Optimization of In-Stage Diversion To Promote Uniform Planar Multifracture Propagation: A Numerical Study

SPE Journal ◽  
2020 ◽  
Vol 25 (06) ◽  
pp. 3091-3110
Author(s):  
Ming Chen ◽  
Shicheng Zhang ◽  
Tong Zhou ◽  
Xinfang Ma ◽  
Yushi Zou
Keyword(s):  
Numerical Study ◽  
High Stress ◽  
In Situ Stress ◽  
Stress Difference ◽  
Multiple Fractures ◽  
Stress Shadow ◽  
Limited Entry ◽  
Fully Coupled ◽  
Flux Redistribution

Summary Creating uniform multiple fractures is a challenging task due to reservoir heterogeneity and stress shadow. Limited-entry perforation and in-stage diversion are commonly used to improve multifracture treatments. Many studies have investigated the mechanism of limited-entry perforation for multifracture treatments, but relatively few have focused on the in-stage diversion process. The design of in-stage diversion is usually through trial and error because of the lack of a simulator. In this study, we present a fully coupled planar 2D multifracture model for simulating the in-stage diversion process. The objective is to evaluate flux redistribution after diversion and optimize the dosage of diverters and diversion timing under different in-stage in-situ stress difference. Our model considers ball sealer allocation and solves flux redistribution after diversion through a fully coupled multifracture model. A supertimestepping explicit algorithm is adopted to solve the solid/fluid coupling equations efficiently. Multifracture fronts are captured by using tip asymptotes and an adaptive time-marching approach. The modeling results are validated against analytical solutions for a plane-strain Khristianovic-Geertsma de Klerk (KGD) model. A series of numerical simulations are conducted to investigate the multifracture growth under different in-stage diversion operations. Parametric studies reveal that the in-stage in-situ stress difference is a critical parameter for diversion designs. When the in-situ stress difference is larger than 2 MPa, the fracture in the high-stress zone can hardly be initiated before diversion for a general fracturing design. More ball sealers are required for the formations with higher in-stage in-situ stress difference. The diverting time should be earlier for formations with high in-stage stress differences as well. Adding more perforation holes in the zone with higher in-situ stress is recommended to achieve even flux distribution. The results of this study can help understand the multifracture growth mechanism during in-stage diversion and optimize the diversion design timely.

scholarly journals The Influence of Bedding Planes and Permeability Coefficient on Fracture Propagation of Horizontal Wells in Stratification Shale Reservoirs

Geofluids ◽  
2020 ◽  
Vol 2020 ◽  
pp. 1-19
Author(s):  
Yuepeng Wang ◽  
Xiangjun Liu ◽  
Lixi Liang ◽  
Jian Xiong
Keyword(s):  
Permeability Coefficient ◽  
In Situ Stress ◽  
Stress Difference ◽  
Strength Parameters ◽  
Breakdown Pressure ◽  
Permeability Coefficients ◽  
Bedding Planes ◽  
Initiation Pressure ◽  
Shale Reservoirs

The complexity of hydraulic fractures (HF) significantly affects the success of reservoir reconstruction. The existence of a bedding plane (BP) in shale impacts the extension of a fracture. For shale reservoirs, in order to investigate the interaction mechanisms of HF and BPs under the action of coupled stress-flow, we simulate the processes of hydraulic fracturing under different conditions, such as the stress difference, permeability coefficients, BP angles, BP spacing, and BP mechanical properties using the rock failure process analysis code (RFPA2D-Flow). Simulation results showed that HF spread outward around the borehole, while the permeability coefficient is uniformly distributed at the model without a BP or stress difference. The HF of the formation without a BP presented a pinnate distribution pattern, and the main direction of the extension is affected by both the ground stress and the permeability coefficient. When there is no stress difference in the model, the fracture extends along the direction of the larger permeability coefficient. In this study, the in situ stress has a greater influence on the extension direction of the main fracture when using the model with stress differences of 6 MPa. As the BP angle increases, the propagation of fractures gradually deviates from the BP direction. The initiation pressure and total breakdown pressure of the models at low permeability coefficients are higher than those under high permeability coefficients. In addition, the initiation pressure and total breakdown pressure of the models are also different. The larger the BP spacing, the higher the compressive strength of the BP, and a larger reduction ratio (the ratio of the strength parameters of the BP to the strength parameters of the matrix) leads to a smaller impact of the BP on fracture initiation and propagation. The elastic modulus has no effect on the failure mode of the model. When HF make contact with the BP, they tend to extend along the BP. Under the same in situ stress condition, the presence of a BP makes the morphology of HF more complex during the process of propagation, which makes it easier to achieve the purpose of stimulated reservoir volume (SRV) fracturing and increased production.

