Transverse Hydraulic Fractures Research Articles

Multi-fracture growth behavior during TPDF in a horizontal well of multi-clustered perforations: An experimental research

Temporary plugging and diverting fracturing (TPDF), involving inner-fracture temporary plugging (IFTP) and inner-stage temporary plugging (ISTP), has been proposed as a widely applied technique in China, for promoting the uniform initiation and propagation of multi-clustered hydraulic fractures (HFs) in a horizontal well of the shale oil/gas reservoirs. However, how the key plugging parameters controlling the multi-fracture growth and the pumping pressure response during TPDF in shale with dense bedding planes (BPs) and natural fractures (NFs) is still unclear, which limits the optimization of TPDF scheme. In this paper, a series of TPDF simulation experiments within a stage of multi-cluster in a horizontal well were carried out on outcrops of Longmaxi Formation shale using a large-scale true tri-axial fracturing simulation system, combined with the acoustic emission (AE) monitor and computed tomography (CT) scanning techniques. Each experiment was divided into three stages, including the conventional fracturing (CF), IFTP and ISTP. Multi-fracture initiation and propagation behavior, and the dominant controlling parameters were examined, containing the particle sizes, concentration of temporary plugging agent (TPA), and cluster number. The results showed that the number of transverse HFs (THFs) and the overall complexity of fracture morphology increase with the increase in TPA concentration and perforation cluster number. Obviously, the required concentration of TPA is positively correlated with the cluster number. Higher peak values and continuous fluctuations of pumping pressure during TPDF may indicate the creation of diversion fractures. The creation of standard THFs during CF is favorable to the creation of diversion fractures during TPDF. Moreover, the activation of BPs nearby the wellbore during CF is unfavorable to the subsequent pressure buildup during TPDF, resulting in poor plugging and diverting effect. Notably, under the strike-slip fault stress regime, the diversion of THFs is not likely during IFTP, which is similar as the results of ISTP to initiate mainly the un-initiated or under-propagated perforation clusters. Three typical pressure curve types during TPDF can be summarized to briefly identify the hydraulic fracture diversion effects, including good (multiple branches or/and THFs can be newly created), fair (HF initiation along the slightly opened BPs and then activating the NFs), and bad (HF initiation along the largely opened BPs and then connecting with the NFs).

Multi-Fracture Growth Law for Temporary Plugging and Diversion Fracturing of Horizontal Well with Multiple Clusters in Shale Reservoir

Temporary plugging and diversion fracturing (TPDF) is a common method to increase production and efficiency in shale gas reservoirs, but the growth law of diversion fractures and the temporary plugging mechanism are still unclear, which restricts the further optimization of temporary plugging fracturing schemes. Therefore, in this study, a series of simulation experiments of TPDF in a horizontal well with multi-clusters were carried out for the Longmaxi Shale outcrop by using a large true triaxial fracturing system. The laboratory method of “inner-fracture + inner-segment” TPDF with multiple clusters of perforation in horizontal wells was proposed, and the fracture initiation law and control factors, including the number of clusters and the method of perforating, were investigated. The experimental results show that the peak pressures of inner-fracture temporary plugging (IFTP) and inner-segment temporary plugging (ISTP) stages are higher, and the number of diversion fractures and the overall complexity of hydraulic fractures (HFs) are higher when the number of perforation clusters is five. The peak pressures of IFTP and ISTP do not increase significantly under the fixed-face perforating condition compared with the helical perforating, but the pressure profile fluctuates more frequently, the overall HF morphology is more complex, and the number of diversion fractures and transverse hydraulic fractures (THFs) is higher. In addition, the diversion of multi-fractures corresponds to a stage of frequent fluctuations in the wellhead pressure, during which the pressure reaches the peak and drops sharply, indicating the generation of diversion fractures with larger fracture widths that grow toward the surface of the rock sample. The results of the study provide a theoretical basis for the design of TPDF schemes in the laminar-rich Longmaxi Formation shale reservoir.

