Enhancement and control of hydraulic fracturing stimulation and overall production by manipulation of casing material properties: techniques and completion assemblies

Abstract

This invention discloses a well completion assembly and an associated method for enhanced hydraulic fracturing control in single or multiple production zones. The well completion assembly utilizes a dual-casing architecture comprising: (i) a primary load-bearing inner casing maintaining wellbore structural integrity, and (ii) a formation-engaged outer casing configured to optimize stress field modulation. Computational modeling demonstrates a reduction in formation breakdown pressure through controlled casing stiffness reduction.

Description

DescriptionTitle Of Invention: Enhancement and Control of Hydraulic FracturingStimulation and Overall Production by Manipulation of Casing MaterialProperties: Techniques and Completion assembliesField of the Invention

[0001] In this patent, we are proposing a method that consist of a technique and a wellbore completion assembly. The aim of this novel method is to significantly reduces the breakdown pressure without (to an extent) intervening with the previously proposed methods in the literature. In addition, it contributes to enhancing the hydrocarbons production over the well life. The study we conducted have shown that wellbore casing stiffness can alter the stress field in the formation. Stresses are the main derive of fracture initiation and propagation. Therefore, we suggested to utilize lower stiffness casing. Nevertheless, lower casing stiffness is not sufficient to support the wellbore structure. In addition, they are prone to failure due to their associated inherent strength (at least from a typical correlation aspect). For this reason, we propose a completion assembly that encounter both considerations: the essential stiffness of a primary casing (e.g., steel) and the luxurious lower stiffness to control the stress field in the formation. A simple embodiment of this idea is by a double / dual casing method where the primary casing is connected conventionally in a continuous manner. Then, each secondary casing is attached to the primary casing from the casing connection in one end to the other. When the dual casing is perforated, the borehole pressure is equalized across the primary casing thickness and applied to the secondary casing and the formation. The secondary casing is purposed to engage with the formation facilitating the energy transformation to the formation which intensifies / alter the stress field to the designed level. As we have shown that stiffness affects the stress magnitude, it was convenient to study the effect of manipulating the casing stiffness on the formation stresses in term of direction. We utilized anisotropic material (i.e., orthotropic material) which has different stiffness values at each orthogonal direction (E1 , E2 and E3). As the “secondary” casing is formed from this material, the highest modulus of elasticity (E1 ) was chosen to be in the tangential direction in the pipe. The longitudinal modulus (E2) was ranged from (7%) to (100%) of (E1 ) to observe the effect on the stress field. It was found that the reduction of E2 caused changes in the stress field in the formation leading to higher chances of creating a transverse fracture. The latter has significant impact on the production of hydrocarbon as it strikes the reservoir in preferable angle. Therefore, utilizing orthotropic materials in a casing can helpcontrolling fracture type / orientation. From material science aspect, controlling orthotropic moduli can be done by number of techniques. One is by orienting fibers in composite materials to the highest desired stiffness direction. In composite materials, selection of matrix and fiber properties in addition to the volume fraction between two is the key to control the magnitude of the desired moduli of elasticity. This orthotropic material technique can be implemented by the dual casing method explained earlier. Other material technologies to control material properties of the secondary casing is also included. Smart and properties adoptive materials are important examples of such deployments.BACKGROUND

[0002] Hydraulic fracturing is a vital wellbore stimulation method used to enhance oil and gas recovery from subsurface formations (Montgomery and Smith 2010; Al-Arnous et al. 2018). It is implemented in conventional and unconventional gas well stimulation. In the latter, the well have many production zones that requires fracturing separately in what is known as plug and perf technique (‘Hydraulic Fracturing Operations-2023’ 2023; Al-Obaid et al., n.d.) (up to 25 zones).

[0003] Hydraulic fracturing involves injecting high-pressure fluid to create fractures in the rock, allowing hydrocarbons to flow more freely. Research highlights the importance of hydraulic fracturing in increasing production rates and improving recovery factors (Wu et al. 2022; Chen et al. 2014; Daneshy 2010). In tight formations, the production potential without fracturing is extremely low due to the poor permeability and complex pore structures. The natural permeability of tight reservoirs is often less than 0.1 millidarcy (mD), making it nearly impossible for hydrocarbons to flow freely without artificial stimulation like hydraulic fracturing. In such reservoirs, there is typically no possibility of achieving high or stable production unless the reservoir is artificially reconstructed to create flow channels for the hydrocarbons. Fracturing enhances the permeability of these formations, which significantly increases the productivity. In fact, without fracturing, many tight oil and gas reservoirs would remain non-productive as the natural flow of oil and gas would be insufficient to sustain commercial production levels (Daneshy 2010; Wu et al. 2022). With the increment of development of the fields, the challenges to achieve the hydraulic fracture escalate. The most critical challenge in achieving a fracture is the high breakdown pressure, the equipment used for hydraulic fracturing may not be capable of reaching the required breakdown pressure, particularly in deep or tight formations where pressures are naturally high. If the pressure applied is insufficient to break down the rock, no fracture will be initiated, leading to a failed stimulation attempt. This is especially problematic in tight reservoirs, where high breakdown pressures are common due to low permeability and strong rock matrices(Fazio et al. 2021 ; Lee and Hopkins 1994). Even if the pressure supplysource (e.g., pumps) is competent to achieve the pressure, the breakdown pressure may exceed the well control equipment rating (e.g., casing and completion) {Recent Advances In Hydraulic Fracturing, n.d.), which typically around 20 Kpsi. Statistically, in a particular field, 50% of zones were left unstimulated due to failure to fracture (Khan et al. 2023). In Saudi Arabia , tight formations, such as shale, Khuff, and Unayzah, present unique challenges for hydraulic fracturing. Shale formations require high pressures to initiate fractures, often exceeding tubular ratings (Taleghani and Klimenko 2015). The Khuff and Unayzah formations in Saudi Arabia are characterized by high in-situ stresses and tight rock properties, making fracture initiation particularly difficult and costly (Khan et al. 2023). The demand is increasing dramatically particularly in shadow of the newly discovered giant discoveries of oil and gas deposits in Saudi {Reuters 2024). In 2024, the oil and gas production is recovering to pre-covid period and heading toward the peak between 2030 and 2040. As production increases and drilling on various environments takes place, more challenges appears. Therefore, efficient solutions and innovative technologies are fetal at this critical era.

