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The top hole pressure or injection pressure is equal to 0. This implies that if the effective stress had a significant effect on the flow behavior, the flow path in a cement plug specimen would be less compli- cated than that in a rock bridge specimen. Discussions below are concentrated on testing of Charcoal granite specimens, which typifies the permeameater test results. The cement used is Ideal Class A cement with an expansive agent and a dispersant added.

Data from the test on the rock bridge are shown as a solid line, curve a. IMPal "'ax..! Inflow rate as a function of top hole injection pressure, for a Charcoal granite specimen with cement borehole plug. The latters a, b, c, d and e are ordered in time i. Curve a, flow rate through the rock bridge, is the baseline. Curve b is derived next.

It shows a lower flow through the cement plug than the rock bridge under similar axial and confining stresses. Curve c repeats the condi- tions of curve b. It is derived to check repeatability; obviously, curve c shows a lower flow rate than curve b. This is believed to be due to decreas- ing cement plug permeability resulting from forcing high-pressure water through the plug.

Reducing the axial and confining stresses increases the flow rate curve d , but not until axial and confining stresses are reduced to about one-third their initial values does flow rate through the plug-rock system exceed the initial flow rate through intact rock. Bottom hole head is zero; the sides and ends of the specimen are modeled as no-flow boundaries. The flow rate is a linear function of the permeability at a given injection pressure.

This is expected as the calculation is based on Darcy's Law. OMPa ,7. CG-I02 3. Based on the measured flow rates from a granite specimen, the rock has permeabilities of The permeability increases with increasing injection pressure because the higher pore water pressure tends to increase the size of the connected pore space in the specimen, increasing its permeability. Microscopic examination of thin sections of the granite indicates that pore space in the granite exists along mineral grain bound- aries and as microfractures through grains.

The design and construction of water impounding plugs in working mines

As the sample is subjected to higher stress conditions, that is, as 11 increases the sample permeability decreases, probably due to decreasing pore sizes and fracture widths within the sample. Permeability increases as the injection pressure increases at a given stress level, 1 1 , because increasing the injection pressure decreases the effective stress; increasing injection pressure tends to open pores and fractures.

The All measured flow rates fell below the theoretical curves, possibly due to stress redistributions resulting from the coring out of the rock bridge and the expansiveness of the cement used as the plug material. These test results supported by the finite element analysis illustrate three points significant to choice of materials for borehole sealing:. Borehole plug materials with permeabilities less than the rock being sealed do not significantly reduce flow in the vicinity of the borehole.

Flow through the borehole plug only begins to increase significantly when the plug material becomes an order of magnitude of greater in permeability than the rock being sealed. Significant reduction of rock stress did not greatly increase flow through a cement plug in granite. However, the granite had a higher Young's modulus than the cement plug.

Two specimens from the same location are obtained by diamond coring in the field. One specimen first had a percussion hole drilled into it in the field; it was then overcored. The other specimen was first cored in the field and returned to the laboratory.

Top and bottom holes were diamond drilled to leave a rock bridge in place. The specimen with the rock bridge was then tested, the rock bridge cored from it and a cement plug placed. A cement plug was also placed in the percussion drilled specimens. Flow rates were measured in both specimens under similar axial and confining stresses with similar injection pressures. This very direct comparison of diamond-drilled with the percussion-drilled specimens indicates drilling method does not create a borehole wall damage zone which significantly affects seal performance.

This finding agrees with the experimental results obtained by Mathis and Daemen and Fuenkajorn and Daemen , a. It should be noted that the effect of drilling fluid mud is not considered here. In deep in situ boreholes the remains of drilling fluid on the hole wall mud cake could degrade the cement plug performance. The fracture was thin, partially healed and less than 1 mm in aperture.

It was barely visible upon visual inspection prior to testing and was not detected during routine core logging.

Jaak Daemen, Ph.D.