A Hybrid Data-Driven/Physics-Based Modeling Approach for Hydraulic Fracture Initiation and Early-Phase Propagation in Shale

Optimal hydraulic fracture (HF) initiation and early-phase propagation results in minimal near-wellbore tortuosity, decreasing the likelihood of screenouts and maximizing the resultant well productivity. While most predictive models for the HF geometry produced in a treatment consider the far-field region, the near-well vicinity should be an integral part of a properly-engineered reservoir stimulation strategy. In this work, a novel hybrid data-driven/physics-based approach is elaborated for modeling HF initiation and early-phase propagation from perforated horizontal wells.A treatment-optimization scheme via oriented perforating is presented using the developed hybrid model, considering the orientation of the induced HF initiation (longitudinal or transverse with respect to a well drilled along the minimum horizontal in-situ principal stress) and the resultant formation breakdown pressure (FBP). Transverse HF initiation and early-phase propagation is ideal for wells drilled in low-permeability “tight” formations, while FBP minimization suppresses the overall on-site hydraulic horsepower (HHP) requirements. The demonstrated optimization scheme is applied separately to the in-situ stress states of seven prolific shale plays from the U.S. and Argentina, suggesting oriented-perforating strategies that target the promotion of transverse HF initiation in two of these (Barnett and Marcellus), while suggesting oriented-perforating strategies that target FBP minimization in the remaining five (Bakken, Fayetteville, Haynesville, Niobrara, and Vaca Muerta). The effectiveness of oriented-perforating can potentially be compromised by fracturing fluid leakages around the casing’s circumference, which is shown to hinder transverse HF initiation.The hybrid-modeling approach is also used to estimate fracture initiation pressure (FIP) values for the seven shale plays studied, indicating significant discrepancies with analytical expressions traditionally used to approximate these FIPs in modern HF computational simulations. This sets the basis for expanding this hybrid-modeling approach over a range of in-situ stress states, incorporating aggregate data-driven (numerically-derived) correction factors in order to compensate for inaccuracies in the analytical approximations, which comprise the physics-based core of the proposed hybrid model.

Методы гидроразрыва труднообрушающейся кровли и угольного пласта для исключения динамических явлений в угольных шахтах

The paper demonstrates the need to develop methods of managing poorly caving roofs in the stoping and development faces as well as the ways of intensifying the degassing process of unrelieved coal seams to improve the technical and economic performance and safety of operation. Methods of the directional hydraulic fracturing (DHF) of the roof and the interval hydraulic fracturing (IHF) of the coal seam are suggested to eliminate the dynamic phenomena in coal mines as well as technological charts of their implementation. The DHF method consists in creating an initiating slot in the borehole walls, which acts as a stress concentrator, followed by its sealing with an insulating device, i.e. a packer. When a fluid is injected into this slot, the tensile stresses are created in the borehole walls and the fracture is forced to develop in a predetermined direction. The IHF method of the coal seam from the development workings using non-cased degassing holes of large and variable diameter consists in creation of transverse hydraulic fractures in the hole relative to its axis and increasing its gas recovery by an order of magnitude. This is achieved by using a double-sided packer with elastically expanding elements and a valve installed between them. Additional tangential stresses created in the borehole walls ensure the development of inelastic strain zone, leading to the formation of transverse cracks in the borehole. The results of numerical modeling of the stress-and-strain state of the rocks near the borehole where packer seals are installed using the finite element method are presented in the paper. Techniques and means to monitor the processes of hydraulic impact on the coal-rock massif are shown.

Completion Designs in Horizontal Well with Transverse Hydraulic Fractures for Enhanced Recovery in Unconventional or Tight Reservoirs

Summary Low recovery, 2 to 15%, in unconventional plays (including tight reservoirs and source rocks) has long been recognized as a business deterrent. The industry applies enhanced oil recovery (EOR) techniques, along with hydraulic fractures in tight/unconventional plays, to improve the recovery. To maximize matrix sweep, the fractures are aligned in a face-to-face assembly. Such an arrangement can be achieved using a vertical or longitudinal hydraulic fracture on horizontal wells, but these, generally, do not provide as effective reservoir contact (hydraulic fracture surface area) as horizontal wells with multistage transverse hydraulic fractures. The multistage transverse hydraulic fracture, however, comes at the costs of conformance issues with early water breakthrough from short-circuiting and inability to achieve fracture face-to-fracture face alignment of the injection and production fractures. The vast majority of wells drilled in unconventional plays are in the transverse configuration; hence, there is a need for an optimal solution for transverse fractures combined with improved oil recovery (IOR)/EOR approaches. In this work, we introduce the multistage enhanced recovery (MS-ER) techniques that enable face-to-face alignment for optimal enhanced hydrocarbon recovery (EHR)/IOR/EOR in horizontal wells with multistage transverse fractures, thereby enabling optimal recovery and mitigating the key risk of fracture short-circuiting.

