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Summary Alternate or out-of-sequence fracturing (OOSF) has been field tested in western Siberia in 2014 and in western Canada in 2017, 2018, and 2019, with operational success and positive well-production performance. It is conducted by fracturing Stage 1 (at the toe) and then fracturing Stage 3 (toward the heel), followed by tripping back to place Stage 2 (center fracture) between Stages 1 and 3 (outside fractures). During placing the center fracture, OOSF can exploit the reduced stress anisotropy to effectively activate the planes of weakness (natural fractures, fissures, faults, and joints) to potentially create failure surfaces with different breakdown angles in virtually all directions. This can potentially lead to branch fractures that can connect the hydraulic fractures to stress-relief fractures that are created while placing the outside fractures, ultimately generating a complex fracture network and enhancing fracture connectivity. Despite prior works on fracture modeling (calibrated by field tests) and geomechanical modeling, a comparative analysis of wellbore-breakdown character and hydraulic-fracture orientation during OOSF is still lacking. Thus, in this study, the solutions to 3D Kirsch equations are provided for both low and high stress anisotropies to analyze the differences in breakdown gradient, failure angle, and fracture orientation under various geomechanical and treatment-design conditions. The consideration is given to an intact rock from an isotropic stress state to high-stress-anisotropy conditions. The results are analyzed in the context of the downhole-measured pressures and temperatures. The results indicate that the reduced stress anisotropy during OOSF leads to favorable treating conditions: With a net fracture-extension pressure greater than the reduced stress anisotropy, fracture complexity can be created by allowing the fracture to grow with different failure angles. Also, a well can be drilled and fractured at any inclination or azimuth with favorable breakdown gradients of 45 to 85% of the overburden gradient. The reduced stress anisotropy can also trigger some challenges. The near-wellstress-concentration effects can become more pronounced, promoting longitudinal fracture creation. For treatments with tortuosity greater than the stress anisotropy, longitudinal fractures can be created instead of transverse fractures because the tortuosity is transmitted to the wellbore body and not into the fractures. In this case, to initiate transverse fractures, either the wellbore must intersect the pre-existing transverse notches or the near-wellpore-fluid pressure must exceed the axial stress and rock strength (before the hoop stress reaches the tensile failure point). In addition, the fracture might lose directional control and follow any path of weakness. Hence, the rock-fabric effects become more dominant under a low-stress-anisotropy regime, which means that with no pre-existing transverse natural fractures or notches, a longitudinal fracture can be generated at the bottom and top of an intact horizontal wellbore. This is the first attempt in identifying the circumstances that should be avoided for optimizing OOSF through geomechanical modeling and the analysis of the downhole-measured pressures and temperatures to reveal the differences in breakdown character using the Kirsch equations under various geomechanical and treatment conditions during the low-stress-anisotropy regime.
Summary One of the considerations in out‐of‐sequence‐fracturing treatment is creating fracture complexity through reducing the in‐situ differential stress to enhance hydraulic‐fracture connectivity by activating natural fractures, fissures, faults, and cleats within the formation to create secondary or branch fractures (induced‐stress‐relief fractures) and connect them to the main biwing hydraulic fractures. In out‐of‐sequence fracturing, this is achieved by beginning fracturing Stage 1 at the toe of the well and then moving toward the heel and fracturing Stage 3 so that there is a degree of interference between the two fractures, followed by placing Stage 2 between the previously fractured Stages 1 and 3. Out‐of‐sequence fracturing in this mode ensures that the fracture in Stage 2 (center fracture) takes advantage of the altered stress in the rock and connects to the stress‐relief fractures from the previous Stages 1 and 3 (outside fractures), thus enhancing the connectivity of the fracture network. The first successful field trial of out‐of‐sequence fracturing was executed by Lukoil in treating eight wells in western Siberia in 2014. The first case of out‐of‐sequence fracturing in North America was later conducted in western Canada in 2017, with eight more trials followed in 2017, 2018, and 2019. In this work, a 3D hydraulic‐fracture‐extension simulator is rigorously calibrated by history matching the observed treatment pressures from the out‐of‐sequence‐fracturing field treatment in western Canada to reliably quantify the effective fracture geometries. Then, a separate set of fracture modeling is conducted to predict the hydraulic‐fracture geometries in a conventional (sequential‐fracturing) treatment of the same candidate well. Finally, production forecasting is used to assess the production potential from the candidate well according to each set of the generated fracture geometries from each of the scenarios (out‐of‐sequence fracturing vs. conventional sequential fracturing). The results of coupling the rigorously calibrated fracture modeling and production forecasting indicate noticeable production‐uplift potential from a carefully designed out‐of‐sequence‐fracturing vs. sequential‐fracturing treatment. Besides, the discovered characteristic trends in fracture geometries in out‐of‐sequence fracturing confirm some of the findings obtained in a previous sensitivity analysis of out‐of‐sequence fracturing. The previous sensitivity study entailed analyzing nearly 200 fracture‐modeling scenarios using a variety of geomechanical properties and treatment‐design variables. These characteristic trends render unique opportunities and advantages for the optimization of fracturing treatments and field development. This work is the first attempt in comparative evaluation of the effect of out‐of‐sequence fracturing by incorporating the actual field data into fracture modeling coupled with production forecasting. The learnings from this multifaceted study are worth sharing with the industry and could be used to guide future successful designs of the out‐of‐sequence fracturing for completion optimization in both unconventional and conventional reservoirs. From a large‐scale field‐development perspective, when conducted in multiple wells, optimized out‐of‐sequence fracturing has the potential of rendering full‐length interference effect and optimizing the stress shadowing while reducing the risk of well bashing.