scholarly journals Apparent Toughness Anisotropy Induced by “Roughness” of In-Situ Stress: A Mechanism That Hinders Vertical Growth of Hydraulic Fractures and Its Simplified Modeling

SPE Journal ◽  
2019 ◽  
Vol 24 (05) ◽  
pp. 2148-2162 ◽  
Author(s):  
Pengcheng Fu ◽  
Jixiang Huang ◽  
Randolph R. Settgast ◽  
Joseph P. Morris ◽  
Frederick J. Ryerson
Keyword(s):  
Hydraulic Fracture ◽  
High Stress ◽  
In Situ Stress ◽  
Height Growth ◽  
Hydraulic Fractures ◽  
Simple Relationship ◽  
Vertical Growth ◽  
Stress Profiles ◽  
Fracture Height

Summary The height growth of a hydraulic fracture is known to be affected by many factors that are related to the layered structure of sedimentary rocks. Although these factors are often used to qualitatively explain why hydraulic fractures usually have well–bounded height growth, most of them cannot be directly and quantitatively characterized for a given reservoir to enable a priori prediction of fracture–height growth. In this work, we study the role of the “roughness” of in–situ–stress profiles, in particular alternating low and high stress among rock layers, in determining the tendency of a hydraulic fracture to propagate horizontally vs. vertically. We found that a hydraulic fracture propagates horizontally in low–stress layers ahead of neighboring high–stress layers. Under such a configuration, a fracture–mechanics principle dictates that the net pressure required for horizontal growth of high–stress layers within the current fracture height is significantly lower than that required for additional vertical growth across rock layers. Without explicit consideration of the stress–roughness profile, the system behaves as if the rock is tougher against vertical propagation than it is against horizontal fracture propagation. We developed a simple relationship between the apparent differential rock toughness and characteristics of the stress roughness that induce equivalent overall fracture shapes. This relationship enables existing hydraulic–fracture models to represent the effects of rough in–situ stress on fracture growth without directly representing the fine–resolution rough–stress profiles.

scholarly journals Comparative Study of the Excavation Damage and Rockburst of the Deeply Buried Jinping II Diversion Tunnels Using a TBM and the Drilling-Blasting Method

2020 ◽  
Vol 2020 ◽  
pp. 1-14
Author(s):  
Jing Yang ◽  
Xing-Guo Yang ◽  
Jia-Wen Zhou ◽  
Yong Liu ◽  
Bao-Shun Dong ◽  
...  
Keyword(s):  
Rock Mass ◽  
Stress Concentration ◽  
High Stress ◽  
Tunnel Boring Machine ◽  
In Situ Stress ◽  
Surrounding Rock ◽  
High In Situ Stress ◽  
Damaged Zone ◽  
Blasting Method

The rock mass failure induced by high in-situ stresses during the excavation of deep diversion tunnels is one of the key problems in the construction of the Jinping II Hydropower Station. Based on the results of acoustic wave tests and rockburst statistical analysis conducted, this study focuses on the excavation damaged zone (EDZ) and rockburst events in the Jinping II diversion tunnels excavated using the tunnel boring machine (TBM) method and the drilling-blasting method. The unloading failure mechanism and the rockburst induced by the two different excavation methods were compared and analyzed. The results indicate that, due to the different stress adjustment processes, the degree of damage to the surrounding rock mass excavated using the drilling-blasting method was more serious than that using the TBM method. The EDZ induced by the TBM was usually distributed evenly along the edge of the excavation surface. While, the drilling-blasting method was more likely to cause stress concentration, resulting in a deeper EDZ in local areas. However, the TBM excavation method can cause other problems in high in-situ stress areas, such as strong rockbursts. The drilling-blasting method is more prone to structural controlled failure of the surrounding rock mass, while the TBM method would induce high stress concentration near the edge of excavation and more widely distributed of stress adjustment induced failure. As a result, the scale and frequency of the rockburst events generated by the TBM were significantly greater than those caused by the drilling-blasting method during the excavation of Jinping II diversion tunnels. The TBM method should be used carefully for tunnel excavation in high in-situ stress areas with burial depths of greater than 2000 m. If it is necessary to use the TBM method after a comprehensive selection, it is suggested that equipment adaptability improvement, advanced prediction, and prediction technology be used.