CO2 Capture and Storage Performance Simulation in Depleted Shale Gas Reservoirs as Sustainable Carbon Resources

Underground carbon capture and sequestration (CCS) is a useful technique for separating this kind of greenhouse gas from atmosphere and store it under the surface of the earth. As a matter of fact, CO2 can be transferred to underground petroleum reservoirs which are initially contained oil and gas, or aquifers which are initially saturated with water. This kind of CCS takes place using an injection well which is drilled from surface to the target underground bedrocks. A shale gas reservoir (SGR) is a type of petroleum gas reservoir in which natural gas is stored in ultra-tight pores of the shale rock. In this study, a flow modeling analysis in SGR with a multi-stage fractured horizontal well (MSFHW) is conducted using numerical simulation. In this shale layer, a horizontal well is drilled and several transverse hydraulic fractures, for increasing the flow efficiency between the well and porous medium, are created. The studied SGR – a depleted reservoir acting as a macroscopic sustainable material for the CCS – is initially saturated with methane gas, and carbon dioxide is required to be injected for the storage. The most outstanding results of this study is about sensitivity analyses for SGR permeability with different conditions of gas adsorption and stress-dependent permeability which are from important features of SGRs. The results show a minor reduction in cumulative gas injection due to the effect of stress-dependent permeability in all measures for reservoir permeabilities. Furthermore, gas sorption shows a considerable positive correlation with CO2 storage response in high-permeability SGR and a minor increasing effect on SGRs with lower permeability values.

A Semianalytical Modeling Approach for Hydraulic Fracture Initiation and Orientation from Perforated Wells

Summary Accurate prediction of fracture initiation pressure and orientation is paramount to the design of a hydraulic fracture stimulation treatment and is a major factor in the treatment's eventual success. In this study, closed-form analytical approximations of the fracturing stresses are used to develop orientation criteria for relative-to-the-wellbore (longitudinal or transverse) fracture initiation from perforated wells. These criteria were assessed numerically and found to overestimate the occurrence of transverse fracture initiation, which only takes place under a narrow range of conditions in which the tensile strength of the rock formation is lower than a critical value, and the breakdown pressure falls within a “window.” For a case study performed on the Barnett Shale, transverse fracture initiation is shown to take place for breakdown pressures below 4,762 psi, provided that the formation's tensile strength is below 2,482 psi. A robust 3D finite volume numerical model is used to evaluate solutions for the longitudinal and transverse fracturing stresses for a variable wellbore pressure, hence developing correction factors for the existing closed-form approximations. Geomechanical inputs from the Barnett Shale are considered for a horizontal well aligned parallel to the direction of the least compressive horizontal principal stress. The corrected numerically derived expressions can predict initiation pressures for a specific orientation of fracture initiation. Similarly, at known breakdown pressures, the corrected expressions are used to predict the orientation of fracture initiation. Besides wellbore trajectory, the results depend on the perforation direction. For the Barnett Shale case study, which is under a normal faulting stress regime, the perforations on the side of the borehole yield a wider breakdown pressure window by 71% and higher critical tensile strength by 32.5%, compared to perforations on top of the borehole, implying better promotion of transverse fracture initiation. Leakage of fracturing fluid around the wellbore, between the cemented casing and the surrounding rock, reduces the breakdown pressure window by 11% and the critical tensile strength by 65%. Dimensionless plots are employed to present the range of in-situ stress states in which longitudinal or transverse hydraulic fracture initiation is promoted. This is useful for completion engineers; when targeting low permeability formations such as shale reservoirs, multiple transverse fractures must be induced from the horizontal wells, as opposed to longitudinal fracture initiation, which is desired in higher permeability reservoirs or “frac-and-pack” operations.

Characterising the application of horizontal wells and indirect hydraulic fracturing for improved coal seam gas drainage

