Hydraulic Fracturing Pressure Research Articles

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.

Analysis of hydraulic fracturing pressure histories: A comparison of five methods used to identify shut-in pressure. Technical note : Aggson, J R; Kim, K Int J Rock Mech Min SciV24, N1, Feb 1987, P75–80

Analysis of hydraulic fracturing pressure histories: A comparison of five methods used to identify shut-in pressure. Technical note : Aggson, J R; Kim, K Int J Rock Mech Min SciV24, N1, Feb 1987, P75–80

Analysis of hydraulic fracturing pressure histories: A comparison of five methods used to identify shut-in pressure

Analysis of hydraulic fracturing pressure histories: A comparison of five methods used to identify shut-in pressure

In situ stress estimates from hydraulic fracturing and direct observation of crack orientation

Estimates of in situ stress in G Tunnel, Rainier Mesa, Nevada Test Site, have been obtained with hydraulic fracturing techniques. This work represents a nontraditional use of hydraulic fracturing in that it was performed primarily in horizontal boreholes drilled into the formation from access drifts tunneled into the mesa rather than the usual operation performed at depth in vertical boreholes drilled from the surface. Several operations were performed in essentially orthogonal triads of boreholes located at a point. A significant feature of this work is the mineback operation in which the borehole is mined out to reveal the actual fracture. During the hydraulic fracturing operation, colored dye was added to the fracturing fluid which left an easily detectable stain on the fracture surface. Direct observation of the fracture orientation away from the borehole establishes the direction of the minimum compressive in situ stress and the plane of the other two principal stresses. In one borehole, which was not aligned along a principal stress direction, a modified mineback operation revealed a fracture plane that twisted as it grew away from the borehole, aligning itself finally in a plane determined by the in situ stresses. This twisting is consistent with theoretical predictions of borehole stresses under these conditions. This same mineback also showed that fracture of the formation initiated at or under one of the packers, indicating that the packers may have an effet on in situ stress estimates. General observations of over 100 tests performed in G Tunnel show that under the sloping portion of the mesa, fracture planes are not vertical but dip in a direction tending to parallel the mesa slope. Deep into the tunnel and well under the flat part of the mesa, fractures are essentially vertical with strikes approximately N45°E, which substantiates the usual vertical fracture assumption in traditional fracture operations performed at depth in vertical boreholes. Advantages, limitations, and problem areas associated with extracting in situ stress fields from hydraulic fracture pressure records are discussed in detail.

Changes in Earth Stresses Around a Wellbore Caused by Radially Symmetrical Pressure and Temperature Gradients

Abstract Pressure and temperature gradients are created around wellbores during waterflooding or when fluids are injected in connection with any other secondary or tertiary recovery process. These gradients result in changes in earth stresses, which in turn cause hydraulic fracturing pressures to change. In this paper, analytical solutions have been used to determine the stresses resulting from radially symmetrical temperature and pressure changes around a wellbore. These stresses are required to predict the change in fracture extension pressure that is caused by the injection process. Exact, closed-form solutions are given for the stresses. These have been evaluated with a computer, and more convenient empirical formulas have been fitted to the calculated results. Solutions for discrete cylindrical or disk-shaped regions of changed temperature and pressure are shown. Also, the solutions can be adapted to annular elements of finite thickness that are convenient for incorporation to an r-z-type computer program. Such a program could then be used to compute the stresses resulting from temperature and pressure fields that vary gradually in the radial direction. This paper gives examples to illustrate the effect of injecting a large volume of liquid that is cooler than the insitu reservoir, as is common when waterflooding. The cooling can have a large effect on lateral earth stresses, and for some conditions vertical hydraulic fracturing pressures can be significantly reduced.

The effects of aqueous chemical environments on crack and hydraulic fracture propagation and morphologies

The role of surface active aqueous environments in chemomechanical weakening of geologic materials is examined in light of the results of hydraulic fracture tests in sandstone, calorimetric studies, and crack propagation tests in synthetic quartz. In hydraulic fracture tests mploying Crab Orchard Sandstone it was found that the effective hydraulic fracture pressure was reduced, over that attained with distilled water, when 5×10−4 M aqueous solutions of dodecyl trimethyl ammonium bromide (DTAB) were used as the hydraulic fracture medium. The degree of branching of the fractures was also increased in the presence of the DTAB solution. Previously reported crack propagation stress values in quartz exposed to distilled water and various DTAB solutions displayed the same trend. When examined in this study, the cracks propagated in the presence of DTAB solutions also displayed a greater degree of branching than those propagated in the presence of distilled water or the ambient atmosphere. These results and results from calorimetric measurements of the heats of adsorption and desorption from quartz of distilled water and DTAB are synthesized in a model relating the weakening and morphological effects to a reduction in the surface free energy of quartz due to adsorption of species from the chemical environment onto the surfaces of the quartz and sandstone.

