Experimental investigation on fracture initiation and non-planar propagation of hydraulic fractures in coal seams

Abstract In this paper, a 3D near-wellbore fracture propagation model is established, integrating five parts: formation stress balance, drilling, casing and cementing, perforating, and fracturing, in order to investigate fracture initiation characteristics, near-wellbore fracture non-planar propagation behavior, and torturous hydraulic fracture morphology for cased and perforated horizontal wellbores in tight sandstone formation. The method is based on the combination of finite element method and post-failure damage mechanism. Finite element method is used to determine the coupling behavior between the pore fluid seepage and rock stress distribution. Post-failure damage mechanism is adopted to test the evolution of hydraulic fractures through simulating rock damage process. Moreover, a user subroutine is introduced to establish the relation between rock strength, permeability, and damage, in order to solve the model. This model could simulate the interaction between fractures during their propagation process because of the stress shadow. The simulation results indicate that each operation could cause redistribution and reorientation of near-wellbore stress. Therefore, it is important to know the real near-wellbore stress distribution that affects near-wellbore fracture initiation and propagation. Initially, hydraulic fractures initiate independently from each perforation and propagate along the direction of maximum horizontal stress. However, hydraulic fractures divert from original direction gradually to interconnect and overlap with each other, because of stress shadow, resulting in non-planar propagation behavior. Individual fractures coalesce into a spiral-shaped fracture morphology. In addition, a longitudinal fracture could be observed because of wellbore effect, which is a result of weak cementing strength or near-wellbore weak plane. Finally, the complex and torturous fracture morphologies are created near the wellbore, incorporating Multi-spiral shaped fracture and horizontal-vertical crossing shaped fracture. However, the propagation behavior of fracture far away from wellbore is controlled by in-situ stress, forming a planar fracture. The highlights of this 3D near-wellbore fracture propagation model are following: 1) it considers near-wellbore stress change caused by each construction to ensure the accuracy of near-wellbore stress distribution; 2) it achieves 3D simulation of fracture initiation and near-wellbore propagation from perforations; 3) the interaction between fractures is involved, resulting in complex and torturous morphology. This model provides the theoretical basis for fracture initiation and propagation, which also could be applied into heterogenous formations considering the effect of discontinuities.

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Fracture Initiation and Propagation in a Deep Shale Gas Reservoir Subject to an Alternating-Fluid-Injection Hydraulic-Fracturing Treatment

SPE Journal ◽

10.2118/195571-pa ◽

2019 ◽

Vol 24 (04) ◽

pp. 1839-1855 ◽

Cited By ~ 12

Author(s):

Bing Hou ◽

Zhi Chang ◽

Weineng Fu ◽

Yeerfulati Muhadasi ◽

Mian Chen

Keyword(s):

Hydraulic Fracturing ◽

Shale Gas ◽

Fracture Initiation ◽

Fluid Injection ◽

Hydraulic Fractures ◽

Stress Difference ◽

Gas Reservoirs ◽

Complex Fracture ◽

Initiation And Propagation

Summary Deep shale gas reservoirs are characterized by high in-situ stresses, a high horizontal-stress difference (12 MPa), development of bedding seams and natural fractures, and stronger plasticity than shallow shale. All of these factors hinder the extension of hydraulic fractures and the formation of complex fracture networks. Conventional hydraulic-fracturing techniques (that use a single fluid, such as guar fluid or slickwater) do not account for the initiation and propagation of primary fractures and the formation of secondary fractures induced by the primary fractures. For this reason, we proposed an alternating-fluid-injection hydraulic-fracturing treatment. True triaxial hydraulic-fracturing tests were conducted on shale outcrop specimens excavated from the Shallow Silurian Longmaxi Formation to study the initiation and propagation of hydraulic fractures while the specimens were subjected to an alternating fluid injection with guar fluid and slickwater. The initiation and propagation of fractures in the specimens were monitored using an acoustic-emission (AE) system connected to a visual display. The results revealed that the guar fluid and slickwater each played a different role in hydraulic fracturing. At a high in-situ stress difference, the guar fluid tended to open the transverse fractures, whereas the slickwater tended to activate the bedding planes as a result of the temporary blocking effect of the guar fluid. On the basis of the development of fractures around the initiation point, the initiation patterns were classified into three categories: (1) transverse-fracture initiation, (2) bedding-seam initiation, and (3) natural-fracture initiation. Each of these fracture-initiation patterns had a different propagation mode. The alternating-fluid-injection treatment exploited the advantages of the two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs; therefore, we concluded that this method is an efficient way to enhance the stimulated reservoir volume compared with conventional hydraulic-fracturing technologies.

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Experimental investigation of cranial fracture initiation in blunt human head impacts

Forensic Science International ◽

10.1016/j.forsciint.2019.04.003 ◽

2019 ◽

Vol 300 ◽

pp. 51-62 ◽

Cited By ~ 1

Author(s):

Mariyam I. Isa ◽

Todd W. Fenton ◽

Alexis C. Goots ◽

Elena O. Watson ◽

Patrick E. Vaughan ◽

...

Keyword(s):

Experimental Investigation ◽

Human Head ◽

Fracture Initiation ◽

Head Impacts

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A Case Study on the Optimal Design of the Horizontal Wellbore Trajectory for Hydraulic Fracturing in Nong’an Oil Shale

Energies ◽

10.3390/en13010286 ◽

2020 ◽

Vol 13 (1) ◽

pp. 286

Author(s):

Ying Zhu ◽

Han Zhang ◽

Dongbin Pan ◽

Lianghao Zhai ◽

Shuai Gao ◽

...