Historically, horizontal wells in coal seam gas (CSG) producing areas of Australia have been limited in their ability to stimulate horizontal wellbores with hydraulic fractures, despite most basins being in favourable stress states capable of generating multiple transverse hydraulic fractures. In some cases, overlapping mining tenements limit the ability to effectively deploy steel casing completions to effectively manage multi-stage fracture stimulations along the lateral. In other cases, potential wellbore instabilities preclude the placement of laterals with adequate length to stage numerous fractures required to make a well economic. Strike Energy’s Jaws 1 well indicated well instability when drilling horizontally in the Patchawarra Vu Coal Seam in Petroleum Exploration Licence 96 of the Weena Trough in the Cooper Basin. To overcome this instability, Strike Energy introduced the application of an innovative process of deploying multiple, indirect hydraulic fractures in Jaws 1ST from a horizontal wellbore underlying the coal. Microseismic and surface deformation tiltmeter data acquired during the treatment confirmed the successful placement of fractures in the Patchawarra Vu Coal Seam. Further, chemical tracing and production testing provide further insight into the placement and effectiveness of the overall stimulation. Through analyses of data from the Jaws 1ST, we provide insight and recommendations regarding horizontal well placement to improve indirect hydraulic fracture effectiveness. These recommendations would be applicable for other Australian CSG basins where direct horizontal well drilling and fracture stimulation has been limited (e.g. overlapping mining tenements) or where horizontal well instabilities limit drilling within the seam (i.e. Cooper Basin, depleted coals).

Experimental study on the feasibility of supercritical CO2-gel fracturing for stimulating shale oil reservoirs

CO2-based fracturing was widely introduced to stimulate shale oil reservoirs for its multiple advantages. However, fracture height containment may become noticeable as the low-viscosity CO2 used in the shale with massive mechanically weak bedding planes (BPs). To address this issue, a novel CO2-gel fracturing design is introduced in this study. The design consists of initially pumping gel to break through the BPs at the near-wellbore zone and then injecting supercritical CO2 (Sc-CO2) to further extend these gel-induced hydraulic fractures (HFs) and to reactivate far-wellbore BPs. In this paper, the design feasibility was demonstrated through laboratory multi-stage fracturing experiment results, and their comparisons to single-phase Sc-CO2, slickwater, and gel fracturing results. Computerized tomography scanning and acoustic emission monitoring were employed to reveal the fracture geometry and the tension, shear and compression failure mechanisms. Fracturing with low-viscosity Sc-CO2 and slickwater encountered the problem of fracture height containment caused by high infiltration ability of these two fluids, which would lead to opening of wellbore-connected BPs dominated by tensile failure. High-viscosity gel fracturing can create multiple transverse HFs but with a few fractures on BPs. When CO2-gel fracturing fluid was used, the fracture height is enhanced and more far-wellbore BPs are activated mostly in slip. The isolation effect of gel on BPs can lower the leak-off amount of Sc-CO2. Consequently, CO2-gel fracturing is relatively far to the leak-off dominated regime, which means more elastic energy of high-compressibility Sc-CO2 could be attained for fracturing the reservoir rocks. CO2-gel fracturing can provide a useful alternative in improving the vertical fracture height during the fracturing treatment.

Experimental Study of the Simultaneous Initiation of Multiple Hydraulic Fractures Driven by Static Fatigue and Pressure Shock

One of the most important challenges in horizontal well fracturing is to create multiple tightly-spaced hydraulic fractures (HFs) within each fracturing stage in tight reservoirs, especially with the strong heterogeneity. This study explores the influences of static fatigue and pressure shock on the simultaneous initiation of multi-HF from an open-hole horizontal wellbore in a formation with the variability in the rock strength. Laboratory-scale fracturing experiments were conducted on cuboid-shaped specimens (8 cm × 8 cm × 10 cm) of varying brittleness under the tri-axial confining stress. The rock strength variability in a specimen was illustrated as the variation in the width or length of the preset natural fractures (NFs) within the open-hole section (OHS). Inside the wellbore, three pressurization regimes, corresponding to the normal, static fatigue and pressure shock conditions, were obtained by the constant-rate injection, constant-pressure injection, and high-pressure build-up and rapid release, respectively. The results demonstrate that under the pressure shock conditions, bottom hole pressure (BHP) can be forced to exceed the normal breakdown pressure in a conventional fracturing, and generate a relatively high breakdown pressure. It has the potential to improve the simultaneous initiation of multiple transverse HFs from NFs of different strengths and that of longitudinal HF within a single fracturing stage. A lower limit BHP is required to overcome the variations in strength across different potential initiation points. Meanwhile, the low-BHP, which is lower than the normal breakdown pressure, is maintained to induce the static fatigue failure of the surrounding rock. Under static fatigue conditions, a single HF is generally initiated from the weak NF. The time to breakdown decreases evidently with the increase in the BHP and rock brittleness. In low-brittle specimens, the time-dependent initiation of HF may be difficult in the OHS without connecting with the NFs. In practice, simultaneous initiation of multi-HF may be enhanced by the combined effects of static fatigue and pressure shock.