The test configuration was different from the tests described in the previ- ous sections; this was tested in divergent and convergent flow. The most significant results regarding borehole plugging for general engineering ap- plications came from the convergent flow tests.

In these tests water flow was from the annulus to the top and bottom holes. Equivalent permeabilities of the top third of the specimen, which contained the fracture, and the bottom third of the specimen were calculated. Using the equivalent permeability of the intact rock as a reference of 1, the rock with the cement plug had an equivalent permeability about times greater. The equivalent permeability of the rock with the frac- ture was about times greater than the permeability of the intact rock, and 7 times greater than the permeability of the plugged rock.

This demon- strates the dominant effect of fractures on flow in the vicinity of a borehole. Results from a similar test configuration obtained by Fuenkajorn and Dae- men , a also indicate that pre-existing fractures intersecting the borehole near the plug location usually become a preferential flow path for fluid to bypass the plug. Variation of the triaxial stress state on the two basalt specimens does not significantly affect flow through the cement plugs, nor is the basalt permeability measurably af- fected. Increased flow rate in the granite specimens with decreasing triaxial stress state is due to variation of rock permeability, rather than plug per- meability.

Interaction pressure at the cement plug-rock interface resulting from the cement expansion is important for maintaining a good hydraulic bond at the interface. For the type of cement used here, the interaction pressure at the interface could be as high as 4 MPa when it is installed in a stiff rock, such as granite. One specimen is dried at room temperature for 27 days. This is found to impair slightly the performance of its cement plug. The other specimen is dried for 42 dyas at 55C. This impairs plug performance, increasing plug permeability from about nanodarcy to an initial value of nanodarcy.


As flow continues the plug becomes less permeable, reaching nanodarcy after 50 days. In- creasing the triaxial stress state on the sample does not greatly affect plug permea bili ty. Microscopic studies on the dried-out cement plugs by Fuenkajorn and Daemen b indicate that widths of the plug-rock interface are about 0.

The dried cement plugs themselves also show shrinkage cracks with an average width of 0. More discussions on the effect of drying on the hydraulic perform- ance of cement plug are given in Chapter 4. Taken together, the laboratory tests and numerical analyses indicate several points to be considered when plugging boreholes: Currently available expansive cements are adequate to provide good per- formance for borehole seal under changing stress conditions. However, if the plug material is stiffer i. Interaction pressure at cement plug-rock interface is governed by the swelling pressure of the cement and stiffness of the rock.

Good hydraulic bond at the interface is expected when cement is installed in stiff rock. Making the plug material less permeable than the surrounding rock, including fractures, will not significantly reduce fluid flow. In fractured rock, flow through the natural fracture system will dominate provided the seal material is no more than one order of magnitude more permeable than the intact rock. Acknowledgements 27 Borehole sealing is not sensitive to the drilling method used to produce the borehole. Drying can significantly increase cement plug permeability by several orders of magnitude.

Performance partially recovered upon resaturation. A combination of cement and bentonite placed in appropriate sections of a borehole is expected to give the best results. Cement should be placed at each end of a section with a bentonite plug between. The cement will provide strength and the bentonite will be able to accommodate strains resulting from stress changes. Support and permission to publish this chapter are gratefully acknow- ledged. Sealing of penetrations e. Penetrations of and near a high-level nuclear waste repository need to be sealed reliably to retard any radionu- clide migration to the accessible environment US Nuclear Regulatory Commission, , Design of seals may also be required in 1 water dams, barriers, water wells, mine drifts or shafts, to prevent flooding of underground operations e.

Garrett and Campbell Pitt, ; Loof- bourow, , 2 diversion tunnels for the construction of hydroelectric power plants e.

HELL Found At Bottom Of Deepest Hole On Earth?!

Mitchell, ; Kinstler, ; Moller et al. Smith, ; Calvert, ; Halliburton Services, un- dated , and 5 blasthole stemming studies e. Axial loads on seals or plugs may be due to water, gas or backfill press- ures, or due to temperature changes induced subsequent to waste and plug emplacement. These axial loads induce shear stresses along the contact between plug and host rock.