scholarly journals Numerical Investigation of Injection-Induced Fracture Propagation in Brittle Rocks with Two Injection Wells by a Modified Fluid-Mechanical Coupling Model

Energies ◽  
2020 ◽  
Vol 13 (18) ◽  
pp. 4718
Author(s):  
Song Wang ◽  
Jian Zhou ◽  
Luqing Zhang ◽  
Zhenhua Han
Keyword(s):  
Fracture Propagation ◽  
In Situ Stress ◽  
Hydraulic Fractures ◽  
Fracture Networks ◽  
Breakdown Pressure ◽  
Injection Wells ◽  
Mechanical Coupling ◽  
Stress Shadow ◽  
Well Spacing

Hydraulic fracturing is a key technical means for stimulating tight and low permeability reservoirs to improve the production, which is widely employed in the development of unconventional energy resources, including shale gas, shale oil, gas hydrate, and dry hot rock. Although significant progress has been made in the simulation of fracturing a single well using two-dimensional Particle Flow Code (PFC2D), the understanding of the multi-well hydraulic fracturing characteristics is still limited. Exploring the mechanisms of fluid-driven fracture initiation, propagation and interaction under multi-well fracturing conditions is of great theoretical significance for creating complex fracture networks in the reservoir. In this study, a series of two-well fracturing simulations by a modified fluid-mechanical coupling algorithm were conducted to systematically investigate the effects of injection sequence and well spacing on breakdown pressure, fracture propagation and stress shadow. The results show that both injection sequence and well spacing make little difference on breakdown pressure but have huge impacts on fracture propagation pressure. Especially under hydrostatic pressure conditions, simultaneous injection and small well spacing increase the pore pressure between two injection wells and reduce the effective stress of rock to achieve lower fracture propagation pressure. The injection sequence can change the propagation direction of hydraulic fractures. When the in-situ stress is hydrostatic pressure, simultaneous injection compels the fractures to deflect and tend to propagate horizontally, which promotes the formation of complex fracture networks between two injection wells. When the maximum in-situ stress is in the horizontal direction, asynchronous injection is more conducive to the parallel propagation of multiple hydraulic fractures. Nevertheless, excessively small or large well spacing reduces the number of fracture branches in fracture networks. In addition, the stress shadow effect is found to be sensitive to both injection sequence and well spacing.

The Cross-Sectional Shapes of Longitudinal Hydraulic Fractures

1984 ◽  
Vol 106 (4) ◽  
pp. 554-561 ◽  
Author(s):  
D. Segalman
Keyword(s):  
Mathematical Formulation ◽  
High Stress ◽  
In Situ Stress ◽  
Hydraulic Fractures ◽  
Stress Distributions ◽  
Type Problem ◽  
Cross Sectional ◽  
The Cross ◽  
Sectional Shape

A mathematical formulation has been developed for calculating the cross-sectional shape of hydraulic fractures. This formulation treats the problem as a free-boundary-type problem and is modeled after mathematical formulations developed for contact and lubrication problems. Numerical solution of the resulting equations has been used to address problems involving particularly difficult in-situ stress distributions, including problems in which the fracture breaks through high-stress barriers. The technique is illustrated on two example problems.

Numerical Study on the Relationship of the Axis Orientation of Deeply Buried Circular Tunnel and the Direction of Horizontal In Situ Stress

2011 ◽  
Vol 117-119 ◽  
pp. 1723-1727 ◽  
Author(s):  
Jun Qi Wang
Keyword(s):  
Principal Stress ◽  
Numerical Study ◽  
High Stress ◽  
Side Wall ◽  
Maximum Principal Stress ◽  
Water Diversion ◽  
Stress Fields ◽  
Axis Orientation ◽  
Plastic Zones

Deeply buried tunnels usually lie in high stress fields, whose horizontal stress which is not uniform is far larger than vertical stress, and their stability is dominated by the original in-situ stresses. With three-dimensional nonlinear finite element method, the axis orientation effects of tunnel on the displacement and stability of two types of surrounding rocks are studied systematically for one water diversion project. The tunnel lies in different original stress fields whose maximum horizontal principal stress is parallel with or perpendicular to the axis and lies in different kinds of rocks. The numerical analysis results show that the plastic zones develop in side wall of tunnel mostly when the horizontal maximum principal stress is parallel with the tunnel axis while the plastic zones distribute in the top and bottom of tunnel when the horizontal maximum principal stress is perpendicular to the tunnel axis. It is concluded that the principle of tunnel axis should be parallel with horizontal maximum principal stress regulated by the “specification for design of hydraulic tunnel” is not available for the stability of tunnel always.

