external diameter of 20 mm and a thickness of 2 mm.
 Both thermal and hydraulic boundary conditions were set considering the lack of information about the groundwater regime of the Doss Trento and the internal aerothermal conditions of the Piedicastello tunnel. Nonetheless, given its unusual conditions (absence of moving vehicles and closed environment), the latter would likely have a minor role in the heat exchange process. Accordingly, setting adiabatic boundary conditions at the tunnel contour and considering a dry rock mass, a preliminary sensitivity analysis was performed with the aim of drawing some meaningful conclusions about two key design aspects of the rBHEs solution. To this purpose, three different borehole length setups (short – 12.50 m, mid – 18.75 m and long – 25.00 m) and three cross section interaxis distances (3.0 m, 5.0 m and 7.5 m) were investigated. The results of the preliminary numerical simulations are shown in Table 1.
 From the sensitivity analysis, it seems evident that, in the considered cases and ranges: (i) as expected, the longer the rBHEs, the higher the heat that the system can exploit, accordingly the rBHEs optimal length should be designed considering also the pumping and drilling costs, (ii) two consecutive instrumented cross sections have virtually no interaction if the interaxis distance is set higher than 5.0 m.
 Starting from these valuable evidences, future research will investigate: (i) the influence of the groundwater regime, as well as the internal aerothermal conditions of the tunnel, (ii) new alternatives for the thermal retrofitting of existing tunnels and (iii) the optimal rBHEs length with respect to a cost-benefit analysis that will consider both drilling and pumping costs.

Fracture geometry and breakdown pressure of radial borehole fracturing in multiple layers

Radial borehole fracturing that combines radial boreholes with hydraulic fracturing is anticipated to improve the output of tight oil and gas reservoirs. This paper aims to investigate fracture propagation and pressure characteristics of radial borehole fracturing in multiple layers. A series of laboratory experiments with artificial rock samples (395 mm × 395 mm × 395 mm) was conducted using a true triaxial fracturing device. Three crucial factors corresponding to the vertical distance of adjacent radial borehole layers (vertical distance), the azimuth and diameter of the radial borehole are examined. Experimental results show that radial borehole fracturing in multiple layers generates diverse fracture geometries. Four types of fractures are identified based on the connectivity between hydraulic fractures and radial boreholes. The vertical distance significantly influences fracture propagation perpendicular to the radial borehole axis. An increase in the vertical distance impedes fracture connection across multiple radial borehole layers and reduces the fracture propagation distance along the radial borehole axis. The azimuth also influences fracture propagation along the radial borehole axis. Increasing the azimuth reduces the guiding ability of radial boreholes, which makes the fracture quickly curve to the maximum horizontal stress direction. The breakdown pressure correlates with diverse fracture geometries observed. When the fractures connect multi-layer radial boreholes, increasing the vertical distance decreases the breakdown pressure. Decreasing the azimuth and increasing the diameter also decrease the breakdown pressure. The extrusion force exists between the adjacent fractures generated in radial boreholes in multiple rows, which plays a crucial role in enhancing the guiding ability of radial boreholes and results in higher breakdown pressure. The research provides valuable theoretical insights for the field application of radial borehole fracturing technology in tight oil and gas reservoirs.

CO2 flooding in shale oil reservoir with radial borehole fracturing for CO2 storage and enhanced oil recovery

This study introduces a novel method integrating CO2 flooding with radial borehole fracturing for enhanced oil recovery and CO2 underground storage, a solution to the limited vertical stimulation reservoir volume in horizontal well fracturing. A numerical model is established to investigate the production rate, reservoir pressure field, and CO2 saturation distribution corresponding to changing time of CO2 flooding with radial borehole fracturing. A sensitivity analysis on the influence of CO2 injection location, layer spacing, pressure difference, borehole number, and hydraulic fractures on oil production and CO2 storage is conducted. The CO2 flooding process is divided into four stages. Reductions in layer spacing will significantly improve oil production rate and gas storage capacity. However, serious gas channeling can occur when the spacing is lower than 20 m. Increasing the pressure difference between the producer and injector, the borehole number, the hydraulic fracture height, and the fracture width can also increase the oil production rate and gas storage rate. Sensitivity analysis shows that layer spacing and fracture height greatly influence gas storage and oil production. Research outcomes are expected to provide a theoretical basis for the efficient development of shale oil reservoirs in the vertical direction.