These shear stresses may cause cracking and increased permeability along the plug-rock interface. Under extreme conditions they could cause dislodging or slipping of plugs. Therefore, the interface between the plug and rock is a critical element for the design and performance of plugs in boreholes, shafts or tunnels.

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Push-out test mechanical interactions 29 The objective of this study is to determine the strength of cement grout borehole plugs in welded tuff cylinders as measured through push-out tests. The push-out test involves the use of a steel rod to dislodge experimentally a cement grout plug emplaced within the coaxial borehole of a hollow rock cylinder Figure 3.

The rock specimens were taken from the densely welded brown unit of the Apache Leap tuff near Superior, Arizona. RP-lOB The push-out tested tuff cylinders had inside radii ranging from 6. The plugs were cured under water for 8 days prior to testing. Loads were incremented stepwise at intervals of 5 min. Figure 3. A cylindrical steel rod applies an axial load to a neat cement grout plug installed in a rock cylin- der. The top L VDT and dial gage displacement monitoring points rest on horizontal brackets clamped to fixed vertical reference bars.

The steel platen underneath the sample has a slit on one side to allow the downward movement of the horizontal arm of the bottom vertical displacement monitoring assembly. A vertical rod, screwed into the bottom of the cement plug, is connected to the horizontal arm which supports the bottom L VDT and dial gage monitoring points for bottom plug displacement measurements. The bottom LVDT and dial gage are clamped to fixed vertical reference bars.

A steel pipe is placed around the rock specimen to provide confinement. The tuff cylinders tested had inside radii of 6. The tuff cores were plugged with nearly centered Self-Stress II cement grout plugs having length-to-radius ratios ranging from 2. The cement grouts of the push-out specimens were initially loaded to N. The load was kept approximately constant and increased by N every 5 min until the plug failed. The load and displacements were recorded every 30 s upon failure.

Experimental details are given by Akgun and Daemen c. Many structures are mechanistically similar to an axially loaded plug em- placed within the coaxial borehole of a hollow rock cylinder, e. Akgun and Daemen c present a review of mechanical analyses of interaction configurations similar to push-out testing. The shear stress distribution induced by push-out loading along the ce- ment plug-rock interface can be calculated in several ways Akgun and Daemen, c. Assuming a uniform shear stress distribution gives an average shear stress. Assuming an exponential shear stress distribution elastic solution results in Ci zo af3 cosh[f3 L-z ] 3.

Equation 3. It follows from Equation 3. Different radius ratios lead to different stiffnesses, and hence to different radial contact stresses at the plug - rock interfaces. As the axial stress applied to a borehole plug is directly proportional to the radial stress through Poisson's effect, the axial stress at failure or the axial strength for a push-out specimen with a finite radius ratio can be normalized for that of an infinite rock mass by the following equation Akgun and Daemen, c : a vpjVSF a 3.

The average Young's modulus and Poisson's ratio of the cement grout plug are 5. The modulus ratio i. The secondary objective is to assess the validity of the analytical interface shear stress distribution solution pres- ented by Equation 3. The finite element analysis simulated a two material push-out test on an elastic material with an applied axial plug stress of 1.

Desai and Abel give the finite element formulation for the four-node isoparametric axisymmetric elements used in this analysis. The finite element model is designed for a tuff cylinder of radius 76 mm, length mm and with a 13 mm radius coaxial hole. The cement grout plug is centered halfway down the hole and has a length-to-radius ratio of 2. The mesh consists of elements and nodal points.

The hatched area in Figure 3. Akgun and Daemen c give the finite element mesh and boundary conditions for plugged tuff cylinders with outside-to-inside radius ratios of 60 and plug length-to-radius ratios of 2. The objective of using a radius ratio of 60 is to simulate an in situ rock mass.

