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7 Induced Changes to Fracture Systems
Pages 405-454

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From page 405...
... Four different ways that fracture systems can be altered are discussed here (Figure 7.1~. The first is deformation through changes in stresses in the rock mass.
From page 406...
... CHANGES IN FRACTURE VOID GEOMETRY DUE TO CHANGES IN EFFECTIVE STRESS A variety of engineering activities cause fractured rocks to deform or fail. Changes in fluid pressures, the addition or redistribution of loads, or changes in temperature can lead to changes in the state of stress in the rock.
From page 407...
... Any excavation in a rock mass under the groundwater table for tunnels, mines, or foundations tends to drain fluid from the rock. Fluid pressures decrease to near-atmospheric levels in the excavation, which increases the hydraulic gradient.
From page 408...
... The fracture network provides an overall in situ permeability that is several orders of magnitude greater than the matrix permeability and a horizontal permeability-anisotropy ratio of about 100:1 (inferred from interference testing with nearby wells)
From page 409...
... Under this condition, the effective reservoir permeability was found to be one to two orders of magnitude greater than that for initial reservoir conditions. The second was for high effective stress t>34.5 MPa (>5000 psi)
From page 410...
... Where there is more than one fracture set, the nature of anisotropy change would depend on which of the sets were more deformable. In the case of decreasing pore pressures in a low-permeability rock matrix, the closing of fractures could lead to a situation where the rock mass permeability is equal to the matrix permeability by reducing fracture interconnectivity below the critical limit for percolation (see Chapter 6~.
From page 411...
... Quasi-continuum models or closedform solutions can be used to calculate fluid pressures. To use these models, it is necessary to relate an equivalent quasi-continuum conductivity to the rock mass, either through single- or double-porosity models (see Chapter 6~.
From page 412...
... The hydrogeological simulation models that have been linked to mechanical deformation models originated primarily in geotechnical engineering fields. Coupled models account for the role of deformation in fluid flow and the stability of the rock mass.
From page 413...
... However, because of the multitude of parameters that determine the nonlinear hydromechanical response of a rock mass containing a network of fractures, site-specific predictive simulation is extremely difficult. The practical significance of stress-sensitive fluid flow in fractured rock needs to be evaluated for a range of field-scale problems.
From page 414...
... For homogeneous isotropic formations, hydrofractures grow in a direction perpendicular to the minimum principal stress. Hydraulic fracturing is also used to increase conductivities around production wells.
From page 415...
... Entirely analogous but more serious conditions exist in the dam abutment slopes because the water pressure produces both high pore pressures and high hydraulic thrusts. The 1959 Malpasset Dam failure is an example of such a failure (Londe, 1987~.
From page 416...
... After the fracture closes, the pressure reflects the increased pore pressure in the reservoir because of fluid filtration. This pressure dissipates with time.
From page 417...
... For each rate the final pressure increase, UP, relative to in situ pore pressure, is plotted versus the flow rate, Q This plot, illustrated on the right side of Figure 7.6, exhibits a characteristic "dogleg" where hydraulic fracturing begins.
From page 418...
... . However, the desired efficiency of the P3D models is achieved by varying the degrees of constraint on fracture shape and fluid flow or by relaxing the lateral extent of elastic coupling along the fracture plane.
From page 419...
... direction and ignore lateral elastic coupling between cells for determining the fracture aperture and vertical height (i.e., the height and aperture for a cell depend only on the fluid pressure in that cell)
From page 420...
... This behavior depends on the boundary conditions and Poisson effects in the reservoir. Changes in fracture systems caused by pore pressure changes associated with dams and reservoirs were discussed previously.
From page 421...
... _ ·4 15 _ 1 2 4 ~1~ 195m ~ L_ __ _ _ _ ~ _ ~ W~ ~ ~ ~ ~ \ , . , _ _~ ~ 1: m ~ III 1 1: _ __ _ __ _ _ - - 1 1 1 ~ \ 1 _ ~ _ To ~ 1- 1- 1- 1- 1- 1- 1- 1- 1 FIGURE 7.7 Flow nets and deformation in fractured rock under a dam.
From page 422...
... Modeling Deformation and Failure Deformation and failure of fractured rock masses can be modeled in three ways: (1) individual block analyses, (2)
From page 423...
... (c) Kinematic instability determined with vector analysis.
From page 424...
... These fluids heat rocks in the upper part of the reservoir, causing them to expand, which closes fractures and increases fluid pressures. Reinjection of cold fluids cools the rock and may lower fluid pressures.
From page 425...
... Injecting relatively cool fluid (compared to in situ temperatures) can induce a significant reduction in stress by thermoplastic behavior In the extreme this can lead to a negative effective stress condition and hydraulic fracturing.
From page 426...
... Figure 7.9 shows a hot dry rock system in its simplest form, a single open fracture embedded in a less permeable rock matrix. Also shown in the figure are the calculated temperature change, fluid pressure, and rock mass displacement 30 years after the system has been in operation.
From page 427...
... -4S 0 25 50 [m] 1E02m 427 Amp, FIGURE 7.9 Simulation of a hot dry rock system in a single open fracture.
From page 428...