Keyword(s):

Oil Shale ◽

Fracture Initiation ◽

Intact Rock ◽

Hydraulic Fractures ◽

Trajectory Design ◽

Rock Matrix ◽

Preferred Direction ◽

Bedding Planes ◽

Horizontal Wellbore

A horizontal well with hydraulic fractures is key to forming a fracture network in oil shale for the generated hydrocarbon flows. By considering the influence of anisotropic strength, a prediction model is proposed for fracture initiation by studying different fracture initiation modes (FIMs) in oil shale: failure of the intact rock matrix and of the bedding planes. Through a case study on Nong’an oil shale, the influences of wellbore trajectory and bedding planes on the fracture initiation pressure (FIP), location (FIL), and FIM were analyzed and the induced changes in wellbore trajectory design were concluded. The preferred angle between the wellbore axis and the minimum horizontal principal stress was the same of 90° or 270°, when the lowest required FIP corresponded to the failure of the intact rock matrix. However, when the angle corresponded to the failure of the bedding planes, the preferred direction of the wellbore axis was away from the fixed direction and not corresponding to the lowest required FIP due to the fracture morphology. The error between the theoretical and experimental results ranges from 7% to 9%. This research provides a framework for the design of horizontal wellbore trajectories in oil shale, for easier fracture initiation and more complex fracture networks.

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An Experimental Investigation of Filtercake Reinforced Wellbore Strengthening and Fracture Sealing

Volume 8: Polar and Arctic Sciences and Technology; Petroleum Technology ◽

10.1115/omae2019-96675 ◽

2019 ◽

Author(s):

Mingzheng Yang ◽

Yuanhang Chen ◽

Frederick B. Growcock ◽

Feifei Zhang

Keyword(s):

Experimental Investigation ◽

Critical Role ◽

Fracture Initiation ◽

Fluid Loss ◽

Lost Circulation ◽

Fracture Sealing ◽

Sealing Pressure ◽

Wellbore Strengthening ◽

Mud Loss ◽

Lost Circulation Materials

Abstract Drilling-induced lost circulation should be managed before and during fracture initiation rather than after they propagate to form large fractures and losses become uncontrollable. Recent studies indicated the potentially critical role of filtercake in strengthening the wellbore through formation of a pressure-isolating barrier, as well as plugging microfractures during fracture initiation. In this study, an experimental investigation was conducted to understand the role played by filtercake in the presence of lost circulation materials (LCMs). A modified permeability plugging apparatus (PPA) with slotted discs was used to simulate whole mud loss through fractures of known width behind filtercake. Cumulative fluid loss upon achieving a complete seal and the maximum sealing pressure were measured to evaluate the combined effects of filtercake and LCMs in preventing and reducing fluid losses. The effects of some filtercake properties (along with LCM type, concentration and particle size distribution) on filtercake rupture and fracture sealing were investigated. The results indicate that filtercake can accelerate fracture sealing and reduce total mud loss. Efficiently depositing filtercake while drilling can reduce the concentration of LCM that is required to plug and isolate incipient fractures.

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Time-Dependent Initiation of Multiple Hydraulic Fractures in a Formation With Varying Stresses and Strength

SPE Journal ◽

10.2118/171030-pa ◽

2015 ◽

Vol 20 (06) ◽

pp. 1317-1325 ◽

Cited By ~ 2

Author(s):

Andrew P. Bunger ◽

Guanyi Lu

Keyword(s):

Tensile Strength ◽

Hydraulic Fracture ◽

Fracture Initiation ◽

Time Frame ◽

Hydraulic Fractures ◽

Static Fatigue ◽

Breakdown Pressure ◽

Initiation Point ◽

Delayed Failure ◽

Wellbore Pressure

Summary The premise of classical hydraulic-fracture-breakdown models is that hydraulic-fracture growth can only start when the wellbore pressure reaches a critical value that is sufficient to overcome the tensile strength of the rock. However, rocks are well-known to exhibit static fatigue; that is, delayed failure at stresses less than the tensile strength. In this paper, we explore the consequences of delayed failure on axially oriented initiation of multiple hydraulic fractures. Specifically, given a certain breakdown pressure, we investigate the conditions under which subsequent hydraulic fracture(s) can begin within the time frame of a stimulation treatment in regions of higher stress and/or strength because of delayed-failure mechanisms. The results show that wells completed in shallower formations are more sensitive to variations in strength, whereas wells completed in deeper formations are more sensitive to variations in stress. Furthermore, cases in which all hydraulic fractures break down according to the same pressurization regime—that is, all are “fast” (nonfluid-penetrating) pressurization or else all are “slow” (uniformly pressurized fluid-penetrating) pressurization cases—are highly sensitive to small stress/strength variability. On the other hand, if the first hydraulic-fracture initiation is in the “fast”-pressurization regime and subsequent fracture(s) are in the “slow”-pressurization regime, then the system is robust to a much-higher degree of variability in stress/strength. Practically, this work implies that methods aimed at moderately reducing the variability in stress/strength among the possible initiation points (i.e., perforation clusters) within a particular stage can have a strong effect on whether multiple hydraulic fractures will begin. In addition, this analysis implies that pumping strategies that encourage “fast,” nonpenetrative breakdown of the first initiation point followed by the opportunity for fluid-penetrating, “slow” breakdown of subsequent initiation points could be effective at encouraging multiple-hydraulic-fracture initiation.

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