[0004] Previously, there are trials to address the issue of high breakdown pressure. One method is the use of notches (AITammar et al. 2022) (one of the research works done by this patent author) or oriented perforations (e.g, planar perforations). Notches can concentrate stress and lower initiation pressures, while oriented perforations align with the stress field to facilitate easier fracture propagation. Nevertheless, notches can be created in open holes by abrasive jets which may not be sufficient to cut a casing in a cased hole. Studies have shown that planar perforations, where perforations are placed in a single plane, can also help in managing fracture pressures (Zeng et al. 2019) . This method cannot be done by perforation guns since bullets cannot be placed in one plane. In addition, it requires accurate azimuth angle and centralization.

[0005] Economically, failure to fracture formation results in financial loses to the service provider company (as they usually paid per successfully stimulated zone) and to the field operator company as they lost production of hydrocarbons.DISCRIPTION OF THE INVENTION

[0006] The main purpose of this invention is to enhance production of hydrocarbons by facilitating fracturing stimulation method and provide flexibility to the formation dilation during production. It addresses a significant problem in the field which is failure to stimulate production zone by fracturing rock formation leading to implications such as complete lose of hydrocarbons production. This in addition of lowering the high residual stresses inducedby high stiffness casing material utilized in normal practices. Furthermore, the invention allows controlling the fracture orientation, placement and pressure range.

[0007] Manipulating casing stiffness by selecting material with modulus of elasticity lower than steel aiming to alter the stress field in the formation to improve the success of stimulating rock formation particularly in impermeable rock types. Along the well life cycle, the selected casing is tuned to improve production by easing casing dilation and reduce residual stresses.

[0008] Current practice uses mainly steel as casing material. Steel has excessive stiffness ratio to the formation (10-100 times) leading to restricting the formation from consuming the applied energy provided by the borehole pressure. Nevertheless, steel is irreplaceable as it provides the necessary support to the wellbore structure and avoid catastrophic failures. It is possible see very few completion designers use materials other than steel to improve corrosion resistance and provide higher weight capacity. This usage is risky since these materials are not sufficient to support wellbore structure namely in high production reservoirs as these reservoirs are mechanically unstable. Hence, we are proposing a combination of the two casings where a primary casing support the wellbore structure, and a secondary casing engages with the formation facilitating the features we emphasized. Examples of this embodiment is by attaching the secondary casing to the outer surface of the primary casing in each pipe solely. The attaching can be done by adherence technique or by clamping with sets of seal kit. A gap between the two casing is preferred to ensure effective transportation of energy to the formation without inducing interferences from the primary casing. This gap can be filled by fluid to equalize the pressure across the thickness of each casing or by allowing wellbore cement to flood through it. In the latter case, the secondary casing can have multiple layers: two attached to the cement in both directions and one in the middle act as a separator.

[0009] Another key feature is the exploitment of orthotropic / anisotropic material to guide the stress field by manipulating the intensified stress directions resulting in controlling the fracture orientation. An example of an embodiment of this technique is by impeding fibers in composite materials. Orienting fibers establishes a final set of selected stiffness properties.

[0010] The economical impact is another aim of this invention. Failure to stimulate rock formation can lead to financial losses to the field operator company and the service provider company. The former, that invested large capital to drill and develop the well, faces risk of abending it due to lack of production. The service providers usually designthe stimulation technique and responsible of the failure of their design. Hence, payment might be lost due to unsuccessful execution.

[0011] This invention is targeting oil and gas wells in addition to water and geothermal wells as they apply similar methods to increase fluid seepage through formations weather by injection or production.