Study on the Interference Law of Staged Fracturing Crack Propagation in Horizontal Wells of Tight Reservoirs.

Horizontal well multistage fracturing technology is a high-efficiency method for tight oil and gas production, which increases the contact area between the wellbore and stratum by forming multiple transverse hydraulic fractures (HFs). The stress interference caused by the created HFs will affect the propagation geometries of the subsequent HFs, and then affect the overall fracturing treatment performance. In order to study the distribution of HFs and law of interference among multiple fractures during horizontal well multistage fracturing in tight reservoirs, in this paper, the effects of horizontal stress difference, perforation spacing, and net pressure in the created fracture on the initiation and propagation of multiple cracks were specially studied through large-scale hydraulic fracturing experiments and numerical simulation. The results showed that under high horizontal stress difference, multiple HFs with small spacing tended to coalesce at a place where the length has extended to a certain level. It was also found that the initiation pressures of HFs within subsequent stages increase with the rise of net pressure in created HFs, which causes the subsequent fractures to deviate from the direction perpendicular to the horizontal wellbore and then to gradually deflect toward it. Moreover, the ratio of fracture spacing to fracture height, and the net pressure are the key parameters to determine the deflection degree of HFs for multistage fracturing. In addition, the deflection degree of subsequent HFs was expected to enhance with the decreasing ratio of fracture spacing to fracture height and increasing net pressure in created HFs. It was useful for mitigating stress interference to lengthen the stage spacing, control the fracture height, and reduce the net pressure. The research results have a reference value for the optimal design of the fracture spacing for multistage fracturing.

Experimental investigation into hydraulic fracture geometry and proppant migration characteristics for southeastern Sichuan deep shale reservoirs

Difficulties in fracture extension and proppants adding were two key factors that affected the stimulated volume of deep shale reservoir. To clarify the mechanism of hydraulic fracture (HF) propagation and proppants migration, several groups of large-scale true tri-axial fracturing tests with an ingenious method of sand adding were performed utilizing deep Longmaxi shale outcrops from southeastern Sichuan. The interaction behavior between vertical HF and bedding plane (BP) was discussed. The results showed that due to the low bonding strength, the BPs could be easily activated even under high vertical stress difference coefficient. Based on the different influenced degrees of BPs, four types of HF geometries in the vertical direction were observed, that is, transverse HF, horizontal HF, step-shaped HF with fissure opening, and multilateral step-shaped HF network. The migration distance of proppants was limited and almost all the proppants were distributed around the wellbore, thus inducing the formation of multiple secondary HFs. When there are natural fractures (NFs) with large aperture around the wellbore, proppants almost entirely distributed along the NFs. The main factor, causing the difficulty of fracture extension and proppants migration, was the complex HF geometry characterized by the multi-branched fractures with slippage of BPs.

Propagation law of hydraulic fractures during multi-staged horizontal well fracturing in a tight reservoir

Abstract A novel laboratory simulation method for modeling multi-staged fracturing in a horizontal well was established based on a true tri-axial hydraulic fracturing simulation system. Using this method, the influences of net pressure in hydraulic fracture, stage spacing, perforation parameter, horizontal stress bias and well cementation quality on the propagation geometry of multiple fractures in a tight sandstone formation were studied in detail. The specimen splitting and analogy analysis of fracturing curve patterns reveals: Multiple fractures tend to merge under the condition of high horizontal stress bias and short stage spacing with pre-existing hydraulic fractures under critical closure situation, and the propagation of subsequent fractures is possibly suppressed because of high net pressure in pre-created fractures and asymmetric distribution of fracture width. And the subsequently created fractures are situated in the induced stress decreasing zone due to long stage spacing, leading to weak stress interference, and perforation with intense density and deep penetration facilitates the decrease of initiation fracture pressure. The deflection angle of subsequent fracture and horizontal stress variation tend to be amplified under low horizontal bias with constant net pressure in fractures. The longitudinal fracture is likely to be initiated at the interface of wellbore and concrete sample with poor cementation quality. The initiation fracture pressure of the different stages increases in turn, with the largest increase of 30%. Pressure quickly declines after initiation with low propagation pressure when the transverse hydraulic fracture is formed. The pressure reduces with fluctuation after the initiation of fracture when the fracture deflects, the extension pressure is high, and the fracture formed is tortuous and narrow. There is a violently fluctuant rise of pressure with multiple peak values when longitudinal fracture created, and it is hard to distinguish the features between the initiation stage and propagation stage.