scholarly journals Modeling Simultaneous Multiple Fracturing Using the Combined Finite-Discrete Element Method

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-20 ◽  
Author(s):  
Quansheng Liu ◽  
Lei Sun ◽  
Pingli Liu ◽  
Lei Chen
Keyword(s):  
Hydraulic Fracturing ◽  
Solid Phase ◽  
Horizontal Wells ◽  
In Situ Stress ◽  
Gas Production ◽  
Hydraulic Fractures ◽  
Multiple Fractures ◽  
Fracture Geometry ◽  
Stress Shadow ◽  
Oil Gas

Simultaneous multiple fracturing is a key technology to facilitate the production of shale oil/gas. When multiple hydraulic fractures propagate simultaneously, there is an interaction effect among these propagating hydraulic fractures, known as the stress-shadow effect, which has a significant impact on the fracture geometry. Understanding and controlling the propagation of simultaneous multiple hydraulic fractures and the interaction effects between multiple fractures are critical to optimizing oil/gas production. In this paper, the FDEM simulator and a fluid simulator are linked, named FDEM-Fluid, to handle hydromechanical-fracture coupling problems and investigate the simultaneous multiple hydraulic fracturing mechanism. The fractures propagation and the deformation of solid phase are solved by FDEM; meanwhile the fluid flow in the fractures is modeled using the principle of parallel-plate flow model. Several tests are carried out to validate the application of FDEM-Fluid in hydraulic fracturing simulation. Then, this FDEM-Fluid is used to investigate simultaneous multiple fractures treatment. Fractures repel each other when multiple fractures propagate from a single horizontal well, while the nearby fractures in different horizontal wells attract each other when multiple fractures propagate from multiple parallel horizontal wells. The in situ stress also has a significant impact on the fracture geometry.

The Effects of Perforation Erosion on Practical Hydraulic-Fracturing Applications

SPE Journal ◽  
2017 ◽  
Vol 22 (02) ◽  
pp. 645-659 ◽  
Author(s):  
Gongbo Long ◽  
Guanshui Xu
Keyword(s):  
Hydraulic Fracturing ◽  
Horizontal Wells ◽  
In Situ Stress ◽  
Fluid Distribution ◽  
Fluid Viscosity ◽  
Appropriate Treatment ◽  
Limited Entry ◽  
Successful Design ◽  
Treatment Designs

Summary Predicting perforation erosion and its effects on fracture dimensions, fluid distribution, and pressure drop can be an essential part of successful design of hydraulic-fracturing treatments, especially for massive treatments along the horizontal wells when limited-entry techniques are implemented. Both the perforation diameter D and discharge coefficient Cd increase dynamically as proppant-laden slurries are pumped through perforations, making it necessary to consider the changes of these two variables in terms of time to predict the perforation-erosion effects. In this paper, we conduct a study of the perforation-erosion effects by implementing our new perforation-erosion model derived from experimentally verified abrasion mechanisms to calculate the rate changes of these two variables and the consequent influence on the fracture dimensions, fluid distribution, and downhole pressure during a treatment. The selected parameters affecting the erosion effects in the study include perforation number, perforation-cluster spacing, in-situ stress difference, and fracturing-fluid viscosity. The results demonstrate that our model can predict the perforation-erosion effects on practical hydraulic-fracturing applications in a physically clear and mathematically concise manner under different circumstances by inspecting the simultaneous increases of D and Cd separately, leading to more-appropriate treatment designs, especially with the limited-entry techniques.

Application of Comprehensive Reinforce Technology in Soft and Crumbly Surrounding Rock in High Stress

2011 ◽  
Vol 393-395 ◽  
pp. 608-613
Author(s):  
Bing Shen Du ◽  
Li Ma
Keyword(s):  
High Stress ◽  
Engineering Application ◽  
Strengthening Mechanism ◽  
In Situ Stress ◽  
Surrounding Rock ◽  
Strengthening Effect ◽  
Geological Conditions

Based on soft and crumbly surrounding rock of roadway, poor integrity of roof, drench water in roadway, the large in-situ stress and other complex conditions, this paper analyzes the cause of roadway destruction and grouting strengthening mechanism, using the coupling of grouting technique to integrated strengthening for the region. Engineering application shows that the integrated strengthening effect is obvious, and the economic and technique effectiveness is remarkable. The successful application of secondary coupling grouting technology provides the reference for the mine laneway construction of similar geological conditions.

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