[0012] The advantages of this invention can be summarized as follow:Reduces fracture initiation pressure (reduce risk of losing production and failure of stimulation). Enhance production of hydrocarbons by providing flexibility in interactions with the reservoir.Protection (provide sort of protection to the primary casing as it is sacrificed in corrosive environment) along the well life cycle. In addition, the secondary casing absorbed shakes, fluctuations and fatigue cycles from the interactions with the formation.Implementation feasibility and adoption convention: possible on-site assembly of the secondary casing to steel casing provided by any party.Maintain the required strength and stiffness to the wellbore structure.Control fracture orientation by the implementation of the novel method of utilizing orthotropic material.Maintain continuity of the wellbore structure without interrupting the well completion tools.Post fracturing enhancement and proppant efficiency: o Steel by high stiffness lead to fracture arresting and create residual stresses. When pressure is released, fracture width reduces. As proppant injected, less volume will be occupied in the fracture surface. In addition, Placed proppant in the fracture will experience excessive compression force. o In the case of lower stiffness casing, the formation dilation is higher allowing for wider fractures as less arresting is applied and residual stresses created. Consequently, larger proppant volume is injected experiencing lower compression force. o Applicable to all fracture orientationsHigh accuracy is not required as the error on depth can be (perforation diameter / casing length) which is a very small value. Therefore, it is very unlikely to perforate in the casing collar region. Fracture placement control: o First consideration: Stress shadows occur when existing fractures alter the stress field around new fractures. This phenomenon affects fracture propagation and wellbore stability. Segall and Pollard (1983) introduced the concept of stress shadows, and Cochran and Dempsey (2006) investigated their impact on fracture propagation and stability. In another word, existing fractures in one zone can create a cloud of stresses, increasing the pressure required to fracture. o Second consideration: in the same zone, formation strength may vary. In order to unify the required pressure to create fractures, the casing stiffnesses are tuned with the formation strength. o Third consideration: by careful and smart design in plug & perf Intervention, the entire unconventional technique can become completely efficient by implementing the patent method. This can be deployed by placing higher stiffness casing at the upper zone and gradually increase the casing stiffness down to the lowest zone. Then, it is possible to fracture all zones in one single pressure increment striking the of them gradually.Available material (for primary or secondary casing) at which they have comparable steel strength: o Composite Material (0.5-100 Gpa). o Aluminum Alloys (70 Gpa). o Titanium and Platinum Alloys (100-200 Gpa).DISCRIPTION OF THE DRAWINGSFigure 1 . Shows the casing effect on formation dilation comparing to an openhole wellbore.Figure 2. Shows the simulation domain emphasizing the block and the perforation (both are representing the rock formation) while the pipe attached to the perforation is the casing.Figure 3. Shows applied confining directions one the wellbore and perforation representing a horizontal well drilled toward the minimum horizontal stress (aft) in a strik-slip regime.Figure 4. Shows 6-perforations clusters in a curse mesh model arranged in helical configuration.Figure 5. Shows perforation near field computational region. Thickness of the region is 2.5 perforation radius.Figure 6. Shows perforation near field refined mesh.Figure 7. Shows perforation and stress directions where (az) is associated with initiation of transverse fracture while (oy) is associated with initiation of longitudinal fracture.Figure 8. Shows the domain of the recorded stresses for all the presented results hereafter. Therefore, it does not account for the perforation / borehole junction.Figure 9. Shows results of Case-1 model (Equal confining): Plot of (ax, ay, ozand oj gainst the casing stiffness.Figure 10. Shows results of Case-Il model (strike-slip confining): Plot ofand a±) against the casing stiffness.Figure 1 1. Shows results of Case-I model (Equal confining): Plot of (ax, ay, azand aj normalized by stresses induced in the situation of steel casing against the casing stiffness.Figure 12. Shows results of Case-Il model (strike-slip confining): Plot of (ax, ay, azand aj normalized by stresses induced in the situation of steel casing against the casing stiffness.Figure 13. Maximum dilation in all three directions in the perforation. The plot is showing consistent trends with the induced stresses emphasizing the role of the transferred strain energy levels provided by the casing stiffness on the formation stress field.Figure 14. Shows results of Case-Il model (strike-slip confining): Plot of normalized ( ax, ay, azand o^ ). Additionally, the horizontal lines represent the stresses in the openhole case where the borehole radius is equal to the casing radius. The vertical line maps the formation stiffness where a transformation of the function from hypo-trend to hyper-trend.Figure 15. Shows results of Case-Il model (strike-slip confining): Plot of normalized (ox, ay, azand 0^ ). Additionally, the horizontal lines represent the stresses in the openhole case where the current borehole radius is equal to the previous borehole radius.Figure 16. Shows results of Case-Ill model (6-perforations with strike-slip confining regime): Plot ofnormalized by stresses induced in the situation of steel casing against the casing stiffness.Figure 17. Shows ayand azcurves on various point along the perforation indicating the influence of casing stiffness on the near field over the perforation length.Figure 18 Shows the average of each of the stress curves (ayand az) calculated from the curves in figure 17.Figure 19. Shows fiber directions in composite materials and fiber directions in a casing pipe. It is indicating the effect of amplifying the modulus of elasticity in the tangential direction while lowering the modulus in thelongitudinal direction on the stress field namely by reducing ayand increasing az. This effect results in growing chances of creating a transverse fractures rather than a longitudinal fracture.Figure 20. Shows results of Case-IV model (Orthotropic_strike-slip) : Plot of (ayand az) against the tangential over the longitudinal moduli ration (R=E2 / E1 ).Figure 21 . Shows results of Case-IV model (Orthotropic_strike-slip) : Plot of (ayand az) normalized by stresses when the tangential over the longitudinal moduli ration (R=E2 / E1 ) is one against the variation of this very ratio.Figure 22. Shows popular casing failure mode by formation movement indicating the importance of using high stiffness casing (e.g., steel) to resist bending and further implications.Figure 23. Shows an example of the concept of implementation for a method to support the borehole structure by a primary casing (e.g., steel) and a selected secondary casing that engages with formation.Figure 24. Shows development of stress in the casing / formation or casing / cement adhesion line when the borehole is subjected to pressure. In the case of large differences in stiffness between the two adhered materials, delamination and shear failure modes probability arises.Figure 25. Shows a high range and magnitude of residual stresses imposed by large stiffness casing reduces the fracture opening and hence lower proppant volume can be placed. Meanwhile, the opposite effect occurs when the casing has lower stiffness.Figure 26. An approach to implement the double casing method proposed by this invention in fracturing namely by placing different stiffnesses in each zone and fracture all the zones in one pressure rise at which each zone has its own breakdown pressure.Figure 27. Shows an example of the embodiment of the invention. In this design, the secondary casing is applied directly to the primary casing. The former is secured to the latter by a clamp and seal kit at the two ends of the primary casing.Figure 28. Shows an example of the embodiment of the invention. In this design, the secondary casing is attached to the primary casing, however, with a gap between them. The former is secured to the latter by a clamp and seal kit at the two ends of the primary casing. The gap can be filled with incompressible fluid to equalize the pressure across each of the two casings during installation. The seal kit is employed to prevent leak to or out of the gap.Figure 29-a. Shows an example of the embodiment of the invention. In this design, the secondary casing is attached to the primary casing, however, with a slightly larger gap between them. The clamp has open windows allowing cement to flood the gap between the two casings. The secondary casing has three layers: one attached to the outer cement and engage with the formation, the lower layer attached to the beneath cement. The third layer in the middle that act as separator until the borehole is perforation. Then, the pressure must displace this layer away from the secondary casing.Figure 29-b. Detailed schematic of the secondary casing assembly shown in Figure 28-a.Figure 30. Shows example of deploying a composite pipe as a secondary casing.DETAILED DISCRIPTION OF THE INVENTION

