• Geomechanics and Engineering
• /
• 제32권3호
• /
• pp.321-333
• /
• 2023

Rock masses often contain natural fractures of varying sizes, and the size of the natural fractures may affect the propagation of hydraulic fractures. We conduct a series of triaxial hydraulic fracturing tests to investigate the effect of the pre-existing fracture size a on hydraulic fracture propagation. Experimental results show that the pre-existing fracture size impacts hydraulic fracture propagation. As the pre-existing fracture size increases, the hydraulic fracture propagates towards the pre-existing fracture tips, evidenced by the decreased distance between the final hydraulic fracture and the pre-existing fracture tips. Furthermore, the attracting effect of pre-existing fracture tips increases when the distance between the wellbore and the pre-existing fracture is short (L/D=2 or 4 in this study). With increased distance between the wellbore and the pre-existing fracture (L/D=6 in this study), the hydraulic fracture propagates to the middle of the pre-existing fracture rather than the tips, as the attracting effect of the pre-existing fracture diminishes.

• 한국가스학회지
• /
• 제19권5호
• /
• pp.13-19
• /
• 2015

본 연구에서는 보다 현실적인 파쇄균열 전파를 묘사하기 위해 수압파쇄균열 전파 모델을 개발하였다. 이 모델에서는 두 가지 균열 전파 기준을 적용하였다. 첫 번째는 균열의 발생각을 결정하기 위한 최대 접선응력 기준과 두 번째는 파쇄균열의 자연균열 통과 여부 기준이다. 본 모델의 검증 결과, 수압파쇄균열이 자연균열을 통과하는 양상이 실험값과 동일함을 확인하였다. 균열의 전파에 직접적인 영향을 미치는 요소인 최대수평응력 방향, 균열면의 마찰계수, 자연균열의 방향성에 대한 민감도 분석 결과, 이론적 기준에 적합하게 균열의 전파 방향과 통과 여부가 결정되는 것으로 나타났다. 기존의 수직 판형 균열 전파 모델과 본 모델을 비교하여 균열의 연결성과 유정 자극부피 측면에서 차이가 있음을 확인하였다.

• 터널과지하공간
• /
• 제5권3호
• /
• pp.251-265
• /
• 1995

One of the most important matters in stress measurement by hydraulic fracturing technique is the determination of the breakdown pressure, reopening pressure, and shut-in pressure, since these values are the basic input data for the calculation of the in-situ stress. The control of the fracture propagation is also important when the hydraulic fracturing technique is applied to the development of groundwater system, geothermal energy, oil, and natural gas. In this study, a laboratory scale hydraulic fracturing device was built and a series of model tests were conducted with cube blocks of Machon gabbro. A new method called 'flatjack method' was adopted to determine shut-in pressure. The initial stress calculated from the shut-in pressure measured by flatjack method showed much higher accuracy than the stress determined by the conventional method. The dependency of the direction of fracture propagation on the state of the initial stresses was measured by introducin g artificial slots in the borehole made by water jet system. Numerical modeling by BEM was also performed to simulate the fracture propagation process. Both results form numerical and laboratory tests showed good agreement. From this study which provides the extensive results on the determination of shut-in pressure and the control of fracture propagation which are the critical issue in the recent hydraulic fracturing, it is conclued that in-situ stress measurement and the control of fracture propagation could be achived more accurately.

• Structural Engineering and Mechanics
• /
• 제80권5호
• /
• pp.553-562
• /
• 2021

In this paper, a hybrid scaled boundary finite element and finite volume method (SBFEM-FVM) is proposed for simulating hydraulic-fracture propagation in brittle concrete materials. As a semi-analytical method, the scaled boundary finite element method is introduced for modelling concrete crack propagation under both an external force and water pressure. The finite volume method is employed to model the water within the crack and consider the relationship between the water pressure and the crack opening distance. The cohesive crack model is used to analyse the non-linear fracture process zone. The numerical results are compared with experimental data, indicating that the F-CMOD curves and water pressure changes under different loading conditions are approximately the same. Different types of water pressure distributions are also studied with the proposed coupled model, and the results show that the internal water pressure distribution has an important influence on crack propagation.

