Effect of Cleavage Rate and Stress Level on Apparent Surface Energies of Rocks

1966 ◽  
Vol 6 (04) ◽  
pp. 308-314 ◽  
Author(s):  
T.K. Perkins ◽  
W.W. Krech
Keyword(s):  
Hydraulic Fracturing ◽  
Stress Level ◽  
Oil And Gas ◽  
Fluid Pressure ◽  
Confining Stress ◽  
Auxiliary Equipment ◽  
Cleavage Rate ◽  
Rock Samples ◽  
Surface Energies ◽  
Fracturing Process

Abstract As fractures are propagated through rocks, energy is absorbed near the extending crack tip. Apparent surface energies for several rocks have been measured by cleavage under dynamic conditions. At nominal crack velocities from 0.5 to 500 in./min. measurements showed that fractures propagated in discrete jumps. Calculated surface energies and moduli were relatively insensitive to nominal rate of cleavage. In another set of experiments, rocks were cleaved under high confining stresses. The rocks were submerged in low leak-off fluids which formed a filter cake on the freshly cleaved surfaces (similar to the hydraulic fracturing process). Apparent surface energies were increased substantially as the surrounding fluid pressure was increased. Moduli in bending increased significantly upon application of the first 1,000 psi but were insensitive to stress level at greater pressures. INTRODUCTION For almost 20 years, hydraulic fracturing processes have been utilized effectively to stimulate oil and gas wells. During this period, some process improvements have resulted from studies of fracture orientation, mechanics of fracturing, areas generated, conductivities of cracks, etc. Yet many questions remain concerning the conditions and pressures needed during fracture propagation. In this paper we will report additional studies of the mechanics of fracture extension. It was shown previously3 that large rock samples could be cleaved under controlled conditions so as to measure the apparent surface energy (that amount of energy absorbed per unit area of new surface created). In this paper we consider the effects of two additional factors on surface energies, viz.:effect of cleavage rate andeffect of confining stress level. THEORY Cleavage experiments were conducted on rock samples similar to that illustrated in Fig. 1. Blocks of rock several inches wide, 2- to 3-in. thick, and up to 3 ft in length were grooved longitudinally with shallow guide slots. A crack was initiated and allowed to extend along the web as the top of the rock specimen was pulled (or pushed) apart. Auxiliary equipment permits the measurement offorce applied at the top,separation at the top andcrack length. (Further experimental details will be given in the next section.) The rock beams created by the crack are considered to' be cantilever beams. The deflection (or separation of the rock beams) at any point is calculated4,5 by the beam Eq. 1.

scholarly journals Three-Dimensional Ultrasonic Imaging and Acoustic Emission Monitoring of Hydraulic Fractures in Tight Sandstone

2021 ◽  
Vol 11 (19) ◽  
pp. 9352
Author(s):  
Wei Zhu ◽  
Shangxu Wang ◽  
Xu Chang ◽  
Hongyu Zhai ◽  
Hezhen Wu
Keyword(s):  
Acoustic Emission ◽  
Hydraulic Fracturing ◽  
Rock Mechanics ◽  
Oil And Gas ◽  
Ordos Basin ◽  
Water Injection ◽  
Three Dimensional ◽  
Hydraulic Fractures ◽  
Tight Sandstone ◽  
Fracturing Process

Hydraulic fracturing is an important means for the development of tight oil and gas reservoirs. Laboratory rock mechanics experiments can be used to better understand the mechanism of hydraulic fracture. Therefore, in this study we carried out hydraulic fracturing experiments on Triassic Yanchang Formation tight sandstone from the Ordos Basin, China. Sparse tomography was used to obtain ultrasonic velocity images of the sample during hydraulic fracturing. Then, combining the changes in rock mechanics parameters, acoustic emission activities, and their spatial position, we analyzed the hydraulic fracturing process of tight sandstone under high differential stress in detail. The experimental results illuminate the fracture evolution processes of hydraulic fracturing. The competition between stress-induced dilatancy and fluid flow was observed during water injection. Moreover, the results prove that the “seismic pump” mode occurs in the dry region, while the “dilation hardening” and “seismic pump” modes occur simultaneously in the partially saturated region; that is to say, the hydraulic conditions dominate the failure mode of the rock.