[0013] In order to provide proper approach to the method, it important to emphasis the observations from dedicated studies and field records that support the claim of this proposal. One important phenomena is the microannuli seepage of fracturing fluid. Microannuli is a small gap between the casing and formation, can influence fracture initiation pressures. These gaps can reduce the pressure required for fracture initiation by providing a path for fluid to bypass the rock (Taleghani and Klimenko 2015). The reason of this reduction is that when the fluid leak around a perforation, it apply pressure in a form closer to an open hole. Regarding openhole fracturing, it is very well known that openhole requires much lower pressure than cased holes which brings us to the second observation. Research comparing fracture initiation pressures in openhole and cased hole configurations shows that openhole environments generally require lower initiation pressures compared to cased holes. This difference is due to the presence of additional stresses and potential for damage in cased holes (Taleghani and Klimenko 2015). It can be stated that the extreme high stiffness of the steel casing is restricting and arresting the energy transfer to the perforation (Figure 1 .) Namely, it dissipates the strain energy of the formation since it owns a stiffness ratio of up to 100 (Ec / Ef=100). Obviously, the casing stiffness is altering the stress field in the formation when pressure is applied particularly in the presence of stress concentrators like perforations.

[0014] Starting from these observations, it is essential to study the effect of casing stiffness on the stress field at the formation and the associate fracturing pressure. It worth mentioning that, trials to change casing stiffness is initially not acceptable as who would dare to replace steel that was used for several decades supporting the wellbore and providing the required strength and stiffness. Hence, for the first time, this patent is providing a solution that allow designing a suitable casing with a selectable stiffness without gabredizing the wellbore integrity by maintaining the ultimate method of “rig id’Vhig h strength casing.• Finite element model

[0015] Finite element modeling has been widely used to simulate stress fields around perforated wellbores. Notable work in this area includes studies that focus on the stress distribution without simulating fracture initiation, providing insights into how stress fields influence fracture behavior (Xi et al. 2024; AITammar et al. 2022). This numerical simulation method was effective to compare the influence of different conditions on the fracture initiation pressure. Therefore, to provide evidence of the validity of the patent and to a sense of quantitative insight, a finite element model was built using the commercialized software (ANSYS APDL). Four different written codes of (18-30) pages. Each of these codes have a section to input the properties and dimensions. When the code is paste andrun in the software, it automatically generates the geometry and mesh, record the material properties and apply the boundary conditions. Finally, it executes the solution in a do-loop matter where at each iteration, it records the stress field parameters and produces a data file allowing the user to generate plots easily.

[0016] Four different models / cases were created where the first three are under one material assumption and the fourth is assuming different material constitutive behavior. Therefore, we will classify the workflow based on these two assumptions.1 ) Linear elastic isotropic model.

[0017] The model is a stress-based model where a perforation / s is / are created in a block of rock (Figure 2.). The casing nodes are coupled to their corresponding rock nodes. Initially, cement was simulated. However, it was observed that it didn’t effect the result as cement is close to the rock material properties and material behavior in addition to having relatively small thickness comparing with the vast volume of the rock. Hence, to reduce the computational complexity, the cement was dropped from the simulation.

[0018] The material is modeled as linear elastic isotropic material in order to eliminate inhomogeneity factors and maintain the focus on the effect of casing stiffness on the stress field. The material properties of the rock represent the highest possible sandstone stiffness and typical passion ratio. The casing stiffness varies at each iteration (independently) as shown in Table-1. The geometry is considering field practice ranges and typical dimensions (Table-2). The applied boundary conditions include the confining pressures which are applied at the territories / faces of the block while the borehole pressure is applied on the casing and the perforation. In the first case, the confining pressures were selected to be identical in order to eliminate the effect of confining direction and perforation azimuth orientation. In the second case, the confining pressures are selected to mimic a strike-slip regime (popular in many regions specially in Saudi). In a typical strike-slip regime the borehole is drilled toward the minimum horizontal stress) (Figure 3.). In this case, the perforation azimuth angle is oriented toward the vertical stress which is the optimum direction as it acts similar to a vertical well in term of stress field intensity. In the first two cases, only one perforation was simulated in order to provide an indication of the effect of casing stiffness on the stress field. To provide a broader indicator, a third model where six perforations arranged in a helical track is implemented (Figure 4.). This arrangement is the typical perforation orientation of a perforation cluster in the field practice. A summary of the three cases is shown in table-3.Table-1 : Material properties of both rock and casing.Table-2: Geometrical dimensionsParameter Field practice Model WhereCasing t / r 0.02-0.05 6.025 t=thickness, r=radius_ P.erf ,orat ..ion r / / DBLHJOR A 0.d15e-7? A 0.d15e-7? r=radius, BHR=borehole radiusPerforation Density 6 spf 6 spf Spf=shot per feetPerforation L / D 20 20 L=length. D=DiameterTable-3: Summary of the three modeling cases under isotropic material assumption.*Unit=psi*Borehole is toward crh

[0019] The rock model consist of two geometrically mapped meshes. The first is around the perforation (2.5 x the radius) to collect the stress field data (Figure 5.), beyond this thickness is the far field rock material.

[0020] It worth mentioning that the mesh was refined to establish batter examination of the stress distribution in the near field (Figure 6.). There are two types of fractures initiated from the borehole, a longitudinal fracture initiated as result of the stress in the y-direction reaching certain threshold. In contrast, the second type is a transverse fracture initiated as the stress in the z-direction reaches the threshold. Hence, in the results, these two stress parameters will be used as a comparison criteria of the varied casing stiffness (Figure 7.). A third parameter is employed which is the principal stress. The principal stress criteria are used to compare fracture initiation pressures across different techniques. (Liu et al. 2019; Xu, Singh, and Zoback, n.d.; Xue et al. 2023) conducted a study comparing initiation pressures using principal stress as a criterion. Their work emphasized how variations in stress orientation influence fracture initiation, providing valuable insights into the factorsaffecting initiation pressures. In order to avoid the stress overshoot due to the concentration of the perforation / casing junction, in the analysis result, the focused domain is selected to be between the tip of the perforation and down to 90% of the perforation length. Nevertheless, he results are recorded of eliminated part of the junction. (Figure 8.).