Randolph and Wroth propose the use of a radius ratio of 50 for in situ rock mass simulation. Finite element analysis and discussion The plot is given as a function of the modulus ratio EplER and plug length-to-radius ratio Lla , and allows a comparison of the results of the finite element analysis with that of the closed-form solution given by Equa- tion 3.

The interfacial shear stress decreases with increased modulus ratio and increased plug length-to-radius ratio. The plot presents results from the finite element analysis along with those obtained through the closed-form solution and is given as a function of plug length-to-radius ratio. The peak shear stresses increase with decreasing plug length-to-radius ratios.

The shear stresses do not distribute over the entire lengths of longer plugs. The determination of these tensile stresses is important since any tensile fracturing may cause preferential pathways around seals. The finite element analysis simulates a push-out test with an axial stress applied to the plug of 1. The finite element meshes given by Akgun and Daemen c were used to analyze the principal tensile stress distribu- tions. The meshes represent 13 mm radius borehole plugs with length-to- radius ratios of 2.

All meshes have cylinder outside-to-inside radius ratios of 60 and represent borehole plugs in an infinite medium. The tensile radial stress zones on the right-hand side of Figure 3. Modulus ratio of 0. The magnitudes of the tensile stresses and the volumes under tension decrease with increased plug length. Table 3. The tensile stresses are presented as a function of the applied axial stress and plug length-to-radius ratio. The modulus ratio of the push-out specimen in Table 3. The Poisson's ratios of the plug and rock are 0. The tensile stresses are reckoned negative. At this location only one stress component is tensile.

The bottom center of the plug is in biaxial tension. The maximum tension in rock occurs at the upper contact between plug and rock. At all three critical locations the magnitude of tension decreases with increasing plug length-to-radius ratio. No axial tensile stresses are observed within the plugs. The most likely axial stress on a borehole plug in rock is due to water pressure. If a borehole in which the plug is emplaced at m below the surface fills up with water, this creates a water pressure of 9.

This water pressure creates a maximum tensile stress of 6. The mean tensile strength of medium- strength concrete is 2. Therefore, tensile failure of plugs with length-to-radius ratios of 2. The tensile strength of plugs with length-to-radius ratios of 8. Hence, a plug with a length-to- radius ratio of at least 8.

In all push-out tests the behavior was elastic. Upon failure the differ- ence between the top and bottom axial plug displacements decreased, prob- ably due to stress relief. The push-out specimens failed after periods ranging from 3 to 30 min. The tests were continued for up to 2 h. One of the 51 mm radius push-out samples showed tensile splitting during push-out testing.

The axial strength, bond strength and the peak shear strength are the applied axial stress at failure, the average shear stress at failure and the peak shear stress at failure, respectively. The bond strength and the peak shear strength are calculated from Equations 3. Values in square brackets represent strength measures that are normalized to an infinite rock mass. The normalized axial strength is used in Equations 3. All three strength measures decrease with increasing plug radius and with decreasing plug length. Equations 3. I - - - Top plug displacement O Both equations obey a power law.

Size effects on the normalized axial strength might follow from the size-strength studies performed on pillars e. Farmer, When a fracture occurs in a cubic pillar with a side length, L, a certain amount of strain energy, U, is required to satisfy the energy balance for each unit area of the fracture surface created. If the fracture results from brittle breakdown and the strain energy per unit fracture area U j L 2 is assumed to be constant, the strength of the pillar is inversely proportional to the square root of one of the pillar linear dimensions or the plug diameter in the case of a push-out specimen.

This might explain the inverse proportionality between the axial strength and plug radius. Acknowledgements The analysis performed herein shows that a borehole plug with a modulus ratio of 0. Plugs with smaller radii and greater lengths give higher axial strengths and lower peak shear stresses. The axial strengths represent lower bounds due to the absence of confining pressure. As Equation 3. The calculation is further conservative due to the utilization of zero confinement and due to ignoring progressive failure which leads to reduced peak shear stresses.