• Geomechanics and Engineering
• /
• 제6권2호
• /
• pp.153-172
• /
• 2014

In order to investigate the effect of different perforation angles (the angle between the perforation direction and the maximum horizontal principal stress) on the fracture initiation and propagation during hydraulic fracturing of highly deviated well in oil & gas saturated formation, laboratory experiments of the hydraulic fracturing had been carried out on the basis of non-dimensional similar criteria by using 400^3 $mm^3$ cement cubes. A plane fracture can be produced when the perforations are placed in the direction of the maximum horizontal principal stress. When the perforation angle is $45^{\circ}$, the fractures firstly initiate from the perforations at the upper side of the wellbore, and then turn to the maximum horizontal principal stress direction. When the well deviation angle and perforation angle are both between $45^{\circ}$ and $90^{\circ}$, the fractures hardly initiate from the perforations at the lower side of the wellbore. Well azimuth (the angle between the wellbore axis and the maximum horizontal principal stress) has a little influence on the fracture geometries; however it mainly increases the fracture roughness, fracture continuity and the number of secondary fractures, and also increases the fracture initiation and propagation pressure. Oriented perforating technology should be applied in highly deviated well to obtain a single plane fracture. If the well deviation angle is smaller, the fractures may link up.

• Geomechanics and Engineering
• /
• 제9권5호
• /
• pp.645-667
• /
• 2015

Single treatment and staged treatments in vertical wells are widely applied in sandstone and mudstone thin interbedded (SMTI) reservoir to stimulate the reservoir. The keys and difficulties of stimulating this category of formations are to avoid hydraulic fracture propagating through the interface between shale and sand as well as control the fracture height. In this paper, the cohesive zone method was utilized to build the 3-dimensional fracture dynamic propagation model in shale and sand interbedded formation based on the cohesive damage element. Staged treatments and single treatment were simulated by single fracture propagation model and double fractures propagation model respectively. Study on the changes of fracture vicinity stress field during propagation is to compare and analyze the parameters which influence the interfacial induced stresses between two different fracturing methods. As a result, we can prejudge how difficult it is that the fracture propagates along its height direction. The induced stress increases as the pumping rate increasing and it changes as a parabolic function of the fluid viscosity. The optimized pump rate is $4.8m^3/min$ and fluid viscosity is $0.1Pa{\cdot}s$ to avoid the over extending of hydraulic fracture in height direction. The simulation outcomes were applied in the field to optimize the treatment parameters and the staged treatments was suggested to get a better production than single treatment.

• Computers and Concrete
• /
• 제18권2호
• /
• pp.267-278
• /
• 2016

In order to observe the internal damage of concrete in real time, we introduced acoustic emission nondestructive detecting technology into a series of fracture tests; the test results revealed the whole process that concrete undergoes when it sustains damage that leads to failure, according to the change rules of the acoustic emission parameters. The results showed that both the initiation and unstable loads can be accurately determined using the abrupt change of the acoustic emission rate curves and the turning point of the acoustic emission parameters' accumulative curves. The whole process, from damage to failure, includes five phases, beginning with damage, such as cracking, a stable crack growth process, a critical unstable stage, and unstable propagation. The brittle fracture characteristics of concrete change when steel bars are joined, because the steel bars and the concrete structure bond, which causes an increase in the acoustic emission signals within the fracture process of the reinforced concrete. The unstable propagation stage is also extended. Our research results provide a valid methodology and technical explanations, which can help researchers to monitor the cracking process of concrete structures, in real time, during actual projects.