[0021] The results of what I call (fish head) plots is showing:The rock stresses in the (x, y, z and S1 ) directions plotted against the casing stiffness in the following cases: o Equal confining (case-1 ): Figure 9 and Figure 11 . o Strike-slip (case-ll): Figure 10.

[0022] In order to quantify the effect of casing stiffness, each stress parameter is normalized by the same parameter when the casing material is steel (e.g., E=200 Gpa). For example, the normalized y-direction stress is (oy / oy s) where oysis the maximum y-stress when the casing stiffness is 200 Gpa. For this reason, all the curves in the plot are meeting at a normalized stress value of unity at E=200 Gpa. It can be observed from (Figure 11 and Figure 12) that the lower the casing stiffness, the higher the stresses required to initiate a fracture.

[0023] In all these plots, the stiffness axis was converted to a log-scale in order to visualize the stress values at a range of stiffness below 20 Gpa where an interesting transmission is happening in the curves. It can be observed that, the relatively low casing stiffnesses are increasing the stresses to fracture a formation by up to more than 2 times. This indication is showing a powerful method to increase the chances of fracturing a formation and avoid risk of losing production due to failure of fracturing or reaching breakdown pressure. The increase of stress levels can be attributed to the dilation of the perforation as can be seen in Figure 13. This support the hypothesis that high stiffness casing is dissipating strain energy generated by the borehole pressure.

[0024] An additional insights are provided when we simulated an openhole case by replacing the casing with rock material (Casing radius = borehole radius) which is shown in Figure 14. In another consideration, the casing was removed, and the pressure was applied on the rock face (borehole radius = previous borehole radius) which is shown in Figure 15. Interestingly, the case where the casing stiffness is lower than the rock, the stresses are still increasing but with a lower rate. This is because the casing has stiffness lower than the rock. In this situation, the casing is applying tensional stresses on the rock while the rock is dissipating the strain energy. From Figure 14., the transition is obviously causing a change in the rate converting the curve from hypo-function to hyper-function.

[0025] The result of the six perforations (i.e., case-3) is consistent with the single perforation as can be seen in Figure 16. In the latter, x-direction stress is also increasing along withthe y-direction stress. This is because the casing hoop stress direction changes from one perforation to another. In another word, x-direction perforation stress is similar to y- direction stress in another perforation. This observation approves that this method can be applied to any borehole regime and arrangement. In another word, it does not interfere with prior art / methods to reduce fracture initiation pressure.

[0026] Stress averaging techniques, as proposed by (Carter 1992; Lu et al. 2022), help estimate fracture initiation pressures by averaging the stress fields around perforations. This method provides a more accurate assessment of initiation pressures under different conditions. Hence, the current study is presenting the stress distribution around the perforation in both y- and x-directions. Different locations were selected to examine the stress distribution to visualize them in curves as shown in Figure 17 curves. The average of these stresses is shown in Figure 18. In these figures, the predicted influences discussed earlier of the effect of casing stiffness on the stress field is indeed revealed from the simulation. Overall, it can be observed that even the stress distribution is affected by the casing stiffness as the stresses are much higher when the casing stiffness are reduced.2) Innovation novel approach of the orthotropic model.

[0027] Geologists and petroleum engineers have observed that although longitudinal fractures are theoretically expected to initiate first, transverse fractures are often observed in wells drilled towards the minimum horizontal stress. This is because natural fractures are typically oriented against the minimum horizontal stress (Zoback & Harper, 1991 ). For certain fracturing design, reservoir engineers aim to find methods to increase chances of creating transverse fractures. Not only to ensure reaching breakdown pressure but the goal is further beyond. For instance, the transverse fracture is striking the reservoir vertically along 360° range of angle (across the wellbore circumference). This improves the possibility of involving all reservoirs directions and formation properties.

[0028] From the previous model, it was observed that the casing stiffness is crucially affecting the stress field in the rock formation. Hence, in this model, we are aiming to manipulate the casing stiffness in order to intensify the stress in the direction we desire by concentrating higher energy proportion toward it. One way to do it is by the orthotropic materials.

[0029] An orthotropic material has different properties in three mutually perpendicular directions, meaning its material response to stress varies depending on the direction. This type of material exhibits directional dependence in its mechanical, thermal, or other physical properties. Unlike isotropic materials, which have uniform properties in all directions, orthotropic materials show distinct variations along three axes — typicallyreferred to as the x, y, and z axes (Barbero 201 1 ; Daniel and Ishai 2006). An example of an orthotropic material is a fiber-reinforced polymer composite. In such materials, fibers are embedded within a matrix (e.g., epoxy) to provide reinforcement. The fibers, usually made of materials such as carbon, glass, or aramid, are aligned along specific directions to enhance mechanical properties. The matrix binds the fibers together and helps distribute loads.

[0030] In a unidirectional fiber-reinforced composite:• The longitudinal direction (fiber direction) exhibits high stiffness and strength due to the fibers.• The transverse direction (perpendicular to the fibers) shows lower stiffness and strength because the matrix dominates the behavior in that direction.

[0031] For example, carbon fiber-reinforced polymers (CFRPs) display very high tensile strength and stiffness in the fiber direction (typically denoted as E1 ) but much lower properties in the transverse directions (E2 and E3) (Gibson 2016; Jones 1998). Fiber- reinforced composite materials are increasingly used in the construction of pipes, particularly in applications requiring high strength-to-weight ratios, corrosion resistance, and flexibility in design (Al-Dakheel et al. 2021 ; Al-Dakheel, Albinmousa, and Temitope 2022). These pipes are commonly found in industries such as oil and gas, water transportation, and chemical processing. One of the most critical factors in determining the mechanical behavior of composite pipes is the orientation of the fibers. The direction in which the fibers are placed has a significant impact on the ability of the pipe to handle stresses (‘Fatigue Analysis of Surface Pretreated Adhesively Bonded CFRP Using FEM and X-Ray Tomography’ 2020). When fibers are oriented circumferentially (perpendicular to the axis of the pipe), the pipe has enhanced strength in resisting hoop stresses, which are crucial for containing internal pressure. This orientation is commonly used in pressure vessels or pipes that must withstand high internal fluid pressures. Circumferentially oriented fibers, though excellent at resisting radial stresses, may have lower resistance to axial tension or bending, making them less ideal for axial loading conditions.