It is recommended that plug design be based on limiting the elastic peak shear stress to well below the peak shear strength. Some guidance about the peak shear strength is given in the last column of Table 3. A conservative design is believed to result if the strengths of the 13 mm radius plugs in Table 3. It is also recognized that the factor 22 is uncertain as it is based on an extrapolation far beyond the range over which measurements have been made.

This uncertainty confirms the desirability of performing some experiments on larger radius borehole plugs. The results of the detailed numerical and experimental analyses of the mechanical performance of seals performed by Akgun and Daemen c and some reported herein indicate that permanent abandonment plugs should be designed with a length-to-radius ratio of 8.

This conservative length criterion will prevent the development of excessively detrimental tensile stresses within and near an axially loaded borehole plug. It is of considerable interest that a very similar geometrical design recommenda- tion results from detailed hydrological analyses of water flow through plugs and host rock Greer and Daemen, , assuming reasonably similar hydraulic conductivities for the plug and the rock.

This may not be surpris- ing in the light of the parallelism between the governing equations for fluid flow and for elastic stress analysis. Support and per- mission to publish this chapter are gratefully acknowledged. One of the concerns with regard to the performance of borehole seals is the impact of earthquakes, or other types of dynamic loading such as large- scale blasting, on seal integrity.

Of all possible scenarios erosion, glaciation, tectonic and other natural processes , seismic motions are the most likely effects to be experienced by borehole and excavation seals. In the United States many locations are sufficiently close to seismic regions to be affected thereby as can be seen from the seismic hazard map shown in Figure 4. The map in Figure 4. A review of available literature indicates that deep underground struc- tures in competent rocks experience less damage than surface structures, openings at shallow depth and openings in fractured rocks, when subjected to earthquakes and subsurface blasts.

This is based on surveys on the effect of earthquakes on wells, tunnels, mines and other underground structures Nazarian, ; Stevens, ; Dowding, ; Dowding and Rozen, ; Pratt et ai. Introduction From Applied Technology Council, While the information may provide considerable guidance about the like- ly response of sealed openings to dynamic loading, the foregoing review also shows that specific data on the effect of dynamic loading on rock and seal permeability are virtually non-existent.

Yet, there is a considerable need for this kind of information. Borehole sealing has long been used in the petro- leum industry to isolate and stabilize the lower portion of a producing well. More recently, a similar technique has been adopted by the solution mining industry. Many abandoned oil and gas wells have been sealed with cement grout.

However, no data are currently available regarding their sealing performance with respect to seismic loading. This information is also crucial in sealing abandoned underground min- es to prevent groundwater migration into the mine and the leakage of harmful acid to the outside environment. Underground space utilization for the storage of oil, natural or liquefied gas, chemicals, wastes and others requires proper sealing of all access boreholes and wells to maintain the integrity of the storage caverns. Burial of high-level nuclear waste in geo- logic media poses perhaps the most important challenge for borehole sealing application.

This is due to the stringent requirement that all access pathways to the deep underground repositories must eventually be sealed, and the sealing performance must not be compromised by events such as earthquakes. Despite the diversity in application, borehole and underground excava- tion seals generally hold a similar function: as a protective measure in containing often volatile commodities or their byproducts acid, crude oil, radionuclides, etc.

In many of these engineering applications, groundwater migration seems to be the most like- ly scenario for the breach in sealing. In this chapter an experimental sealing performance assessment of cement borehole plugs that have been subjected to dynamic loading is provided. This includes a study of plugs that have dried, as well as of plugs that have remained wet throughout the testing period.

The basic approach in this study was to establish steady-state flow through a cement borehole plug in a rock cylinder, subject the plugged cylinder to dynamic loading using a shaking table, and assess the influence of shaking on plug permeability. The parameter measured during the flow test prior to and after dynamic loading was the flow rate at various injection pressures.