Abstract Out-Of-Sequence Fracturing has been field-tested in Western Siberia (2014) and Western Canada (2017/2018) with operational success and positive well production performance. It is conducted by fracturing Stage 1 (at the toe) and then fracturing Stage 3 (toward the heel), followed by tripping back to place Stage 2 (Centre Frac) between Stages 1/3 (Outside Fracs). During placing the Centre Frac, Out-Of-Sequence Fracturing can exploit the reduced local stress anisotropy to effectively activate planes of weakness (natural fractures/fissures/faults/joints) to create failure surfaces with different breakdown angles in all directions. This results in branch fractures that can connect hydraulic fractures to stress-relief fractures created during placing the Outside Fracs, ultimately creating a complex fracture network, and thus, enhanced fracture connectivity. Despite the published fracture modeling works (calibrated by field tests data) by this author, comparative analyses of wellbore breakdown character and hydraulic fracture orientation during Out-Of-Sequence Fracturing are still lacking. Thus, solutions to 3-D Kirsch Equations are provided for both low and high stress anisotropies to analyze the differences in breakdown gradient, failure angle and hydraulic fracture orientation under various geomechanical and treatment design conditions. Consideration is given to a jointed and intact rock from an isotropic stress state to a reverse faulting condition. Results indicate that reduced stress anisotropy during Out-Of-Sequence Fracturing leads to favourable treating conditions: With a net fracture extension pressure greater than the reduced stress anisotropy, fracture complexity can be created via allowing the fracture to grow with different failure angles. Also, a well can be drilled and fractured at any inclination or azimuth with favorable breakdown gradients of 0.55-0.85 psi/ft. The reduced stress anisotropy may also trigger some challenges. Near-well stress concentration effects may become more pronounced, promoting longitudinal fractures initiation. For treatments with tortuosity greater than stress anisotropy, longitudinal fractures can be initiated instead of transverse fractures, since the tortuosity is transmitted to the wellbore body and not into the fractures. In this case, to initiate transverse fractures, wellbore must intersect pre-existing transverse notches or near-well pore fluid pressure must exceed the axial stress and rock strength (before hoop stress reaching the tensile failure point). Additionally, fracture may lose directional control and follow any path of weakness. Hence, rock fabric effects become more dominant under a low stress anisotropy regime, which means that with no pre- existing transverse natural fractures or notches, a longitudinal fracture can be generated at the bottom and top of an intact horizontal wellbore. This is the first attempt in identifying the circumstances that should be avoided for optimizing an Out-Of-Sequence Treatment by examining the differences in breakdown gradient, failure angle and fracture orientation under various geomechanical and treatment design conditions during the low stress anisotropy regime.