[0032] In our model, we are providing and example of controlling the stress field by orienting the fibers in a direction that arrest the associated stress (Figure 19). Here, the fibers are oriented in the tangential direction to reduce the hoop stress (Le., parallel to y-direction in the previous model) and allow strain energy to transfer to the axial direction (Le., parallel to z-direction in the previous model). Consequently, there will higher chances of creating a transvers fracture due to the intensified stresses in the z-direction. In the model, a cylindrical coordinate system is utilized to simulate material orthotropy. The modulus of elasticity E1 is equated to the tangential direction modulus Eewhile E2 is equated to the axial modulus Ea:

[0033] In order to study the effect of altering the stiffness of the axial direction, the tangential modulus is fixed while the tangential modulus is ranged from 7% to 100% of the tangential modulus. For automating purpose, we set a ratio (R) that is changing in the iteration as follow:

[0034] The stiffness matrix of orthotropic material is shown below. It is essential to ensure that the matrix is positive definite. Hence, the below relation must be satisfied for any selection of properties.being ,

[0035] The three dimensional stiffness matrix (Jones 1998) is

[0036] While the inverse / compliance matrix (Wickeler and Naguib 2020; ‘2.1. StructuralFundamentals’, n.d.) is

[0037] In order to deploy a realistic material model, they must be related accordingly. Properties of composite materials are derived from their raw origins (the fibers and the matrix). Hence, the following parameters are related (Christensen and McCoy 1980):"Vf = f(El,E2,Em)""Ef = f(El,Em,Vfj""vij = f (V f , vf , vm)""Gij = Ei / (2(1 + vij)""E3 = f(Em)Where,Vf: Volume fraction based on selected properties vf: Fibers Poisson ratioVm: Matrix Poisson ratioEf: Fibers modulus of elasticityEm: Matrix modulus of elasticityEi: Orthotropic modulus of elasticity in “i” directionVij: Orthotropic Poisson in “ij” directionGij: Orthotropic shear modulus in “ij” directionThe final selection of main properties (where the rest are derived from) is listed in table-4.Table-4: Selected major properties to simulate composite material.Property _ Magnitude _El 9 GpaEf 12.89 GpaEm 0.2 Gpa vf 0.22 vm 0.35Vf 0.693

[0038] The results of the model are shown in Figure 20 and Figure 21 . Both y- and z-directions are plotted against the ratio of longitudinal modulus over the tangential. At ratio of 60%, the stress in the formation starts to be influenced by the variation of stiffnesses and significant influence started from 20% and below. However, as the stiffnesses get closer in magnitude, the solution marching to a platue approaching the solution of isotropic material. At a ratio of 10% and below, the stress inequality shift, making the stress values in the z-direction higher than the y-direction. To set a sense of evaluation, a normalization was established where we divided all the stress magnitudes by their corresponding stress value when the ratio is unity (Figure 21 ). It can be seen that, the stress in z-direction overshoot to up to 4.5 times. This provides an indication that the proposing method can increase the chances of creating a transverse fracture in wellbore allowing full control of fracture placement and orientation and striking the reservoir at a desired angle.• Concept of implementation

[0039] From the previous study, we have shown that the casing stiffness magnitude, direction and anisotropy effect the stresses developed in the formation resulted from the appliedborehole pressure. As promising the method to be, it faces numerous of challenges. The main challenge is that steel as casing material is irreplaceable to many experts in petroleum and petrophysics engineering as it provides the necessary stiffness and strength to support the wellbore structure. Here, we will present number of scenarios that jeopardize the well integrity in the case of considering lower casing stiffness or particularly replacing steel as casing material.Casing bending during drifting: Casing bending during drifting operations poses a significant risk. Research by (Zhang et al. 2023) highlights the factors leading to casing bending and the associated risks. Low stiffness casing may suffer from buckling as well.Internal Deformation (Collapse): Internal deformation or collapse of casing can result in borehole clogging. (Shi et al. 2024) explored the causes of internal casing deformation and strategies to mitigate these issues.Formation weight changes and geological movements can cause casing bending. (Mohammed et al. 2019) examined the effects of these factors on casing integrity. One example of casing bending due to formation movement is when a transverse fracture is initiated in horizontal well, the overburden pressure deforms the casing causing severe failure that prevents access to the wellbore and lose of production (Figure 22).Corrosion and erosion are critical factors leading to casing failure. (Kiran et al. 2017) reviewed the causes and effects of casing corrosion and erosion, providing mitigation techniques.Reliability is deriving many field developers to select steel as casing material to support the wellbore structure. The certainty / confident is moving from the success of using steel for many decades and various environments.

[0040] For these reasons, we introduce the dual casing completion solution that allow using casing to support the wellbore structure from impediment to the well life cycle. Meanwhile, permitting lower casing stiffness engaging with the formation. The two casing are attached one to the other or combined somehow. As can be seen in Figure 23, steel or any high stiffness casing is attached to another lower casing where the former support the wellbore structure while the latter engage with the formation. The steel casing is not effected by the applied pressure as the pressure across the thickness is equal. The lower casing is attached to the formation providing all the necessary properties to allow and increase energy transfer. In addition, a lower casing stiffness provide flexibility during production by ease interaction with formation minor displacements (e.g., dilation). Here, we advise to maintain a gap between the two casings for sake of precaution as:Our modeling attempts has shown that attaching the two casings doesn’t provide the interesting results we have shown.If any of the casing failure modes we provided previously occurred, consequences can be catastrophic. For example, if formation movement due to transverse fracture occurred, an excessive unfilled gap between the two casing will prevent the steel casing from supporting the formation. Hence, implications of failure can take place leading to more severe failure modes to the formation and even the steel casing.