This was used to determine hydraulic conductivity of the cement plug. In the dynamic loading phase the tests were carried out with increasing duration up to 5 min and peak acceleration up to 2 g. The experimental work performed in this chapter is based on the assump- tion that external shaking of a plugged rock cylinder will adequately simu- late some aspects of dynamic waves impacting a sealed opening.

Of importance in sealing performance testing, analysis and design consider- ations is the scaling effect of size, which will influence the acceleration and duration of the dynamic loads as well as the dynamic displacement and diffusion processes that control fluid flow. A total of eight granite cylinders of Precambrian age from Charcoal Black Quarry in Minnesota were used in this study.

Their typical dimensions were 15 cm diameter by 30 cm long. Each cylinder had a 2. In the middle portion of the hole, either a rock bridge was left for baseline rock permeability measurements or a cement plug was installed to seal the borehole. The cement mixture was poured on top of a stopper in the hole and cured for at least 8 days under water, at room temperature and atmospheric pressure.

These plugs, as well as the rock bridge, were of varying lengths, from 3 cm to 11 cm. Four of the cement plugs remained cured under water wet until testing commenced. Three others were allowed to dry after curing at ambient room temperature for a period of months, before they were rewetted and tested. During the flow test, which lasted up to 9 months, distilled water was injected under pressure on top of the plug and the outflow was collected underneath it.

Hydraulic conductivities were calculated from the measured flow rates through the plug or rock bridge. The schematic diagram of flow test lay-out is described in detail in Figure 4. Once a long-term steady-state flow trend had been established, the cylin- ders were subjected to dynamic loading on a shaking table.

Shaking was performed at various accelerations up to 2 g , and for different durations up to 5 min. Flow testing was continuing before, during and after the shaking. Figure 4. Upon completion of the test these specimens were sawed in half lengthwise and visual observations of the flow patterns were conducted. Test procedures, equipment and materials used in this study are described in detail elsewhere Adisoma, ; Adisoma and Daemen, Nitrogen oos tank 2. Pressure reoulator 3. Low-pressure oos cylinder of pressure intensifier 4.

High-pressure water cylinder of pressure intensifier 5. Woter injection pressure goge 6. Rotameter flowmeter 7. Rock sample 8. Borehole plug 9. Measuring pipets for outflow collection Stainless steel connector Rubber stopper. Using two Permatex-sealed rubber stoppers, the longitudinal flow through the plug used to calculate hydraulic conduc- tivity and the peripheral flow through the rock around the plug are collected separately in the Rand L pipets, respectively.

Four plugged rock cylinders un- dergoing simultaneous flow testing are visible in the background. During each individual flow test which may last from a few minutes for a dried plug to a couple of days for a wet plug , inflow and outflow were read at a given time interval. The flow rate was determined by linear regression, i. Injection pressure is maintained constant during each flow test. For a given specimen the flow test was repeated many times, often using different injection pressures.

The resulting flow rate from each test was plotted as a function of the elapsed time in days since testing on that specimen commenced. Each data point in Figure 4. Flow rate is determined from the slope of the best-fit line. This is an example from a dried cement plug; 1. Injection pressure was 1. Also known as coefficient of permeability or simply per- meability, it is calculated from the flow rate using Darcy's law for one- dimensional flow through a porous medium, and is independent of distance and pressure gradient.

Its derivation can be found in most hydrology tests e. Harr, ; Freeze and Cherry, Hydraulic conductivity has a unit of velocity; often times darcy is also used 1 darcy is approximately equal to 3 cmls for water at 20DC. For flow through a preferential path such as that of dried cement plug-rock interface section 4. The model can be conveniently analyzed by the equivalent parallel plate concept. Darcy's law can then be applied to determine a coefficient of fissure permeability K j as a function of an equivalent parallel plate aperture. It indicates a very low permeability rock, from 4 to 30 nanodracy 4 x 9 to 3 x 10 - 8 darcy.