Abstract Out-Of-Sequence (OOS) Fracturing can potentially maximize reservoir contact and fracture conductivity/connectivity by creating fracture complexity via reducing the stress anisotropy. It is initiated by fracturing two "book-end" frac stages (Outside Fracs), followed by a ‘middle" stage (Centre Frac) between them. The Center Frac is theorized to utilize the reduced stress anisotropy to activate pre-existing failure surfaces oriented at various azimuths and dip angles, thereby connecting bi-wing fractures to planes of weakness (natural fractures/fissures/faults/joints/cleats) and resulting in a complex fracture network that enhances connectivity and fracture area within the Stimulated Reservoir Volume (SRV). OOS Fracturing can mitigate possible issues in treatments aiming at creating fracture complexity, including zipper frac (fracture tip interference and blunting inhibiting fracture extension), modified zipper frac (risks of well bashing and fractures growing asymmetrically opposite of the induced stress from prior stage in the adjacent well), simultaneous frac (middle clusters experiencing larger stress interference inhibiting their growth), and high-rate fracturing (risk of cluster erosion reducing the limited entry effect and premature screenout due to inconsistent diversions inside fractures). Since its inception in early 2010s, OOS Fracturing has not gained considerable attention due to previously-existing operational limitations in fracturing out-of-sequence. It is reported to have been field tested in Western Siberia in 2014 with claimed well performance success. Operational limitations of the system employed in that trial is believed to have prevented its commercial development at that time. With the advent of Multicycle Sleeves and Shift-Frac-Close operation with a single Bottom-Hole Assembly to open and close sleeves, previous operational limitations of OOS Fracturing have been resolved. OOS Fracturing has since been trialed in three formations in Western Canada (2017/2018). This work analyzes the fracture treatment pressures and well performance of these trials. Five OOS Fracturing trials in these three formations reveal that normalized 15-month/18-month production from out-of-sequence-fractured wells outperform that of sequentially-fractured offsets, with similar formation properties and treatment designs. Instantaneous Shut-In Pressures (ISIP) of Centre Frac are generally higher than that of either Outside Fracs. Breakdown pressures for Centre Fracs exhibit a mixed trend, confirming that reducing stress anisotropy could lower the breakdown gradient (based on Kirsch Equation) if rock fabric permits. Well performance and treatment pressures appear to be more sensitive to Centre Frac proppant tonnage/fluid volumes and uneven sleeve spacing. This is the first attempt in analyzing the five OOS Fracturing trials, with encouraging well performance and operational execution in conventional reservoirs where it was deployed. Despite uneven sleeve spacing, depletion due to offset production, and less favorable geomechanical properties (high Poisson’s Ratio and low Young’s Modulus), field trials produced favorable results. True potential of non-sequential fracturing is potentially more promising in unconventional reservoirs with formation properties more conducive to complex fracture generation.
Summary In out-of-sequence (OOS) pinpoint fracturing, Stage 1 is fractured, followed by Stage 3, after which Stage 2 (center fracture) is placed between Stages 1 and 3 (outside fractures). The center fracture can exploit the reduced stress anisotropy to activate planes of weakness (e.g., fissures) and create branch fractures that can connect hydraulic fractures to stress-relief fractures, ultimately enhancing fracture connectivity and complexity. It has been trialed in western Siberia (2014) and western Canada (2017 to 2019) with overall operational and production performance success. Previous fracture-modeling works calibrated by OOS fracturing trials have either used shear-decoupled planar-fracture models (in which slippage along the shear planes restricts the displacement to a limited area because of displacement damping)—which are unable to reproduce out-of-plane fracture complexity, and to dynamically track the change in stress anisotropy and orientation—or discrete-fracture-network (DFN) models, which often exaggerate the fracture-network connectivity, and reproduce unrealistically high fracture-network-extension pressures in the stimulated reservoir volume (SRV). This work attempts to resolve the issues in planar-fracture and DFN models by more realistically addressing the dominant mechanisms of OOS fracturing, dynamic changes in the stress anisotropy and orientation, activation of pre-existing planes of weaknesses, and poroelasticity using an iteratively coupled flow–geomechanical model that uses the dual-lattice implementation of the synthetic-rock-mass (SRM) model with a robust, fully coupled, iterative flow/stress solution to capture the following: Nonlinear deformations caused by induced tensile- and shear-fracture-complexity propagation Induced stress shadowing in and around the SRV Sliding of opened, pre-existing joints, fractures, and fissures using the smooth-joint model (SJM) Propagation of the hydraulic fracture as an aggregate of intact matrix fracturing and opening and slip of pre-existing fluid-filled planes of weakness (e.g., joints, fractures, fissures) Permeability enhancement in the main tensile and complex fractures following the updated deformation aperture from the coupled solution The results (fracture geometries and treatment pressures) of the three models (planar-fracture, DFN, and SRM with lattice models) are compared after using each model for treatment-pressure history matching of an OOS-fracturing trial. The calibrated, coupled SRM with lattice model more reasonably reproduces the measured fracture-extension pressures and end-of-job pressures from OOS pinpoint fracturing treatments, and it reveals the following: The dynamic change in the stress-field orientation and magnitude during OOS fracturing leads to a reduction in stress anisotropy and complex out-of-plane fracturing in the SRV for center fractures. Center fractures tend to be narrower and shorter if sufficient out-of-zone growth is attained in the absence of strong vertical containment, making OOS fracturing an option for penetrating multistacked zones in one treatment. Where center fractures are shorter or near-well fracture complexity is generated, OOS fracturing can be considered in treating the child wells to reduce fracture hits. Compared with planar-fracture and DFN models, this coupling technique achieves the following: Accounts for dominant mechanisms of complex shear and tensile fracturing Renders fast computation in simulating large 3D models with dual-lattice implementation of SRM with SJM Reproduces fracture surface area and SRV permeability more realistically Leads to a more reasonable history match of the measured OOS-fracturing pressures