CO2 huff-n-puff combined with radial borehole fracturing to enhance oil recovery and store CO2 in a shale oil reservoir

This study pioneers the integration of CO2 huff-n-puff and radial borehole fracturing, enhancing shale oil recovery and facilitating CO2 storage. To evaluate the performance of CO2 huff-n-puff with radial borehole fracturing, a numerical compositional model is developed, enabling a comparison of CO2 huff-n-puff performance between radial borehole and horizontal well fracturing, while assessing the influence of various parameters, including CO2 diffusion, lateral configuration, fracture parameters, cycle number, soak duration, production time, injection pressure, and production pressure on oil and gas production, as well as CO2 storage. The results indicate that increasing the number of layers in radial borehole construction can effectively enhance the vertical exploitation efficiency of oil reservoirs. The cumulative oil production of the three-layer, four-branch radial borehole fracturing pattern is 1.66 times higher than that of the horizontal fracturing with 20 fractures. Moreover, it also identifies the principle of enhancing the number of radial boreholes per layer and increasing fracture length to optimize horizontal reservoir exploitation. A 5% reduction in individual cyclic duration from the base case significantly influences the initial high-flow phase, yielding a hydrocarbon production 1.35 times higher. Further strategies to enhance CO2 huff-n-puff efficacy include diminishing the soak stage's proportion and adjusting the temporal proportion of the production stage to 75% for oil extraction and 30% for CO2 storage. Increasing the injection pressure impacts the storage capacity of CO2 significantly, whereas reducing production pressure affects oil and gas production. The findings of this study present a novel method and provide a theoretical foundation for the efficient and economical development of shale oil.

Numerical simulation of three-dimensional fracturing fracture propagation in radial wells

A fracture propagation model of radial well fracturing is established based on the finite element-meshless method. The model considers the coupling effect of fracturing fluid flow and rock matrix deformation. The fracture geometries of radial well fracturing are simulated, the induction effect of radial well on the fracture is quantitatively characterized, and the influences of azimuth, horizontal principle stress difference, and reservoir matrix permeability on the fracture geometries are revealed. The radial wells can induce the fractures to extend parallel to their axes when two radial wells in the same layer are fractured. When the radial wells are symmetrically distributed along the direction of the minimum horizontal principle stress with the azimuth greater than 15°, the extrusion effect reduces the fracture length of radial wells. When the radial wells are symmetrically distributed along the direction of the maximum horizontal principal stress, the extrusion increases the fracture length of the radial wells. The fracture geometries are controlled by the rectification of radial borehole, the extrusion between radial wells in the same layer, and the deflection of the maximum horizontal principal stress. When the radial wells are distributed along the minimum horizontal principal stress symmetrically, the fracture length induced by the radial well decreases with the increase of azimuth; in contrast, when the radial wells are distributed along the maximum horizontal principal stress symmetrically, the fracture length induced by the radial well first decreases and then increases with the increase of azimuth. The fracture length induced by the radial well decreases with the increase of horizontal principal stress difference. The increase of rock matrix permeability and pore pressure of the matrix around radial wells makes the inducing effect of the radial well on fractures increase.

Productivity enhancement in multilayered coalbed methane reservoirs by radial borehole fracturing