[0041] From shell analysis point of view, the outer casing does not necessarily have to have a lower stiffness, it can be at any stiffness, however, with features that allow lower resistance to stresses or energy transfer (e.g., smaller thickness). This might lower thestrength of the outer casing. In the worst-case scenario, possible failures in the secondary / outer casing will be terminated at the edges as this casing is not necessarily continuous along the series of the inner / primary casings. Meaning that at the primary casing collar / connection, the casing and cement are stabilized and isolated even from one neighboring casing to another.

[0042] Another important aspect is the popular failure of steel casing being engaged with production zone. Detachment between steel casing and the formation is occurring because of the vast different in stiffness. This is known in many engineering applications. As can be seen in Figure 24, the applied pressure is causing perforation to expand which impose stresses at the junction. The high difference in stiffness between the casing and formation, leading to shear and delamination modes at the adhesion line. However, a lower casing stiffness has the advantage to reduce this effect as it has closer stiffness to the formation.• Material technology

[0043] Today, material stiffness control is widely studied and implemented in various engineering applications. For this reason, we are suggesting considering these techniques. They can be utilized in the primary casing, in a secondary casing or even further beyond. Examples of the material technology:• ElectroActiv Polymers (0.01 -1 GPA)• Shape Mamory Alloy (70-80 GPA)• Magnetrorheological Materials• Piezoelectric Material (50-1 OOGpa)

[0044] Advantages of the invention:Reduces fracture initiation pressure (reduce risk of losing production and failure of stimulation). Enhance production of hydrocarbons by providing flexibility in interactions with the reservoir.Protection (provide sort of protection to the primary casing as it is sacrificed in corrosive environment) along the well life cycle.Implementation feasibility and adoption convention: possible on-site assembly of the secondary casing to steel casing provided by any party.Maintain the required strength and stiffness to the wellbore structure.Control fracture orientation by the implementation of the novel method of utilizing orthotropic material.Maintain continuity of the wellbore structure without interrupting the well completion tools. Post fracturing enhancement and proppant efficiency (Figure 25).■ Steel by high stiffness lead to fracture arresting and create residual stresses. When pressure is released, fracture width reduces. As proppant injected, less volume will be occupied in the fracture surface. In addition, placed proppant in the fracture will experience excessive compression force.■ In the case of lower stiffness casing, the formation dilation is higher allowing for wider fractures as less arresting is applied and residual stresses created. Consequently, larger proppant volume is injected experiencing lower compression force.■ Applicable to all fracture orientationsHigh accuracy is not required as the error on depth can be (perforation diameter / casing length) which is a very small value. Therefore, it is very unlikely to perforate in the casing collar region. Fracture placement control:■ First consideration: Stress shadows occur when existing fractures alter the stress field around new fractures. This phenomenon affects fracture propagation and wellbore stability. (Han, Cui, and Zhang 2020) one of the latest studies on the stress shadows phenomena, and (Pidho, Cheng, and Yan 2022)investigated their impact on fracture propagation and stability. In another word, existing fractures in one zone can create a cloud of stresses, increasing the pressure required to fracture.■ Second consideration: in the same zone, formation strength may vary. In order to unify the required pressure to create fractures, the casing stiffnesses are tuned with the formation strength.■ Third consideration: by careful and smart design in plug & perf Intervention, the entire unconventional technique can become completely efficient by implementing the patent method. This can be deployed by placing higher stiffness casing at the upper zone and gradually increase the casing stiffness down to the lowest zone. Then, it is possible to fracture all zones in one single pressure increment striking the of them gradually. An approach to visualize the multistage fracturing is shown in Figure 26.Available material (for primary or secondary casing) at which they have comparable steel strength:■ Composite Material (0.5-100 Gpa)■ Aluminum Alloys (70 Gpa)■ Titanium and Platinium Alloys (100-200 Gpa)• Examples of the embodiment

[0045] Here, we are offering examples of embodiment of the proposed method where it is applicable but not limited to:1 - The first embodiment involves a primary casing (e.g., steel) surrounded by a secondary casing. The latter is secured to the former by a clamp where seal kit is placed to prevent cement leak (although it is unlikely due to the cement high viscosity being injected at low pressure). Between the two casings, the gap can be filled with incompressible fluid to equalize the pressure across the secondary casing (Figure 27).2- Although our previous modeling attempts have shown ineffective performance of the proposed method when the two casings are attached. Yet this embodiment still can be considered due to its simplicity. Hence instead of filling the gap, the gap is eliminated (Figure 28). This embodiment can be implemented by either a clamp or an adhesive.3- The secondary casing is attached to a clamp that is connected to the primary casing. The clamp allows the borehole cement to fill the gap between the two casings. The secondary casing is covered in the inner diameter surface by a sheet (Figure 29-a). A spacer (rubber or adhesive) between the sheet and the casing can be placed. This sheet will eventually be attached to the cement. When the borehole is loaded, the pressure separates the sheet from the secondary casing permitting the secondary casing to engage with the formation (Figure 29-b). This embodiment can have another deployment technique. That is the deploying the secondary tube as a continuous pipe and cement it to the formation. Then deploying the primary pipe (as production casing or liner or other form) and cement it to the secondary pipe. The drawback of this technique is that for any reason that cause failure to the secondary pipe, this failure propagates along the pipe leading to implications on the wellbore structure. Hence, we advise to limit the secondary casing deployment to the edges of the primary casing.4- The primary casing possesses both properties of high stiffness to support the wellbore and low stiffness to treat the formation (e.g., anisotropic material). This is either done by anisotropic material (e.g., composites) or by smart material technology. An example of the former is steelfiber reinforced matrix where steel support longitudinally while the lower tangential stiffness enhances the stress field in the formation. (Figure 30)5- Although the continuous primary casing supports the completion structure while drifting and during operation, the secondary casing can be further supported. This can be achieved by either placing a steel part (sheet or rod) between the two clamps. Another way is by using a groove in the primary casing to support the clamps.• Bibliography

