ABSTRACT: Because of limited direct observation, understanding of the interior conditions of the massive storage caverns constructed in Gulf Coast salt domes is realizable only through predictions of salt response. Determination of the potential for formation of salt spalls, leading to eventual salt falls, is based on salt creep and fracture using the Multimechanism-Deformation Coupled Fracture (MCDF) model. This is a continuum model for creep, coupled to continuum damage evolution. The model has been successfully tested against underground results of damage around several test rooms at the Waste Isolation Pilot Plant (WIPP). Model simulations, here, evaluate observations made in the Strategic Petroleum Reserve (SPR), storage caverns, namely, the accumulation of material on cavern floors and evidence of salt falls. A simulation of a smooth cavern wall indicates damage is maximum at the surface but diminishes monotonically into the salt, which suggests the source of salt accumulation is surface sluffing. If a protuberance occurs on the wall, fracture damage can form beneath the protuberance, which will eventually cause fracture, and lead to a salt fall.
ABSTRACT: Centrifuge physical model tests were performed to study the mode of failure during sand production and its concomitant impact on the productivity index. The tests simulated seepage-induced failure around a multi-perforated vertical well. Results indicate that in the presence of a competent cap rock (1) sand production results in the formation of a (truncated) cone-shaped enlarged cavity, (2) surface subsidence of the reservoir due to loss of sand mass may result in opening of flow channels under the cap rock, (3) for a given applied head difference, sand production ceases once the enlarged cavity lowers the flowrate to sub-critical level,. (4) flow becomes diverted towards the upper perfs where the cavity radius is largest, (5) flow rate increase varies between 5 to 10 times depending on whether the mode and volume of sanding is sufficient to result in the formation of flow channels. The study performed shows that (1) the location of perfs affects the mode and magnitude of sand production, and (2) long-term productivity can be improved through managed sand production. Presence of a competent cap rock is the key for maximizing the productivity via sanding.
ABSTRACT: Rock is a very complex and heterogeneous material, containing structural flaws at all scales. Because of those structural flaws, deformation and failure of rock when subjected to differential compressive stresses is non-linear. To simulate the non-linear behavior of rock, mechanical crack models, that is, extensile and shear crack models have been used in several studies. In those studies, non-linear stress-strain curves and various behaviors of rock including the changes of effective elastic moduli due to crack growth were simulated. Most of the studies have mainly focused on the verification of the mechanical crack model with relatively less attempt to apply it to practical purposes such as numerical analysis for underground or slope design. In this study, the validity of mechanical crack model was checked out by simulating the non-linear behavior of rock and consequently it was applied to a practical numerical analysis, finite element analysis commonly used.
Kwon, S. (Korea Atomic Energy Research Institute) | Park, B.Y. (Korea Atomic Energy Research Institute) | Kang, C.H. (Korea Atomic Energy Research Institute) | Chang, K. (Korea Electric Power Corporation)
ABSTRACT: Two-dimensional and three-dimensional DEM programs, UDEC and 3DEC, were used to investigate the mechanical stability of the conceptual design of deposition drift and deposition holes constructed in a crystalline rock mass. From the simulations, the influence of discontinuities, number of deposition holes, and deposition hole interval on the stability of deposition drift and deposition holes could be determined. It was also found that that the deposition hole interval changed from 8 m to 3 m does not influence on the mechanical stability of the deposition drift.
ABSTRACT: A series of thick-walled hollow cylinder tests was conducted on a synthetic shale. The tests were performed under both hydrostatic (essentially isotropic) and plane strain (with elevated initial axial deviatoric stress) conditions. The purpose of the tests was to experimentally examine the differences in the hollow cylinder behaviour under the two test conditions. The experimental results showed that considerable differences could be generated by the two conditions. The axial stress (intermediate principal stress, σ2) has a significant stiffening and strengthening effect on the borehole. Numerical analyses were conducted which showed that a failure criterion incorporating σ2 was required to model the observed strength and deformation behavior of the hollow cylinders.
ABSTRACT: This paper discusses how careful evaluation of the geomechanical properties of gob can be applied to gob degasification techniques in order to improve gas production and quality. It further describes the "best practices" for each method that may increase methane capture and purity. In both the commonly used methods, the cross-measure boreholes and vertical gob wells, the factors affecting gas purity and capture efficiency are geologic and reservoir conditions, appropriate borehole siting, orientation and spacing, and bore-hole integrity. The best practices for the techniques that might improve gas production and/or quality include appropriate siting of the boreholes to maintain integrity, proper borehole inclination, adequacy of well spacing, and targeting stress relief zones. An important implementation with significant potential improvement is continuous monitoring of the gas flow rates, quality and system pressure.
ABSTRACT: This paper summarises the results of numerical studies on drift stability in moderately jointed rock masses. It investigates the relative pertinence of joint orientation, spacing, stress regime, etc. This is undertaken using a distinct element model and a series of parametric studies.
ABSTRACT: Stability of some rock slopes is often influenced by heavy rain during summer in Korea. One of the main process involved in this, is the slaking of highly weathered rock. Such change of engineering behavior due to slaking can not be easily detected by the measurement of intact rock strength only. In this study, the possibility of using slake durability as an indicator for slope stability has been sought by conducting several tests using samples from unstable slope faces. After second cycling of slaking, the durability index were often still too high but the index has been reduced further after one more cycle of slaking. This implies that it would be necessary to define the proper number of slaking cycles for better representation of slope stability. However, such change of engineering behavior can be correlated to the decrease rate of slaking durability against number of slaking cycles.
ABSTRACT: There is a fairly common belief that any rock excavation task can be accomplished with explosives if standard cautious blasting formulas are used appropriately, especially when accompanied by vibration limits Unfortunately, that conclusion is often not true. In some cases, that approach will not give satisfactory results, and vibration restrictions may serve no purpose but to shut down the job. At times, the sequence of the work is at least as important as the blast designs. Sometimes, success may depend on the method of excavation. And in some cases, the rock excavation task could not be accomplished by any means whatever, without some extra mechanical stabilization. Successful work in sensitive rock requires a careful study of the geological and physical characteristics of the site and a determination of the potential failure mechanisms involved. This paper describes several case histories to illustrate some of these important issues.
ABSTRACT: Laboratory hydraulic fracturing tests were performed in cylindrical specimens of highly porous Berea sandstones (17% and 25%). We were able to raise the borehole pressure so as to induce hydraulic fractures by increasing the viscosity of the borehole injection-fluid as well as the controlled flow rate. A series of tests in unjacketed specimens yielded apparent breakdown pressures that increased linearly with the pore (=confining) pressure, and were well approximated by the two classical criteria. Tests in jacketed specimens, in which the far-field stresses were kept constant, revealed that the apparent breakdown pressure decreased near-linearly with the increase in the initial pore pressure, as expected theoretically. The two hydrofrac criteria served as the upper and lower limit of the experimental data points. The shut-in pressures recorded yielded excellent approximations of the horizontal stress, regardless of porosity, fluid viscosity, or pore pressure. Our results suggest that hydraulic fracturing stress measurements in highly porous sandstone are feasible. However, a more appropriate relationship between in situ stress and breakdown pressure requires better understanding of the poroelastic behavior due to injection-fluid infiltration into the surrounding permeable medium..