Coalbed methane (CBM) is an important unconventional natural gas. Exploitation of multilayered CBM reservoir is still facing the challenge of low production rate. Radial borehole fracturing, which integrates radial jet drilling and hydraulic fracturing, is expected to create complex fracture networks in multilayers and enhance CBM recovery. The main purpose of this paper is to investigate the mechanisms and efficacy of radial borehole fracturing in increasing CBM production in multiple layers. First, a two-phase flow and multi-scale 3D fracture network including radial laterals, hydraulic fractures and face/butt cleats model is established, and embedded discrete fracture model (EDFM) is applied to handle the complex fracture networks. Then, effects of natural-fracture nonuniform distribution are investigated to show the advantages of targeted stimulation for radial borehole fracturing. Finally, two field CBM wells located in eastern Yunnan-western Guizhou, China were presented to illuminate the stimulation efficiency by radial borehole fracturing. The results indicated that compared with vertical well fracturing, radial borehole fracturing can achieve higher gas/water daily production rate and cumulative gas/water production, approximately 2 times higher. Targeted communications to cleats and sweet spots and flexibility in designing radial borehole parameters in different layers so as to increase fracture-network complexity and connectivity are the major reasons for production enhancement of radial borehole fracturing. Furthermore, the integration of geology-engineering is vital for the decision of radial borehole fracturing designing scheme. The key findings of this paper could provide useful insights towards understanding the capability of radial borehole fracturing in developing CBM and coal-measure gas in multiple-thin layers.

Experimental investigation on the breakdown pressure and fracture propagation of radial borehole fracturing

Radial borehole fracturing is a technology combing hydraulic fracturing and slender boreholes which are radially drilled from the main wellbore by water jets. This paper aims to investigate how the radial boreholes influence the fracture extension when they are pressurized by the fracturing fluids. Concrete blocks containing radial boreholes were cast, the mechanical properties of such blocks were tested, and these samples were hydraulically fractured through a laboratory tri-axial fracturing apparatus. Experimental results show that increasing the azimuths of the radial boreholes enhances the breakdown pressures, but it will reduce the fracture extension distances (deviation distances) paralleling to the radial borehole axes. Similar trends appear when the horizontal stress differences diminish and the distances between adjacent layers of radial borehole axes increase. When changing the numbers of the radial boreholes, the variations of the breakdown pressures depend on the layouts of the radial boreholes. Besides, increasing the pump rates lifts the breakdown pressures and enhances the deviation distances. Compared to perforation fracturing, radial borehole fracturing has higher breakdown pressures and stronger control to fracture propagation. It is concluded that the extrusion forces exist in the horizontal zones between the radial boreholes. A conceptual model is proposed which can explain the variations of the deviation distances. When the injection time of the fracturing fluids is identical, the higher breakdown pressures will decrease the deviation distances because of the extrusion forces. There is a balance between the lower breakdown pressures and the higher pore pressures both caused by the more penetration of the fracturing fluid, which can either increase the deviation distances or reduce the deviation distances. • The variations of breakdown pressures and fracture geometries were concluded. • The existence of the extrusion force between radial boreholes was proved. • A conceptual model explaining the variation of fracture geometries was proposed.

Co2 Flooding in Shale Oil Reservoir with Radial Borehole Fracturing for Co2 Storage and Enhanced Oil Recovery

Co2 Flooding in Shale Oil Reservoir with Radial Borehole Fracturing for Co2 Storage and Enhanced Oil Recovery

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

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

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

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

Fracture Initiation Mechanisms of Multibranched Radial-Drilling Fracturing

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

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

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

A Method of Quick and Safe Coal Uncovering by Hydraulic Fracturing in a Multibranch Radial Hole with a Coalbed Methane Well

Coal and gas outburst accidents occur most frequently during coal uncovering operation, and uncovering-induced outburst accidents have caused great economic losses and casualties. In order to quickly and safely uncover the outburst coal seam at the air intake shaft in panel 2, Zhaozhuang Coal Mine has come up with the technology of multibranch radial borehole hydraulic fracturing (MRBHF) coal uncovering technology in the surface coalbed methane (CBM) well. By conducting fracturing of constructed radial boreholes in the CBM well, the technology increases fractures, enhancing coal permeability and raising gas drainage volume in the uncovering area. It brings the following three major benefits: (1) uncovering coal efficiently, rapidly, and safely; (2) greatly reducing engineering quantity, coal uncovering period, and construction cost, i.e., significantly improving the economic benefits; and (3) efficiently reusing the CBM well. Compared with conventional uncovering measures, the proposed technology shortens the uncovering period by 118 d, enhances the coal seam permeability by 3.55 times, raises the gas drainage rate by 22%, increases the gas drainage volume by 1.93 times, and reduces the engineering volume of drilling by 12,000 m. The safe, rapid, economical, and applicable technology provides scientific guidance and reference for domestic and foreign shaft coal uncovering engineering and possesses important economic and social values and application prospect for safe and efficient production of coal mines.

