Mechanism Of Hydraulic Fracturing Research Articles

Dehydration reaction and isotope front transport induced by CO2infiltration at Nuliyam, South India

The field relations from a quarry at Nuliyam, South India, illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by CO2 influx, over a scale of 30 m. Both the calc-silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section. Fluid flow is by a hydraulic fracture mechanism, but is thought to be pervasive. The sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed. The front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock, although the divariant nature of the reaction may also have a limited effect. The time-integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol. % adjacent to the calc-silicate. Carbon isotope budgets suggest that decarbonation of the calc-silicate by a Rayleigh fractionation process provides a sufficient source for the CO2 influxing into the gneiss. Graphite abundances vary from 0.01 to 0.1% (by weight), it is principally derived by precipitation from the fluid and may be modelled from phase equilibria. Carbon isotope fronts coincide with the reaction front on the scale of sampling, although isotopic disequilibrium between graphite and inclusion-CO2 also implies local fluid-rock disequilibrium.

Borehole yield and hydraulic fracture initiation in poorly consolidated rock strata—part I. impermeable media

The stress distributions around a circular well in granular elastoplastic impermeable media are calculated. Yielding may develop during drilling as deviatoric stress in increased during cavity creation. Hydraulic injection imposes an unloading action on the existing yielded medium because of reduction of deviatoric stress. Thus, it is unlikely that the stress-strain path returns along the original elastoplastic trajectory; instead, it follows an unloading path which is different from the active loading path followed during drilling. The effects of initial yielding due to excavation or drilling disturbance on the local stress state complicate stress measurements, and may even change hydraulic fracturing mechanisms in such media.A non-associated flow rule is used to calculate the plastic deformation in order to obtain a reasonable deformation field. The final volumetric strain around the borehole is solved as the summation of the incremental strains due to the injection preceding hydraulic fracture, and the existing plastic strains created during borehole excavation.

On non-linear effects in unsteady flows of non-newtonian fluids through fractured porous media

The question of the non-linear effects on unsteady flows of non-Newtonian fluids through fractured porous media is addressed. These effects are illustrated on the pressure and velocity distributions for a power-law fluid of shear thinning behavior. The governing equations and their self-similar solutions, for the first boundary value problem, are presented. The rheological effects and those associated with the presence of a fracture network, into the porous medium, on the flow behavior are shown and discussed. The effects of non-Newtonian fracturing fluid on the fracture dimensions in hydraulic fracturing mechanism are also investigated. Contributed by W.F. Ames.

Regional Fractures I: A Mechanism for the Formation of Regional Fractures at Depth in Flat-Lying Reservoirs (1)

Regional fractures may form slowly, at depth, in extension, and parallel to regional horizontal tectonic compression. Such fracture sets can be common in reservoirs despite the absence of flexure of the strata, and are commonly important contributors to reservoir permeability. Such fractures make good targets for deviated wells. High formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs (conditions conducive to fracturing) despite deep burial, but pore pressure does not exceed the least principal stress and does not cause tensile fracturing: the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets. Rather, many regional fractures form duri g far-field compression, initiate at locally induced tensile stresses caused by flaws in the rock, and propagate in the plane of the maximum and intermediate compressive stresses. The differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure. In the presence of high pore pressure, fractures can be held open at depth by a tectonic, basinwide dilatancy of the strata, and the open void space commonly becomes mineralized.

Laboratory Studies on Hydraulic Fracturing Criteria in Soil

ABSTRACT In order to investigate the mechanism of hydraulic fracturing in soils, several laboratory tests are performed on compacted hollow cylindrical specimens of a cohesive soil under different confining pressures. The results indicate that hydraulic fracturing in soil initiates when total stress near the borehole reaches to the failure condition of the soil in unconsolidated undrained state. Based on Mohr-Coulomb failure criteria, theoretical solution of hydraulic fracturing pressure is derived and physical meanings of empirical equations appeared in previous papers are discussed. Then the general expression of hydraulic fracturing in pore pressure existance is obtained.