The Energy Balance Concept of Hydraulic Fracturing

1968 ◽  
Vol 8 (01) ◽  
pp. 1-12 ◽  
Author(s):  
T.K. Perkins ◽  
W.W. Krech
Keyword(s):  
Energy Balance ◽  
Stress Distribution ◽  
Hydraulic Fracturing ◽  
High Stress ◽  
Fluid Pressure ◽  
Leading Edge ◽  
Energy Balance Equation ◽  
Surface Energies ◽  
New Energy ◽  
Laboratory Models

Abstract This paper explains the concept of a damaged region arising from high stress concentration at the leading edge of a hydraulically created fracture. Approximate stresses near the tip of the crack are calculated, and it is shown that a stable crack shape is possible for which all stresses are finite. A new energy balance is derived incorporating these thoughts, and it is shown that predicted fracturing pressures (using surface energies determined by cleavage) agree with experimental fracturing pressures determined in models. All calculations apply to the case of a nonpenetrating fluid. It is concluded from these studies that in some cases, particularly in small laboratory models, these phenomena significantly affect extension pressures and crack widths. Introduction One of the perplexing questions about hydraulic fracturing that has not been satisfactorily answered is, what pressure is necessary to extend a fracture? For many engineering problems involving failure, it is sufficient to calculate those loading conditions which would bring a stress or elastic strain within the material to a level that could not be tolerated. However, this approach is not useful when considering a sharp-edged crack; calculated stresses and elastic strains always reach infinitely large values near the tip of the crack if fluid pressure is applied all the way to the crack extremity. This difficulty has led to the concept of cohesiveness or absorption of surface energy, implying that behavior near the tip of the crack is not purely elastic. Additional note of the nonideal behavior of rocks will be made in this paper. Then, by simplifying and dealing with an average stress in an inelastic region, the approximate stress distribution around a hydraulic fracture will be calculated and the conditions under which a stable fracture can exist will be shown. A new energy balance equation is then derived incorporating the modified stress picture. Finally, predicted fracture extension pressures are compared with breakdown pressures obtained in laboratory models. This comparison shows that surface energies measured by the cleavage technique are consistent with those values manifested during fracture extension. PROBLEMS OF INDUCED STRESS It will be revealing to consider first the calculated stresses around a penny-shaped line crack, assuming that the rock behaves as a linear, elastic material. Fig. 1 shows the stress distribution in the plane of the crack as calculated with Sneddon's equation. If pressure p is applied uniformly within the crack, then infinitely large tensile stresses would be induced in the rock near the crack tip. Such stresses could not be sustained in a real material. Two approaches have been proposed to explain this dilemma. SPEJ P. 1ˆ

scholarly journals Influences of Hydraulic Fracturing on Fluid Flow and Mineralization at the Vein-Type Tungsten Deposits in Southern China

Geofluids ◽  
2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Xiangchong Liu ◽  
Huilin Xing ◽  
Dehui Zhang
Keyword(s):  
Fluid Flow ◽  
Pressure Drop ◽  
Hydraulic Fracturing ◽  
Numerical Experiments ◽  
Southern China ◽  
Fluid Pressure ◽  
Nanling Range ◽  
Fracturing Process ◽  
Vein Type ◽  
Tungsten Deposits

Wolframite is the main ore mineral at the vein-type tungsten deposits in the Nanling Range, which is a world-class tungsten province. It is disputed how wolframite is precipitated at these deposits and no one has yet studied the links of the mechanical processes to fluid flow and mineralization. Finite element-based numerical experiments are used to investigate the influences of a hydraulic fracturing process on fluid flow and solubility of CO2and quartz. The fluids are aqueous NaCl solutions and fluid pressure is the only variable controlling solubility of CO2and quartz in the numerical experiments. Significant fluctuations of fluid pressure and high-velocity hydrothermal pulse are found once rock is fractured by high-pressure fluids. The fluid pressure drop induced by hydraulic fracturing could cause a 9% decrease of quartz solubility. This amount of quartz deposition may not cause a significant decrease in rock permeability. The fluid pressure decrease after hydraulic fracturing also reduces solubility of CO2by 36% and increases pH. Because an increase in pH would cause a major decrease in solubility of tungsten, the fluid pressure drop accompanying a hydraulic fracturing process facilitates wolframite precipitation. Our numerical experiments provide insight into the mechanisms precipitating wolframite at the tungsten deposits in the Nanling Range as well as other metals whose solubility is strongly dependent on pH.