Transient pressure analysis of horizontal well with nonorthogonal transverse fractures and drainage volume characterization using Fast Marching Method

Horizontal well drilling and multistage hydraulic fracturing are essential to economic recovery of unconventional tight/shale reservoirs. Multiple transverse fractures are ideally supposed to be perpendicular to the minimum stress direction, but nonorthogonal transverse fractures might be seen in practice, which might bring a better connection between wellbore and fractures. However, for the same fracture half-length, the stimulated reservoir area might be less. Thus, the well performance with these nonorthogonal transverse fractures is full of uncertainty. Little research work is currently conducted on the nonorthogonal transverse fracture scenario.Our transient pressure analysis of horizontal well performance with nonorthogonal transverse hydraulic fractures utilizes the concept of rate-normalized pressure and transient productivity index. The novel Fast Marching Method (FMM) is adopted to characterize the drainage volume evolution for multi-stage horizontal wells and validate the proposed methodology. Sensitivity studies contrast transient pressure behavior and productivity index for azimuth angles from 15 to 90°, e.g. from nearly longitudinal to truly transverse fracture angles. Additionally, transient pressure behavior and long-term reservoir performance of horizontal well for different scenarios are investigated, such as reservoir anisotropy and different proppant allocation strategies under the constraints of given horizontal well spacing and total proppant amount.Contrast to what we are usually expecting, results show that for the same fracture spacing, and the same total proppant amount, the well with nonorthogonal transverse fractures will have a higher productivity index than that for truly transverse fractures. Different from productivity index, the long-term reservoir performance, however, turns out to play a different game, given specific horizontal well spacing and the same total proppant amount. Reservoir performance for horizontal well with nonorthogonal transverse fractures can be significantly improved by optimally allocating the proppant and designing hydraulic fracture placement strategies through this research.

Applied method for production design optimization under geologic and economic uncertainties in shale gas reservoirs

In the last decades innovations in horizontal drilling and hydraulic fracturing enabled production of commercial quantities of natural gas from many unconventional reservoirs. Reservoir management and development strategies for shale and tight gas plays have evolved from ad hoc approaches to more rigorous strategies that involve simulation and numerical optimization in presences of multiple economic and production objectives, constraints and uncertainties. Application of an automated integrated optimization framework for placement of transverse hydraulic fracture stages along horizontal wells might increase shale gas reserves and revenue from unconventional assets even further. However, the uncertainty present in many reservoir properties and economic parameters might have tremendous effect on success or failure of an unconventional project. In this study, we assess the uncertainty in key parameters using a full factorial design of experiment and a stepwise regression analysis and combine this assessment with an optimization strategy based on a genetic algorithm to identify the effects of uncertainty on optimal production design and revenue of an unconventional project.

Transverse Hydraulic Fracture Initiation by Indentation in an Uncased Borehole

The hydraulic fracturing technique has applications in the development of solid mineral deposits. The particularities of the method performed in a mine condition include confined space and high requirements for the safety of the equipment used in underground workings. One of the most relevant tasks in in-mine hydraulic fracturing is creating parallel cracks oriented across the axis of the well. In-seam boreholes with this type of fracture system can be used for coal bed methane production, sealing of degassing boreholes, controlled collapse of the roof and other applications. In this paper we consider transverse crack formation on borehole wall in a given plane when the fracture initiation occurs due to an indenter impression located between two inflatable packers. During indentation fracturing, fluid under high pressure is supplied and this contributes to crack propagation in the desired direction. On the basis of the calculation of the stress intensity factor for the semi-infinite plane with the edge crack affected by the forces applied at points on the plane surface the length of the formed crack is obtained. The results show that the force acting on the wall of the borehole with a 76–105mm diameter will be sufficient to create an initiation fracture in a predetermined direction. To implement this approach an experimental device is developed. It is implemented as an additional module of the hydraulic fracturing tool and equipped with a system of three indenters. The design of the device allows to obtain a total acting force in one plane and this will increase the effectiveness of the impact.

