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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.
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.
Shin, K. (Central Research Institute of Electric Power Industry, Abiko) | Ito, H. (Central Research Institute of Electric Power Industry, Abiko) | Okubo, S. (University of Tokyo) | Li, F. (Institute of Crustal Dynamics)
ABSTRACT: Hydrofracturing breakdown has been investigated probabilistically. Numerical analysis based on weakest link theory revealed that Pb and variation in fracture azimuth are affected by stress difference, σH -σn , This relationship is used to estimate σH Independent of Pb and Pr.
RÉSUMÉ: Resultat d'une etude de probabilite de rupture par hydrofracturation. L'analyse numerique basee sur la theorie du lien le plus faible a montre que Pb et la variation dans l'azimuth de fracture sont affectes par la difference de tension, σH -σn . Cette relation permet d'estimer la valeur σH independamment de Pb et de Pr.
ZUSAMMENFASSUNG: Hydraulische Felsspaltung wurde wahrscheinlichkeitstheoretisch untersucht. Die auf der Theorie des schwachsten Gliedes basierende numerische Analyse zeigte, daß die Parameter Pb und Variationen im Bruchazimuth durch die Belastungsdifferenz σH -σn beeinflußt werden. Diese Beziehung wurde dann dazu eingesetzt, σH unabhangig von Pb und Pr zu berechnen.
1 . INTRODUCTION
Hydrofracturing is widely used for stress measurement due to its simplicity and high applicability at great depths. In conventional hydrofracturing, assuming an axi-parallel fracture plane in intact rock, the minimum stress σh in the plane normal to hole axis is calculated from the shut-in pressure Ps ,and the maximum stress σH from breakdown pressure Pb or re-opening pressure Pr . However, σH is less reliable because the breakdown pressure depends on water permeation. Furthermore, reopening is difficult to detect and the mechanism by which reopening occurs remains unclear. Therefore, information concerning σH that could be obtained independent of Pb and Pr would be extremely helpful.
In the present paper; the effects of stress difference, (σH-σh), on the probability distribution of the azimuth of vertical fractures is investigated as a new source of information about σH. Although the azimuth of fracture has been thought to coincide with that of maximum stress σH, one can easily expect that the azimuth of fracture would be randomly scattered when the stress difference is significantly small. Therefore, probabilistic distribution of fracture azimuth may provide information about σH.
In order to analyze the distribution that occurs due to locational variation in strength, the present paper adopts the weakest link theory to investigate the breakdown phenomenon probabilistically. Based on the proposed theory, the probability of breakdown and the p.d.f of fracture azimuth are determined. In addition, the mean and the 90% confidence interval of Pb (or tensile strength n and the 90% confidence interval of fracture azimuth are calculated numerically, The obtained results indicate that the calculated values are closely related to the stress difference and thus provide new information about σH. This approach to stress estimation has been applied in an in-situ test of a granitic rock mass having scarce joints.
2 ANALYTICAL METHOD
2.1 Weakest Link Theory
According to weakest link theory, the weakest among the innumerable defects present in a material determines the strength of the material. When the defects are distributed uniformly throughout the specimen and the strength of each defect is given according to Weibull distribution, the probability distribution of the strength, S, of the specimen is given as follows, where m, V, b are the Weibull's coefficient of uniformity, the volume of the specimen and a constant, respectively (Weibull,1939).
Kashnikov, Yu. A. (Perm National Research Polytechnic University) | Ashikhmin, S. G. (Perm National Research Polytechnic University) | Shustov, D. V. (Perm National Research Polytechnic University) | Kukhtinskii, A. E. (Perm National Research Polytechnic University) | Kondratiev, S. A. (PermNIPIneft)
ABSTRACT: Western Ural (Russia) oil fields have been developed since 1960s. Hydraulic fracturing is the main method of increasing well productivity in the past 15 years. Currently re-fracture treatments are being used to sustain the level of oil production. The analysis of the performed treatments shows that re-fracturing is not as effective as the initial fracturing. The technology of oriented re-fracturing is being developed to increase the effectiveness of re-fracturing treatment. Creation of the hydraulic fracture which is pre-oriented has a number of advantages: a possibility of accessing poorly drained parts of the formation, decreasing the risk of water breakthrough etc. The main idea is to create a system of lateral horizontal boreholes situated in a vertical plane and oriented in the desired direction. The number of lateral boreholes and the space between them is determined based on geomechanical calculations. One industrial experiment was performed on the well to this date. The results are found to be optimistic.
Many oil and gas reservoirs need to be hydraulically fractured for the economic development. Re-fracturing is a quite efficient way to control production decline after the first fracturing treatments. As discussed by He Liu et al., 2008, conventional techniques used for refracture treatments can lead to the following results:
Re-fracturing is not effective in the first and second cases, or it can have a negative effect such as water intrusion. The technology of oriented hydraulic refracturing aimed at the increase of re-fracturing efficiency in the Western Ural oil fields is being developed. Creation of the hydraulic fracture which is pre-oriented allows to increase the treatment efficiency and to decrease the risk of negative consequences. The main idea of oriented hydraulic fracturing in a vertical well is to create a system of lateral horizontal boreholes situated in a vertical plane and oriented in a desired direction. The number of lateral boreholes and the space between them is determined on the base of geomechanical calculations. In order to realize this technology, the following studies are carried out: determination of the in-situ stress state; study of physical and mechanical properties of rocks; development of theoretical and practical basics of oriented hydraulic fracturing.
Abstract Lost circulation caused by low fracture gradients is the cause of many drilling related problems. Typically the operational practice when lost circulation occurs is to add loss circulation materials (LCM) to stop mud from flowing into the formations. To improve the treatment for lost circulation caused by low fracture gradients, especially designed materials in mud system are used to seal the induced fractures around the wellbore. This operation is in the literature referred to as wellbore strengthening that has been found to be a very effective in cutting Non-Productive Time (NPT) when drilling deep offshore wells. Size, type and geometry of sealing materials are debating issues when different techniques are applied. Also the phenomenon is not truly understood when these techniques applied in different sedimentary basins. This paper presents development and simulation results of a three-dimensional Finite-Element Model (FEM) for investigating wellbore strengthening mechanism. This study also describes a procedure for designing Particle Size Distribution (PSD) in field applications. To better understand the numerical results, the paper also reviews the connection between Leak of Tests (LOTs) and wellbore hoop stress and how these LOTs can mislead in fracture gradient determination. A comprehensive field database was collected from different sedimentary basins for this study. Results demonstrate that the maximum attainable wellbore pressure achieved by wellbore strengthening is strongly controlled by stress anisotropy. Results also show that Particle Size Distribution (PSD) of wellbore strengthening should be designed in order to seal the fractures close to the mouth and at fracture tip. This will result both in maximizing hoop stress restoration and tip-screening effects. In addition this model is able to show the exact fracture geometry formed around the wellbore that will help to optimize the sealing materials design in wellbore strengthening pills. To support numerical modeling results, near wellbore fracture lab experiments on Sandstone and Dolomite samples were also presented. Laboratory experiments results reveal importance of rock permeability, tensile strength and fluid leak-off in wellbore strengthening applications.