Packer-Induced Stresses During Hydraulic Well Fracturing

Well bore stresses induced by inflatable packers during hydraulic fracturing operations are investigated. The geologic formation is modeled as an unbounded homogeneous isotropic linear elastic solid containing an infinitely long circular cavity, while the packer is modeled as a semi-infinite thin-walled circular cylindrical shell. For given packer properties, these induced stresses are shown to depend on the difference between packer pressure and fracturing pressure and can become significant. Typical numerical results are obtained and presented graphically. Analytical approximations for the maximum values of these stresses are also presented. While these effects are of no importance in the usual application of hydraulic fracturing to enhance oil and gas recovery, they are crucial in attempts to estimate in-situ stresses from hydraulic fracturing pressure data.

Stress measurements in anisotropic rock by hydraulic fracturing : HAIMSON, BC UNIV. WISCONSIN, MADISON, WI, USA AVASTHI, JM UNIV. WISCONSIN, MADISON, WI, USA 13F, 1T, 7R. APPLICATIONS OF ROCK MECHANICS. PROC 15TH. SYMP. ROCK MECH., SOUTH DAKOTA. ASCE, 1975, P135–156

Many practical applications of rock mechanics require knowledge of the in-situ state of stress. Conventional stress measuring methods, such as borehole deformation gage and door stopper techniques, are severely limited in depth, rock quality, and stress level. Hydraulic fracturing has the potential of overcoming most of these limitations, and so far laboratory experiment and field measurements have demonstrated the soundness of the technique in intact quasi-isotropic rock. Recently an experimental program was undertaken to study the effectiveness of hydraulic fracturing in extremely anisotropic rock. Thinly foliated Martinsburg slate was hydrofractured at 4 different cleavage inclinations, under a set of 3 unequal simulated principal in-situ stresses. The results showed a predictable relationship between in-situ stresses and hydraulic fracturing pressures dependent on filiation inclination. Under normal in-situ stress conditions, and by hydrofracturing through a thin lining of Plaster of Paris, all the induced fractures were vertical and perpendicular to the direction of the smaller horizontal principal stress, just as in the isotropic case.

Hydraulic Fracturing in Porous-Permeable Materials

Laboratory tests show that in porous-permeable rock both vertical and horizontal hydraulic fractures are possible, and that the breakdown pressure is lower than in an impermeable but otherwise identical pressure is lower than in an impermeable but otherwise identical formation. The direction of vertical fractures and the magnitude of breakdown pressures can be predicted from theoretical considerations. Introduction Hydraulic fracturing is a stimulation method used by the petroleum industry to boost oilwell production. In an open-hole completion, the method consists of searing off a section of the well, injecting it with pressurized fluid, and raising the pressure until the pressurized fluid, and raising the pressure until the well fractures. The fracture is then extended by pumping in high volumes of water or other fluids; the pumping in high volumes of water or other fluids; the permeability of the formation is thus improved, often permeability of the formation is thus improved, often resulting in a substantial increase in oil output. However, not all fracturing jobs have been successful. Even today, 20 years after the method was introduced, no certain predictions can be made as to what critical pressure will be required to initiate a fracture, whether pressure will be required to initiate a fracture, whether it will be horizontal, vertical or inclined, what will be its direction, its length, its height, etc. Articles have been published on the theoretical relationship between the tectonic (in-situ) stresses and the hydraulic fracturing pressures, but no thorough attention has been paid to the stresses due to possible infiltration of the formation by the injected fluid. Haimson and, independently, Geertsma have pointed out that this fluid penetration may significantly affect the critical (breakdown) pressure. This paper presents the criteria for fracture initiation in porous-permeable materials by considering all the possible stress fields around the wellbore. There has been very little laboratory experimenting on hydraulic fracturing, and only few reports have been published. This paper summarizes some laboratory tests on hydrostone performed at the U. of Minnesota as a part of an extensive research program in hydraulic fracturing. The experimental set-up allowed for the application of three mutually perpendicular, unequal compressive loads on a cubical sample having a central vertical hole, thus realistically simulating principal tectonic stresses in the earth. The pressurization of the central hole until fracture occurred provided the value of the critical pressure. This value and the direction of the crack were then related to the simulated in-situ stresses. The purpose of the tests was to verify some of the theoretical predictions regarding the relationship between critical pressures and fracture directions vs tectonic stresses in porous rock. porous rock. Theory It is assumed that the rock is brittle, linearly elastic, homogeneous, isotropic and porous, and that the fluid flow through the pores obeys Darcy's law. It is also assumed that one of the principal tectonic stresses acts in a vertical direction, i.e., parallel with the axis of the wellbore. The total stresses around the wellbore can be found by superposing the three individual stress fields generated by: the three principal tectonic stresses, the pressurization of the open hole of the wellbore, and the fluid flow from the pressurized hole into the formation. pressurized hole into the formation. JPT P. 811