• Computers and Concrete
• /
• 제18권3호
• /
• pp.337-354
• /
• 2016

To study the influence on fracture properties of reinforced concrete wedge splitting test specimens by the addition of reinforcement, and the restriction of steel bars on crack propagation, 7 groups reinforced concrete specimens of different reinforcement position and 1 group plain concrete specimens with the same size factors were designed and constructed for the tests. Based on the double-K fracture criterion and tests, fracture toughness calculation model which was suitable for reinforced concrete wedge splitting tensile specimens has been obtained. The results show that: the value of initial craking load Pini and unstable fracture load Pun decreases gradually with the distance of reinforcement away from specimens's top. Compared with plain concrete specimens, addition of steel bar can reduce the value of initial fracture toughness KIini, but significantly increase the value of the critical effective crack length ac and unstable fracture toughness KIun. For tensional concrete member, the effect of anti-cracking by reinforcement was mainly acted after cracking, the best function of preventing fracture initiation was when the steel bar was placed in the middle of the crack, and when the reinforcement was across the crack and located away from crack tip, it plays the best role in inhibiting the extension of crack.

• 지질공학
• /
• 제27권3호
• /
• pp.233-244
• /
• 2017

주입액의 점성도와 응력상태에 따른 균열전파 특성을 분석하기 위해 실험실 규모의 수압파쇄시험을 실시하였다. 시험에 사용된 시료는 시멘트 몰탈을 사용하여 제작되었으며, 각 변의 길이가 20 cm인 정육면체 형태이다. 제작된 시료는 최대강도를 갖기 위해 수중에서 약 1달간 양생과정을 거쳤다. 독립적인 가압시스템을 가지고 있는 진삼축압축장치로 시료에 압력을 가하여, 실제의 지반에서 작용하는 원위치응력 상태를 재현하였다. 시추 환경 재현을 위해 시료에 소형 시추공을 천공한 후, 일정한 주입속도로 수압파쇄시험을 실시하였다. 수압파쇄시험 과정에서 시추공에 주입된 유체의 압력을 실시간으로 측정하였으며, 동시에 미소파괴음(AE) 신호를 측정하였다. 수압파쇄시험의 모든 과정이 끝난 후 생성된 균열의 형태를 육안으로 관찰하였다. 일차파쇄압력은 주입액의 점성도 증가에 따라 지수형태를 보이며 증가하였다. 수압파쇄시험으로 인해 생성된 균열의 형태는 최대주응력과 최소주응력의 차이인 편차응력의 크기에 따라 서로 다른 양상을 보였다. 낮은 편차응력의 조건에서는 단일의 균열이 아닌 다중 균열이 생성되거나, 균열 성장과정에서 방향이 휘어지는 경향을 보였고, 이에 반해 높은 편차응력의 조건에서 생성된 균열은 단일 면상의 균열이 발생하였다. AE 분석에서도 편차응력이 클수록 미세균열이 단일 면상으로 집중되어 발생되는 경향을 보였다. 이러한 연구결과는 수압파쇄 방법을 이용한 암반파쇄에서 편차응력이 클 때보다 작을 때 더 복잡한 균열이 발생된다는 것을 보여준다. 따라서 셰일가스를 개발할 때 생산량을 높이기 위해서는 복잡한 균열을 발생시킬 수 있는 편차응력이 작은 조건에서 수압파쇄가 적용되는 것이 효과적일 것으로 판단된다.

• 터널과지하공간
• /
• 제21권1호
• /
• pp.66-81
• /
• 2011

암반 내 균열 생성, 진전, 파괴 등과 같은 지하 암반의 역학적 거동과 이들 균열을 통한 지하수 유동 및 온도 변화에 기인한 열응력이 역학적 거동에 미치는 상호작용을 모델링하기 위한 수치해석코드를 개발하였다. 개발된 수치해석코드에서는 기존의 2차원 FRACOD(Shen & Stephasson, 1993)에 열-역학 및 수리-역학 상호 거동을 모델링하기 위한 해석모듈을 개발하여 추가하였다. 열-역학 연계를 위해서는 가상열원법과 시간전진기법을 도입하였으며, 수리-역학 연계에서는 양해법에 의한 반복기법을 적용하였다. 수치해석결과와 해석해와의 비교를 통해 개발된 해석모듈의 유용성을 검증하고 해석사례를 통해 온도변화에 따른 열균열 발생 및 수압파쇄과정에서의 균열 진전 양상과 같은 절리암반 복합거동을 잘 표현할 수 있음을 확인하였다.