M N Gray

A study of the compaction properties of potential clay—sand buffer mixtures for use in nuclear fuel waste disposal

Abstract In-situ emplacement of clay-based buffers in a nuclear fuel waste disposal vault limits the maximum attainable buffer density. This will vary with the composition of the buffer. A study of the maximum attainable densities of candidate Na bentonite/sand and illite/sand buffers is described. The addition of sand significantly increases the achievable compacted density. This increase may be obtained without any decrease in the swelling pressures developed by Na bentonite buffers. Sand decreases the shrinkage potential of the buffer and may also decrease the mass diffusion coefficient. A mixture of 50% sand and 50% clay by mass appears to optimise the physical properties of the buffer.

Why Oilwells Leak: Cement Behavior and Long-Term Consequences

Oil and gas wells can develop gas leaks along the casing years after production has ceased and the well has been plugged and abandoned (P&A). Explanatory mechanisms include channelling, poor cake removal, shrinkage, and high cement permeability. The reason is probably cement shrinkage that leads to circumferential fractures that are propagated upward by the slow accumulation of gas under pressure behind the casing. Assuming this hypothesis is robust, it must lead to better practice and better cement formulations Introduction, Environmental Issues This discussion is necessarily superficial, given the complexity of the issue and attendant practical factors such as workability, density, set retardation, mud cake removal, entrainment of formation gas, shale sloughing, pumping rate, mix consistency, and so on. A conceptual model will be developed in this article to explain slow gas migration behind casing, but we deliberately leave aside for now the complex operational issues associated with cement placement and behavior. In 1997, there were ~35,000 inactive wells in Alberta alone, tens of thousands of abandoned and orphan wells, plus tens of thousands of active wells. Wells are cased for environmental security and zonal isolation. In the Canadian heavy oil belt, it is common to use a single production casing string to surface (Figure 1); for deeper wells, additional casing strings may be necessary, and surface casing to isolate shallow unconsolidated sediments is required. As we will see, surface casings have little effect on gas migration, though they undoubtedly give more security against blowouts and protect shallow sediments from mud filtrate and pressurization. To form hydraulic seals for conservation and to isolate deep strata from the surface to protect the atmosphere and shallow groundwater sources, casings are cemented using water-cement slurries. These are pumped down the casing, displacing drilling fluids from the casing-rock annulus, leaving a sheath of cement to set and harden (Figure 1). Casing and rock are prepared by careful conditioning using centralizers, mudcake scrapers, and so on. During placement, casing is rotated and moved to increase the sealing effectiveness of the cement grout. Recent techniques to enhance casing-rockcement sealing may include vibrating the casing, partial cementation and annular filling using a small diameter tube. Additives may be incorporated to alter properties, but Portland Class G (API rating) oil well cement forms the base of almost all oil well cements. Generally, slurries are placed at densities about 2.0 Mg/m, but at such low densities will shrink and will be influenced by the elevated pressures (10-70 MPa) and temperatures (35 to >140oC) encountered at depth. The consequences of cement shrinkage are non-trivial: in North America, there are literally tens of thousands of abandoned, inactive, or active oil and gas wells, including gas storage wells, that currently leak gas to surface. Much of this enters the atmosphere directly, contributing slightly to greenhouse effects. Some of the gas enters shallow aquifers, where traces of sulfurous compounds can render the water nonpotable, or where the methane itself can generate unpleasant effects such as gas locking of household wells, or gas entering household systems to come out when taps are turned on. Methane from leaking wells is widely known in aquifers in Peace River and Lloydminster areas (Alberta), where there are anecdotes of the gas in kitchen tap water being ignited. Because of the nature of the mechanism, the problem is unlikely to attenuate, and the concentration of the gases in the shallow aquifers will increase with time. This implies that current standards for oilwell cementing and P&A are either not well founded, or the criteria are based on a flawed view of the mechanism. This is not a condemnation of industry: all companies seek to comply with standards. Nevertheless, we believe that the AEUB Interim Directive 9903 is flawed with respect to gas leakage around casings. To rectify this, the mechanisms must be identified correctly. Practise can then be based on correct physical mechanisms, giving a better chance of success (though we do not believe SPE 64733 Why Oilwells Leak: Cement Behavior and Long-Term Consequences Maurice B. Dusseault, SPE, Porous Media Research Institute, University of Waterloo, Waterloo, Ontario; Malcolm N. Gray, Atomic Energy of Canada Limited, Mississauga, Ontario; and Pawel A. Nawrocki, CANMET, Sudbury, Ontario 2 DUSSEAULT, GRAY AND NAWROCKI SPE 64733 that the problem can be totally eliminated because of the vagaries of nature and human factors, despite our best efforts). There is also need for better quality oil-well cement formulations that can resist thermal shocking. For example, leakage of fluids along thermal wells in cyclic steam operations in Alberta has proven a challenging problem for Imperial Oil. If poor quality or poorly constituted cement is used, high injection pressures, thermal shocking, plus non-condensible gas evolution lead to leakage behind the casing that could break to surface under exceptional conditions. Finally, in production management for conservation purposes, zonal isolation is multiple-zone wells. There are initiatives to identify old leaking wells and undertake mitigating action in Alberta and Saskatchewan, the “orphan well” program of the AEUB, initiatives by the Petroleum Technology Alliance Centre in Calgary, and so on. This article is to try and clarify the mechanisms involved.