Feasibility of monitoring hydraulic fracturing using time-lapse audio-magnetotellurics

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. WB119-WB126 ◽  
Author(s):  
Lanfang He ◽  
Xiumian Hu ◽  
Ligui Xu ◽  
Zhanxiang He ◽  
Weili Li
Keyword(s):  
Hydraulic Fracturing ◽  
Case History ◽  
Oil And Gas ◽  
Signal To Noise Ratio ◽  
Time Lapse ◽  
Microseismic Monitoring ◽  
High Signal ◽  
Hydraulic Fractures ◽  
Industry Waste ◽  
Fracturing Process

Hydraulic fracturing is widely used for initiating and subsequently propagating fractures in reservoir strata by means of a pressurized fluid to release oil and gas or to store industry waste. Downhole or surface microseismic monitoring is commonly used to characterize the hydraulically induced fractures. However, in some cases, downhole microseismic monitoring can be difficult due to the limitation imposed by boreholes. Surface microseismic monitoring often faces difficulties acquiring high signal-to-noise ratio data because of the on-site noise from hydraulic fracturing process. Research and field observations indicate that injecting conductive slurry or water into a strata may generate distinct time-lapse electromagnetic anomalies between pre- and posthydraulic fracturing. These anomalies provide a means for characterizing the hydraulic fracturing using time-lapse electromagnetic methods. We examined the time-lapse variation over an hour, one day, one month, and two years of observed audio-magnetotellurics (AMT) resistivity and the 1D and 3D AMT modeling result of the variation pre- and posthydraulic fracturing. There is also a successful case history of applying the time-lapse AMT to map hydraulic fractures. Observed data indicate that the variation of AMT resistivity is normally less than 6% apart from the data of the dead band and some noisy data. Modeling results show the variation pre- and posthydraulic fracturing is larger than 30% at the frequency point lower than 100 Hz. The case history indicates that time-lapse magnetotelluric monitoring may form a new way to characterize the hydraulic fracture.

Shale, Quakes, and High Stakes: Regulating Fracking-Induced Seismicity in Oklahoma, USA and Lancashire, UK

2019 ◽  
Vol 3 (1) ◽  
pp. 1-14
Author(s):  
Miriam R. Aczel ◽  
Karen E. Makuch
Keyword(s):  
United States ◽  
Hydraulic Fracturing ◽  
Shale Gas ◽  
Seismic Activity ◽  
Oil And Gas ◽  
Oil And Gas Industry ◽  
High Volume ◽  
The United States ◽  
Horizontal Drilling ◽  
Induced Seismic Activity

High-volume hydraulic fracturing combined with horizontal drilling has “revolutionized” the United States’ oil and gas industry by allowing extraction of previously inaccessible oil and gas trapped in shale rock [1]. Although the United States has extracted shale gas in different states for several decades, the United Kingdom is in the early stages of developing its domestic shale gas resources, in the hopes of replicating the United States’ commercial success with the technologies [2, 3]. However, the extraction of shale gas using hydraulic fracturing and horizontal drilling poses potential risks to the environment and natural resources, human health, and communities and local livelihoods. Risks include contamination of water resources, air pollution, and induced seismic activity near shale gas operation sites. This paper examines the regulation of potential induced seismic activity in Oklahoma, USA, and Lancashire, UK, and concludes with recommendations for strengthening these protections.

Simulation of proppant transport and fracture plugging in the framework of a radial hydraulic fracturing model

2020 ◽  
Vol 35 (6) ◽  
pp. 325-339
Author(s):  
Vasily N. Lapin ◽  
Denis V. Esipov
Keyword(s):  
Numerical Model ◽  
Hydraulic Fracturing ◽  
Oil And Gas ◽  
Oil And Gas Industry ◽  
Injection Rate ◽  
Fluid Injection ◽  
Subtraction Technique ◽  
Gas Industry ◽  
Proppant Transport ◽  
Hydraulic Fracture Propagation

AbstractHydraulic fracturing technology is widely used in the oil and gas industry. A part of the technology consists in injecting a mixture of proppant and fluid into the fracture. Proppant significantly increases the viscosity of the injected mixture and can cause plugging of the fracture. In this paper we propose a numerical model of hydraulic fracture propagation within the framework of the radial geometry taking into account the proppant transport and possible plugging. The finite difference method and the singularity subtraction technique near the fracture tip are used in the numerical model. Based on the simulation results it was found that depending on the parameters of the rock, fluid, and fluid injection rate, the plugging can be caused by two reasons. A parameter was introduced to separate these two cases. If this parameter is large enough, then the plugging occurs due to reaching the maximum possible concentration of proppant far from the fracture tip. If its value is small, then the plugging is caused by the proppant reaching a narrow part of the fracture near its tip. The numerical experiments give an estimate of the radius of the filled with proppant part of the fracture for various injection rates and leakages into the rock.

Sign in / Sign up

Export Citation Format

Share Document