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Claims

AMENDED CLAIMS received by the International Bureau on 31 October 2025 (31.10.2025)Claims

1. A well completion assembly for hydraulic fracturing of a subterranean formation, comprising: a primary, load-bearing casing string formed of a first material and configured to maintain wellbore structural integrity; and a secondary casing section arranged radially outside the primary casing and configured to be engaged to the formation across perforations, wherein a hydraulic equalization path is provided such that internal borehole pressure is substantially equalized across a thickness of the primary casing during fracturing, whereby the primary casing is substantially decoupled from pressure-induced radial strain while the secondary casing is configured to transfer strain energy into the formation to modulate the local stress field and reduce breakdown pressure.

2. The assembly of claim 1 , wherein the hydraulic equalization path comprises an annular gap between the primary and secondary casings filled with fluid during fracturing.

3. The assembly of claim 1 or 2, wherein the secondary casing is cemented to the formation and a separator layer is configured to debond or displace under pressure to activate engagement of the secondary casing with the formation.

4. The assembly of any of claims 1-3, wherein the secondary casing exhibits an elastic modulus lower than steel and within 0.5-100 GPa.

5. The assembly of any of claims 1-4, wherein the secondary casing comprises a fiber-reinforced composite.

6. The assembly of any of claims 1-5, wherein the secondary casing is orthotropic, having a tangential (hoop) modulus E1greater than an axial modulus E_z such that E_z / E1is between 0.07 and 0.6.

7. The assembly of claim 6, wherein the fiber orientation is substantially circumferential to increase tangential stiffness and bias fracture initiation toward a transverse orientation relative to a wellbore axis.

8. The assembly of any of claims 1—7, wherein the secondary casing comprises a property-adaptive material selected from electroactive polymer, shape memory alloy, magnetorheological, or piezoelectric material to tune stiffness during or after fracturing.

9. The assembly of any of claims 1-8, wherein the secondary casing is provided as discrete sections positioned at respective fracturing stages along a horizontal well.

10. The assembly of any of claims 1-9, further comprising clamps and seal kits at ends of each secondary section to hydraulically isolate an annular gap and maintain the equalization path during stimulation.

11. The assembly of any of claims 1-10, wherein the primary casing comprises steel and the secondary casing is non- continuous between adjacent primary joints.

12. A method of hydraulic fracturing using the assembly of any of claims 1-11 , comprising:(a) providing the assembly in a wellbore;(b) forming perforations that communicate borehole pressure to the secondary casing and the formation;(c) hydraulically equalizing pressure across the thickness of the primary casing during pumping so that strain energy is preferentially transmitted by the secondary casing into the formation; and(d) injecting fracturing fluid to initiate and propagate fractures at a reduced breakdown pressure relative to a steel-only casing configuration.

13. The method of claim 12, further comprising orienting the secondary casing fibers to provide E1> E_z and thereby biasing fracture initiation toward a transverse fracture relative to the wellbore axis.

14. The method of claim 12 or 13, wherein zonally varying the stiffness of the secondary casing along multiple zones equalizes or staggers breakdown pressures such that a single pressure ramp fractures a plurality of stages sequentially.

15. The method of any of claims 12-14, wherein reduced residual stresses after pressure release increase proppant placement volume and reduce proppant crushing.

16. A multi-zone fracturing system comprising a plurality of secondary casing sections according to any of claims 1- 11 arranged at respective zones, each section having a predetermined stiffness so that, under a monotonic pressure schedule, the zones initiate in a predetermined order.

17. The system of claim 16, wherein at least one section is orthotropic with E_z / E1between 0.07 and 0.6 to promote transverse fracture initiation, and at least one different section has a higher axial modulus to promote longitudinal fracture initiation.

18. The assembly of any of claims 1-11 , wherein the hydraulic equalization path renders a radial pressure differential across the primary casing of less than 10 of borehole pressure during pumping.

19. The assembly of any of claims 1-11 , wherein the annular gap is configured to receive cement or an incompressible fluid and includes windows to allow controlled flooding of cement between the primary and secondary casings.

20. Use of the assembly of any of claims 1-11 for enhancing hydrocarbon production by reducing fracture breakdown pressure and increasing post-fracture conductivity through reduced residual formation stresses.STATEMENT UNDER ARTICLE 19 (1 )This statement is submitted under Article 19(1) of the Patent Cooperation Treaty to provide a concise explanation of the amendments made to the claims and their technical significance. The amendments clarify and reinforce the inventive concept relating to hydraulic fracturing enhancement and control through the manipulation of casing material properties.The amended claims emphasize the explicit hydraulic decoupling of the primary casing via pressure equalization across its thickness, allowing the casing to respond dynamically to the formation pressure. This functional decoupling ensures a more uniform pressure profile along the borehole and reduces stress concentrations that could otherwise lead to premature casing failure.A further inventive aspect lies in the introduction of a secondary, lower-stiffness — optionally orthotropic — casing that engages directly with the formation. By tailoring the tangential-to-axial stiffness ratio, the structure promotes controlled energy transfer and preferential transverse fracture propagation, leading to improved fracture initiation and placement.Finally, the concept of zonal stiffness programming is introduced, enabling multiple fracture zones to initiate sequentially under a single pressure schedule. This design achieves operational simplicity while maintaining precise fracture staging, thus enhancing efficiency and reliability of hydraulic stimulation.These amendments do not introduce new matter but clarify and refine the inventive scope originally disclosed. The invention as claimed contributes to more predictable fracture control, reduced energy loss, and improved production performance.No disparaging comments are made regarding the International Search Report or the cited prior art.

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