Research and Application of Radial Borehole Fracturing Based on Numerical Simulation

The radial borehole fracturing technology has been applied in a certain number of oilfields with good results being achieved. However, the morphology and variation of fracture still require further study. In this paper, the reservoir model based on formation fluid-solid coupling equation is established with the extended finite element method (XFEM) in ABAQUS, and the fracture morphologies in the single-radial borehole, vertical multiradial borehole, and horizontal multiradial borehole are simulated and analyzed with criteria of maximum principal stress and maximum energy release rate as the damage mechanism. Moreover, the accuracy of numerical simulation results is verified with the large-scale true 3D physical simulation experiment. The results show that the induced stress field along the radial borehole during fracturing is the root cause of fracture directional propagation along the radial borehole whose effective guidance distance reaches 40 m. The vertical multiradial borehole can effectively enhance fracture directional propagation and is capable of reducing fracture initiation pressure. In the horizontal multiradial borehole, the major fracture propagating along each radial borehole is formed in the remote-borehole area, and the secondary fracture connecting the neighboring radial boreholes is formed in the near-borehole zone. Coordination of major and secondary fractures can effectively increase the drainage area and reduce the flow resistance in the near-borehole zone. Based on the research on fracture morphology of multiradial borehole fracturing, the scheme of radial borehole arrangement is optimized and verified through numerical simulation of deliverability. The final optimum borehole arrangement scheme is the intersectional angle of 45° between four orthogonal radial boreholes and horizontal maximum principal stress.

Calculation of the wellbore temperature and pressure distribution during supercritical CO2 fracturing flowback process

Supercritical CO2 fracturing is a new type of waterless technology developed in recent years. When the supercritical CO2 flows back after fracturing, the formation water is lifted by the upward movement of CO2. Since the pressure gradually decreases along the wellbore and the temperature also decreases due to the CO2 expansion, it is easy to result in the formation of hydrate under the conditions of high pressure and low temperature. In order to prevent the clogging of the wellbore due to CO2 hydrate, the wellbore temperature and pressure need to be predicted and regulated. Based on the Span-Wagner CO2 gas state equation and the Fenghour gas transport equation, combined with the classical wellbore flow heat transfer model, a supercritical CO2 fracturing flowback wellbore flow model with heat source and sink considered is developed, the dual coupling solution of axial and radial borehole is realized by iterating the pressure and temperature and coupling of tubing-annulus-formation. The results show that the wellbore pressure and temperature both decrease from the bottom to the top of the well, which is similar to the flow in oil and gas production. The parameters such as discharge output, tubing size, formation temperature gradient and pressure gradient have a great influence on the wellbore temperature and pressure. The reduction of the discharge output and the increase of the tubing size can effectively keep the wellbore temperature high and reduce the risk of CO2 hydrate formation. The discharge time has almost no effect on the wellbore pressure, and only slightly affects the wellbore temperature. The results can provide guidance for the study of supercritical CO2 fracturing flowback.

Numerical simulation study on fracture propagation of zipper and synchronous fracturing in hydrogen energy development

Until now, hydraulic fracturing has played prominent role in increasing production of hydrogen energy such as natural gas, geothermal energy, natural gas hydrate. Hydrogen energy stimulation is realized by many methods. Such as multiple-clusters in staged fracturing, and fracture propagation guided by combined radial boreholes in different azimuths. In order to extend lateral stimulation area, the zipper fracturing is investigated continuously. However, the report on the zipper fracturing is limited to the post-frac productivity. In this study, model of simultaneous and zipper fracturing was established in ABAQUS to investigate the effect of six factors on the fracture propagation. The results showed that in simultaneous fracturing, the fracture network is formed spontaneously. With same brittleness of reservoir, fracture in zipper fracturing always propagates slightly longer than that in simultaneous fracturing. The results provide theoretically support for both fracturing modes, which helps design of well completion and fracturing operation parameters.