... One explanation is that two-phase flow conditions decreased the inflow. The performance of nuclear waste repositories may be affected by phase changes in the fluids of surrounding rock.
From page 429...
... However, the complexity of the calculations increases rapidly with the number of fluid phases, especially if different parts of the system are not in equilibrium. Disequilibrium may be common in fractured rock because of the large differences in permeability between fractures and the pores in the matrix.
From page 430...
... The introduction of solids into a fractured rock mass will usually decrease its bulk permeability. This permeability decrease is one of the desired results for grouting but is generally undesirable in cases of inadvertent injection of suspended solids.
From page 431...
... Grouting Grouting is used in civil engineering, engineering geology, mining engineering, and petroleum engineering to reduce inflow from the rock mass by reducing
From page 432...
... by chemical reaction of the grout with the rock mass. The particle size of suspensions and the viscosity of solutions in relation to fracture aperture and grout pressure govern their injectability.
From page 433...
... Grout flow into fractures is governed by the same principles as fluid flow and can be predicted by using the methods discussed in Chapter 6, as well as the coupled methods discussed in this chapter. Complicating factors are the aforementioned characteristics of sedimentation/ flocculation of suspensions and the viscosity increase of solutions.
From page 434...
... This procedure actually characterizes grouting in general; a grout is injected, and its behavior is observed during injection (flow/ pressure behavior) and afterwards (reach, effect on rock mass properties)
From page 435...
... INDUCED CHANGES TO FRACTURE SYSTEMS a b 435 ~ l ~ 2`
From page 436...
... From this description of impermeabilization grouting for dams, excavations, and tunnels, it should be evident that the same procedures can be used to strengthen and stiffen weak or deformable fractured rock masses. Most clay-based suspensions and "metathetical precipitation" solutions can only be used for impermeabil
From page 437...
... (c) Grouting with radial arrangement of grout holes.
From page 438...
... Grout pressures may hydraulically fracture the rock mass and inadvertently worsen the condition they are intended to improve. Leaching and erosion, particularly if high water pressures exist, may reduce the intended effect of the grout.
From page 439...
... Chemical Mobilization and Swelling of Clays Many fractured rocks contain clay in the rock matrix or as fillers in fractures and pores. Clay particles can be mobilized by changes in fluid chemistry and transported in suspension, or the clay can swell.
From page 440...
... Similar species-specific differences in solubility are observed for fluid pressure changes.
From page 441...
... Mass will be transferred spontaneously from the phase with the higher chemical potential to the phase with the lower chemical potential. The direction of change induced by perturbations in stress, temperature, fluid pressure, or composition introduced by an engineering operation is easily determined by calculating the changes in chemical potentials.
From page 442...
... Solid-state phase transitions may also be important at the elevated rock temperatures expected around nuclear waste repositories. In this case the time of interest is many thousands of years rather than a few decades, so even a small increase in reaction rate could produce significant effects.
From page 443...
... The fracture geometry, mechanical properties, water pressure, flow, and chemical conditions in a rock mass are usually ill defined. Design and construction or, more generally, engineering decisions must consider this intrinsic uncertainty.
From page 444...
... SUMMARY OF DEFICIENCIES AND RESEARCH NEEDS The preceding discussion of rock-fracture-related problems in civil and mining engineering and engineering geology identified a number of problems: · It is not possible to completely characterize the geometric, mechanical, and chemical properties of rock masses. Exploration methods often have inadequate ranges of resolution and penetration and are too expensive.
From page 445...
... INDUCED CHANGES TO FRACTURE SYSTEMS OBSERVATIONAL METHOD L Exploration Feedback Design for Most Likely Condition and Contingency Designs for Other Conditions I Consign ,tion l 1 _ Performance Monitoring During Construction 445 Design includes: Location and frequency of performance monitoring · Determination of critical perfomance parameters and prediction of their values FIGURE 7.13 Knowledge of design parameters is updated through monitoring and feedback.
From page 446...
... The vertical gradient of stress is proportional to rock density and is larger than the vertical fluid pressure gradients, which means that the effective stress is decreasing in the upward direction. This stress gradient promotes upward propagation of hydrofractures, as illustrated in Figure 7.A1, which shows results from 600 400 ~ 300 0 C In ._ Q \ 200- air 100 O -100- ,, , , , ~ 0 100 200 300 (Distance x, It)
From page 447...
... Additional features of natural hydraulic fracture networks can be illuminated by the comprehensive numerical models used in the petroleum industry (Gidley et al., 1989, Chapters 3, 4, and 5~.
From page 448...
... Methods for accomplishing this involve the creation of new fractures by blasting or hydraulic fracturing and increasing fracture conductivity by propping. These methods are discussed earlier in this chapter, under "Creation or Extension of Fractures Owing to Increases in Fluid Pressures."
From page 449...
... (c) Drainage through short holes drilled into the rock mass.
From page 450...
... 1989. A numerical model of fluid flow in deformable naturally fractured rock masses.
From page 451...
... 1971. Analytical and graphical methods for the analysis of slopes in rock masses.
From page 452...
... International Journal of Rock Mechanics and Mining Sciences and Geomechanics Abstracts, 29(4)
From page 453...
... 1994. Comparison study of hydraulic fracturing models test case: GRI staged field experiment no.


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