Figures 4. During 9 months of flow testing the values remain fairly con- stant with time. One exception is the specimen in Figure 4. Compared to the permeability of gran- ite in Figure 4. These plugs dried out in ambient room temperature 7 months for the specimen in Figure 4. This caused the cement plug to shrink and the plug-rock interface to open, creating a preferential flow path. The dye injection test section 4. The plots in Figure 4. They continue to decrease, albeit at a slower rate, until the flow test is concluded months later.

This suggests renewed cement expansion upon resaturation, which results in partial clos- ing of the gap in the plug-rock interface. However, the final permeabilities are still several orders of magnitude higher than those of cement plugs which are always wet. During this period fissure permeability decreases from 20 to 0. Flow test results The results indicate a very low-permeabil- ity granite, and an even lower-permeability cement seal when maintained wet throughout.

This composite plots shows the permeabilities of Charcoal gran- ite and of wet and dried cement plugs as a function of time. Drying the cement plugs increases their previously very low permeabili- ties by seven to nine orders of magnitude, depending on the drying period and temperature. The dried cement plugs exhibit a similar response when they are resaturated.

The flow rates decrease rapidly for the first 2 months, and continue to decrease at a slower rate thereafter. The permeability of specimen in a , which was dried for 7 months, is 25 times that in b which was dried for 3 months. These values are several orders of magni- tude higher than those of wet plugs. Drying is potentially very detrimental to cementitious plugs. Sealing performance is only partially recovered when the plugs are re- saturated. A liquid concentrate dye marker was injected into the permeant water during the later stage of flow testing in two rock cylinders with wet cement plugs.

Dye was also injected into a rock cylinder with a room-dried cement plug and another with an oven-dried cement plug. This test was performed upon completion of the dynamic loading test and the subsequent post- shaking flow test. After the dye injection test had been concluded the rock specimens were sawed in half along their lengths. This allowed visual obser- vation of the flow pattern of both wet and dried plugs, as well as verifica- tion of results obtained from the flow tests. Dye was injected during the last 41 days of flow testing on this specimen.

The dye-colored permeant water penetrated the cement grout seal uniformly. The plug body remains intact and no visible cracks can be observed. Preferential flow path along the plug-rock interface is non-exist- ent. This observation confirms the very low flow rate observed during flow testing on this specimen. Judging from the similar flow rate in other speci- mens with wet cement plugs, this flow pattern seems to be typical for all wet cement borehole seals as well. This cement plug was left to dry at room temperature for 7 months and was subsequently flow tested for 8 months.

Dye-colored permeant penetrated the wet cement plug body uni- formly, without any preferential flow path a. In the dried plug, dye traces in the interfacial fissures and in a crack across the plug body indicate preferential flow paths b. The photograph clearly shows traces of dye along the plug-rock interface.

The dye also highlights a crack which is visible across the plug body, extending from the left to the right interface. This crack and the interfacial fissures clearly acted as preferential flow path, since none of the dye marker penetrated the main body of the plug. This experiment has shown that when a cement seal is dried it shrinks and cracks.

This is especially true in cement grout with an expansive agent added to it. The rate of shrinking and cracking is apparently related to the temperature during the drying process and the duration of drying. Cement shrinkage causes the plug-rock interface to open, thus creating a preferen- tial flow path.

Cracking in the plug body creates additional flow paths. The presence of preferential flow paths seems to be typical in dried cement plugs. They explain the high flow rates observed during the flow tests on these specimens. Okrajni, S. Mud cuttings transport in directional well drilling. Ortlepp, W. More O'Rerrall, and J. Support methods in tunnels. Min Managers S. Paillet, F. Acoustic modes of propagation in the borehole and their relationship to rock properties.

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While a considerable data base has been gathered over the last two decades or so with regard to the performance of seals, most of the information is presented in research reports and widely scattered papers in journals and proceedings of conferences. Hence, the materials are not readily accessible to potential users such as designers, contractors or regulators who are not familiar with nuclear waste disposal programs.

Seismic Risk and Instrumentation.