MECHANISM OF HYDRAULIC FRACTURING IN SANDY GROUND

In order to elucidate the mechanism of hydraulic fracturing in sandy ground, we investigate the relation among the hydrofracturing pressure, the confining pressure and the permeability of sand through the injection tests on several kinds of sands, including the cemented sands as well as the cohesionless sands. The result is the following equation.P′f=mσ′3+σt+RP′f: hydrofracturing pressure, m: constant (1<=m<=2)σ′3: confining pressure, σt: tensile strengthR: resistance of crack progressionThe value of R depends upon the permeability of sands and increases as the permeability increases.

The wet seam and the Teton Dam failure

During the discussion of the June 1976 Teton Dam failure at the 13th ICOLD Congress in New Delhi in October 1979 (1), I concluded that the most probable trigger mechanism for the failure of that dam was the construction defect represented by the wet seam. This conclusion was based on the fact that even after intensive post-failure investigations, the previously postulated trigger mechanisms of hydraulic fracturing and differential settlement could not be substantiated, while the wet seam was found to occur throughout the core material of the dam remnant close to and parallel with the 1974--1975 winter shutdown surface. This wet seam was characterized by low density, high moisture content, and significantly lower temperature than the normal compacted silt (6°C cooler in drill hole PR 100) (2). I stated that the temperature difference implied that frozen ground or frozen borrow soil was present near the 1974--1975 winter shutdown surface and that when this layer melted an avenue of easy access of reservoir water through the core resulted. Since that time the Final IRG Report of the failure has been published, and it attributes the wet seam to causes other than frost action. This International Workshop on Dam Failures presents an oppor tuni ty for me to take another look at the wet seam as a failure mechanism. I find that additional data and information were available which had not been adequately considered in the IRG Final Report or which postulated that Report. This discussion is concerned with this new or not-adequately considered data. Also I feel obliged to respond to some of the criticism of the Teton Dam design, which I believe to have been unfair, by providing pertinent facts that should be considered.

Rheological behaviour effects of non‐Newtonian fracturing fluids on propagation of a vertical hydraulic fracture

AbstractThis communication investigates the effects fo the non‐Newtonian behaviour of the fracturing fluids in the hydraulic fracturing mechanism. These effects are illustrated on the propagation of a vertical hydraulic fracture into an oil reservoir, in which the fracturing fluid is of power law and of pseudo‐plastic type. The analytical solutions for evaluating the fracture lengrhs are presented for the cdases of large fluid loss and for no fiuid loss. The results obtained should be useful in the design of fracture treatments, permitting the finding of the rheological properties of the injected fluid for obtaining the desired fracture length and width.

Explicit Approximate Solutions Associated With the Mechanics of Hydraulic Fracturing

Governing equations, based on a Lagrangian formulation, for the assumed case of a hydraulically induced vertial elliptic fracture propagating in a uniformly confined, linear, elastic, homogeneous medium are derived. Explicit solutions for the generalized coordinates designating the time-dependent crack width, major axis, and minor axis are obtained for one-dimensional fracture fluid flow with and without leak-off effects.

Fluid Flow and Structural Response Modeling Associated With the Mechanics of Hydraulic Fracturing

Summary A review of hydraulic-fracture modeling is given. Equations governing pertinent fluid-flow, structural, and fracture-mechanics responses are presented. The finite-element method is used to discretize the field equations and to compute the fracture dimensions, fluid leakoff, and stress intensity factors. In addition, the effects of fracture-fluid properties, layered strata, and in-situ stresses are characterized, and numerical examples are presented.

On the limit for the size of hydraulic fractures near the earth's surface

Mechanics of hydraulic fracturing is a great branch of the application of fracture mechanics theory, since hydraulic fracturing was considered as a technique for recovering geothermal energy from hot dry rocks. A substantial amount of geothermal energy could be extracted from hot dry rocks near the earth's surface by circulating water through crack-like hydraulic fractures. Because of the low thermal conductivity of rock, relatively large fractures are necessary to gain an economically acceptable thermal energy rate.