Chalk reservoir of the Ockley accumulation, North Sea: in situ stresses, geology and implications for stimulation

Abstract The Ockley discovery is a gas condensate accumulation contained within tight chalks of the Hod Formation. The observed faults and fractures are a combination of features radial to the main periclinal structure and parallel to the local structural grain 100° from north. Most natural fractures appear to be healed or cemented. The pore pressure gradient at Ockley is c. 0.199 bar/m. The in situ stresses are estimated to all be within c. 46 bar of each other, indicating a near-isotropic in situ stress system. Therefore the orientation of S Hmax is hard to define and drilling optimally oriented wells to create transverse hydraulic fractures is difficult. The estimated intact rock fracture initiation pressure in a horizontal well would exceed all the in situ stress gradients. Therefore, even if a vertical planar induced fracture were created at the wellbore wall, it would probably exploit natural fractures and bedding planes, leading to complex fracture geometries. Acid fracture stimulations are plugged by the insoluble clay residue within the clay-rich Hod chalk, so this is not an optimal strategy. Proppant fracturing has more merit, but complex fracture geometries present significant challenges for successful treatment design while trying to avoid or minimize extremely costly early screen-outs.

Rigorous Integrated Evolutionary Workflow for Optimal Exploitation of Unconventional Gas Assets

Applications of stochastic evolutionary algorithms in engineering are gaining more attention in practical applications in the oil and gas industry. An important factor to consider when implementing stochastic algorithms is its ability to find the global optimum efficiently. In this study the authors formulate, implement, and test a genetic algorithm with strong elitism to solve a critical problem in the upstream oil industry: how to develop economically an unconventional gas asset. This problem involves finding the optimal number of horizontal wells, the number of transverse hydraulic fracture stages along them, and stage half-length. The described problem is inherently discrete or mixed optimization problem for which the authors develop a conceptually new evolutionary integrated framework that addresses all production design questions. They outline the range of applicability of their workflow and provide ample test cases and results. Their rigorous formulation performs well for a given problem statement and finds the optimal solution that is consistent with the industry accepted optimum.

Effect of discontinuity stress shadows on hydraulic fracture re-orientation

Hydraulic fracturing has been applied to the cave mining industry as a pre-conditioning method to improve rock mass caveability and fragmentation sizes. In theory, the hydraulic fracture orientation is dictated by and is perpendicular to the minimum in-situ stress orientation. Such orientations will not always result in the creation of a blocky rock mass to aid caveability. The understanding of hydraulic fracture re-orientation induced by the stress shadow effect is important for either avoiding the undesirable interaction between multiple transverse hydraulic fractures or taking advantage of this phenomenon to create prescribed hydraulic fractures that result in the creation of blocky rock masses. In this paper, the existing knowledge of the stress shadow effect around a pre-existing discontinuity is reviewed. A 3D numerical modelling code, Rock Failure Process Analysis (RFPA) is used to investigate factors influencing hydraulic fracture re-orientation. It is established that hydraulic fractures can be forced to propagate in desired directions if advantage is taken of the stress shadow effect in combination with knowledge of rock mass homogeneity and site far field stress conditions. Both the differential stresses between σ2 and σ3 and between σ1 and σ3 affect the hydraulic fracture re-orientation geometry. The minimum far field stress magnitude has no direct impact on hydraulic fracture re-orientation. Rock mass homogeneity significantly influences how far a hydraulic fracture propagates along its predefined orientation from its initiation point before re-orientation.

Stochastic optimization of hydraulic fracture and horizontal well parameters in shale gas reservoirs

Multistage hydraulic fracturing is one of the most important techniques in the successful exploitation and development of shale gas reservoirs. Most unconventional reservoirs rely on multistage hydraulic fracturing for commercial success. This success is accomplished by drilling horizontal well of appropriate length and creating transverse hydraulic fractures in stages across the well. Shale-gas reservoir's contact with the horizontal well bore is improved by using optimal well length, optimal fracture conductivity, optimum fracture length and optimum number of fracture stages. Because multistage fracturing of long horizontal wells increase the cost of field development, the economics of field development can be improved by using global optimization algorithms to estimate the optimum values of these operational parameters. In shale gas reservoirs, hydraulic fracture parameters such as fracture half-length, amount of proppant and fracture spacing should be optimized to maximize the net present value (NPV).In this work, differential evolution (DE) is implemented on a more realistic LGR-based shale gas reservoir simulation model. The objective is to maximize the net present value by optimizing hydraulic fracturing parameters and horizontal well length. Results obtained indicate that significant increase in NPV can be realized by using the optimization algorithm to estimate the operational parameters of hydraulic fracturing process in shale gas reservoirs.