Conditions Under Which Fractures Form and Create Conduits for Fluids: ABSTRACT

The underground occurrence of open extension fractures is important in petroleum exploration because the fractures provide plane conduits for the migration and storage of fluid. Extension fractures are considered to be natural hydraulic fractures that form like the artificial hydraulic fractures produced during well stimulation. Mechanics of natural hydraulic fracturing are discussed, so that the implications of the theory for the migration and accumulation of hydrocarbons are apparent. Development of open extension fractures in the earth's crust is inhibited by gravitationally induced confining pressures; there should exist a depth above which extension fractures could form and below which faulting would be the dominant mode of failure. Mechanical considerations indicate that for the development of extension fractures on a regional scale this limiting depth is rather shallow--on the order of a few hundred meters to a few kilometers. Alternatively, mechanical considerations indicate that extension fractures could form locally in anomalously stressed regions at much greater depths if the ratio of pore fluid pressure to overburden weight approaches one. Thus, open extension fractures will be most likely to occur at depth in the earth's crust in places where the mean to al stress is anomalously low, and where the pore pressure is anomalously high. Fracture porosity depends critically on fracture aperture, and on degree of infilling with secondary mineral matter. Observed apertures of fractures are usually one to three orders of magnitude larger than that predicted by theory, suggesting that pressure solution is important to the aperture of fractures. End_of_Article - Last_Page 990------------

The age of the Roberts Arm Group, north-central Newfoundland: Discussion

content over the entire stratigraphic limits of the Muncho-McConnell, Stone interval is a meaningless exercise within the context of this discussion -breccias do occur within sandy dolomite beds containing more than 30% by weight of quartz sand and it appears to us that special pleading is required if only the quartz sand-free beds are to be dissolved; (3) corrosion, solution, and resultant insoluble residue or deposits are not ruled out by a fracture mechanism, but the quantity of such lags is expected to be small (as is the case at Robb Lake) and not comparable in quantity, or areal extent with lag deposits associated with well-documented solution-collapse terranes such as those described by Sando (1974); (4) Figures 8 and 9 together with the accompanying discussion (Macqueen and Thompson 1978, pp. 1751-1752) demonstrate large scale geological relations that are not fully explained by a solution-collapse mechanism; (5) we did not make a direct comparison with the work of Phillips (1972) and we do not accept Sangster's (1979) contention that the geological relationships described by Phillips (1972) constitute the necessary and limiting conditions for a fracture mechanism to be operative; and (6) we suggested r? hydraulic fracturing mechanism as a pussihle alternative that meets the criteria set out on p. 1752 (Macqueen and Thompson 1978) as well as providing greater flexibility with regard to the timing of brecciation. We leave Si~ngster with the dilemma of timing: the occurrence of bitumen within the breccia matrix suggests breccia matrices may have been open long after deposition of the

気中高速水噴流特性に関する研究

The breakup of turbulent water jets in air is especially of technical importance. A familiar example of application is the effective jet length for the hydraulic fracturing in coal mining. In spite of this importance, there is little understanding of the breakup mechanism, no general theory, nor even a reasonably successful method of correlation in the jet cutting technology.In this paper, the authors gave theoretical analysis of the breakup characteristics of the water jet atomizer and verified their results by high-speed photography, electric device and laser velocimeter. From the results of such a investigation, the authors proved the relationship between the characteristics of water jets in air and the hydraulic fracturing mechanism.

Analysis of factors influencing fracture initiation and orientation in oil reservoir sandstone : Komar, CA Frohne, KH 11F. US Bur. Mines, RI 7813, 1973, 22P

Hydraulic fracturing has developed into a widely applied technique for increasing production from petroleum and natural gas reservoirs. Although more than a million wells have been fractured, the degree of understanding of the mechanics of hydraulic fracturing is still incomplete. Laboratory-scale hydraulic fracture tests were conducted on directionally oriented sandstone material, along with measurements of directional rock properties such as tensile strength, permeability, ultrasonic velocity, and dynamic elastic constants. Fracture orientation was determined as a function of various combinations of stress, rock saturation, rate of borehole pressurization, and direction of minimum tensile strength relative to minimum horizontal compressive stress. The objective was to investigate whether horizontal stresses always controlled the orientation of induced vertical hydraulic fractures, or if under conditions of small differences in stress and/or slow borehole pressurization rates, the induced fracture direction was influenced by directional properties of the rock matrix. This study shows that the expected influence of horizontal earth stress on the orientation of an induced hydraulic fracture becomes negligible whenever the magnitude of the difference between the compressive stresses is 200 psi or less. When this occurs, fracture orientation is controlled by the directional properties of the rock--primarily permeability and tensile strength. (auth)

A Study of Inclined Hydraulic Fractures

Abstract The results of a theoretical and experimental investigation of inclined hydraulic fractures, reported in this paper, indicate that such fractures do not generally initiate perpendicular to the maximum tensile stress induced on the borehole wall. Unlike axial or normal hydraulic fractures, a degree of shear failure seems to be associated with the initiation and extension of almost all inclined hydraulic fractures. These fractures often intersect the borehole along two diametrically opposite axial lines, thus giving it the appearance of an axial fracture. Inclined hydraulic fractures generally change their orientation as they extend away from the wellbore until they become perpendicular to the least compressive in-situ principal stress. Therefore, the borehole trace of such fractures cannot be used for their positive identification. Introduction The process of hydraulic fracturing of a formation essentially consists of injecting a fluid inside the borehole and pressurizing it until the induced stresses exceed the strength of the formation and cause failure. Failure is generally indicated by a sudden major drop in the variations of the borehole fluid pressure with time. In general in an isotropic medium, the over-all plane of a hydraulic fracture is either parallel, plane of a hydraulic fracture is either parallel, inclined, or perpendicular to the axis of the borehole from which it is extending. Accordingly, these fractures will be called axial, inclined or normal, respectively. This classification of hydraulic fractures refers them to the borehole where they are observed rather than the ground surface or the bedding planes. In case of vertical boreholes, axial and normal fractures become identical with vertical and horizontal fractures (which are the terms often used in petroleum industry). In the first comprehensive analysis of the mechanics of hydraulic fracturing, Hubbert and Willis proposed that axial or normal hydraulic fractures initiate when the maximum tensile stress induced around the borehole exceeds the tensile strength of the formation, and that such fractures extend in a plane perpendicular to the least compressive in - situ principal stress. The correctness of this proposal has since been verified by Haimson and Fairhurst, who conducted an extensive series of laboratory experiments on the subject. In their theoretical and experimental work, Haimson and Fairhurst assumed that one of the in-situ principal stresses is parallel to the borehole axis. Under such a condition, one can only create an axial or a normal hydraulic fracture in an isotropic medium. For the case when none of the in-situ principal stresses are parallel to the borehole, Fairhurst derived mathematical expressions for the stress components on the borehole wall, in isotropic and transversely isotropic media. Experimentally, von Schonfeldt and Daneshy independently observed that under such a condition the fracture orientation is influenced by the borehole in its vicinity. The trace of inclined hydraulic fractures at the wellbore was found to be misleading if used for the purpose of determining the over-all fracture orientation. The research reported here is an extension of a previous work on the subject of inclined hydraulic previous work on the subject of inclined hydraulic fractures. It includes the computation of the magnitude and the orientation of the maximum tensile stress induced at the borehole wall, for each experiment, and the resulting fracture shape. Such investigations can, in the course of time, provide means of determining the over-all fracture provide means of determining the over-all fracture type at great depth, which has significant importance in many fields, such as geophysics, petroleum, geological and civil engineering.