Suctions, stresses and strengths in unsaturated sand–bentonite

Abstract This paper presents suction and strength characteristics for a dense, compacted, unsaturated sand–bentonite mixture under a variety of preparation and stressing conditions. Suctions were determined experimentally using thermocouple psychrometers and the filter paper method. They are shown to be related to water contents, saturation, dry densities and osmotic agents. The influence of the initial suctions on strength was evaluated using quick undrained triaxial compression tests, here called ‘constant-mass’ tests. Examination of the suction response to applied external stresses was carried out using stress-controlled triaxial tests along selected stress paths. These tests measured suctions in tests where the mean stress p and deviator stress q were changed systematically to give a series of constant values of Δ q /Δ p . The results showed that suction decreased as mean stress loading increased. Suction changes appear to be produced only by the mean stress component of the stress tensor and not by the shear stress component. In this compacted sand–bentonite material, for the pressure range at which the tests were performed ( p ′≤3 MPa), changes in suction produced by changes in mean stress are largely reversible.

Swelling capacity and permeability of an unprocessed and a processed bentonitic clay

Abstract Research on the longevity of potential bentonite-based barrier materials is an important part of the Canadian Nuclear Fuel Waste Management Program. Valuable information on the longterm effectiveness of bentonitic barriers can be obtained by examining the properties of unprocessed bentonites from natural deposits. This study compares the swelling capacity (Ps) and hydraulic conductivity (K) of an unprocessed (clay that has not been ground and dried by the supplier) and a processed bentonite from south-central Saskatchewan, Canada. The clay deposit is 75 to 85 Ma old. At a given clay density, the processed bentonite exhibits a greater P3 and a lower K than the unprocessed clay. This can be at least partially attributed to the partial cementation of the unprocessed bentonite particles; this cementation is likely ruptured when the clay is ground during processing. Even though K for the unprocessed clay is higher than that of the processed clay, it is still low enough (

A capillarity-advective model for gas break-through in clays

Abstract Laboratory testing has investigated how gases can break through compacted specimens of illite, bentonite, and sand–illite or sand–bentonite mixtures. Specimens were formed with a wide range of initial clay densities, water contents and degrees of saturation. Tests were done using two different test procedures. In one, equal increments of gas pressure were applied at constant time intervals until break-through was observed. In the second, the pressure was held constant, and the time required for break-through recorded. Results show that the pressure at break-through increases with clay density and decreases with degree of saturation. When the degree of saturation is below about 85% in illite and clay–illite, and 93% in bentonite and sand–bentonite, there is only a small resistance to gas migration. Above these degrees of saturation, break-through pressures rise sharply. In an approach that differs from some others that have been reported, it is postulated that gas migration is only possible when its pressure is higher than a Gas Entry Value (GEV) that is related to capillarity effects in the largest pores of the material. Thereafter, the rate of advance of the gas–water interface depends on advective flow, that is, on the pressure (hydraulic) gradient across the specimen. Analysis shows that times to break-through should decrease inversely with pressure increase and this was observed in the experiments. Tests were also done on specimens made with non-polar paraffin instead of water. This inhibited the development of bound water in diffuse double layers (DDLs) and led to break-through at much lower pressures.

High-pressure triaxial testing on the canadian reference buffer material

Abstract Triaxial testing is being carried out on a mixture of Na-bentonite and sand to provide parameters for computer modelling of a nuclear-fuel waste disposal vault when the material is used as a sealant. Drained and undrained tests with porewater pressure measurement have been performed at confining pressures up to 6 MPa. The program is now being extended to 10 MPa and elevated temperatures. In the long term, at room temperatures, it is predicted that the material will behave like a normally consolidated clay and strain harden plastically with increasing mean effective stress. In shear, its undrained strength will decrease slightly after failure. Working relationships, based on critical state soil mechanics, have been established for the normal consolidation and failure conditions of the material. The behaviour of the material in drained conditions can be predicted from these relationships.

Mechanism of Ionic Diffusion in Dense Bentonite

A new model based on ionic diffusion in surface (bound) and interstitial (free) water in compacted bentonite systems has been developed. The model is supported by data obtained from flow-through diffusion experiments using Cs + , I - and Cl - . The results show that diffusion depends on the charge of the diffusing species: cations are attracted to the clay particle surface, but are not immobilized, whereas anions are repelled from the clay particle surface. Cations diffuse through both surface and interstitial water, and anions diffuse mainly through interstitial water. The clay structure has been found to affect the diffusion of cations and anions.

Modelling hygro-thermo-mechanical behaviour of engineered clay barriers — validation phase

Abstract The Canadian Nuclear Fuel Waste Management Program (CNFWMP) is evaluating the concept of disposal of nuclear fuel waste in an engineerted vault at a depth of 500 to 1000 m in the plutonic rock of the Canadian Shield. In common with engineered barrier system designs being developed in other countries, the waste would be contained within durable containers that, in turn, would be isolated from the host rock by clay-based materials. The objective of the CNFWMP is to develop a disposal concept that will protect human health and the natural environment far into the future. Assessments of the conceptual vault designs are based on system theory in which an attempt is made to correlate experiences with theoretical concepts of planned systems in such a way that the resulting coordination is sound and convincing. By necessity, since experiments with a total disposal system can never be performed, both the design and the performance assessment rely on experiments performed on physical models of vault elements over relatively short times and on information inferred from calculations (mathematical models) that simulate the probable behaviour of the system in the space-time domainof interest. For a simulation model to be successful, that is applied within a real world situation, the model must provide information regarding the behaviour of the system of interest that is clearly better, in some way, than the mental image or other abstract model that would be used instead. The results of a series of tests performed within the activity known as validation serve as tangible evidence regarding the success of a model in representing the system of interest. This paper focusses on the validation of the models that describe the hygro-thermo-mechanical behaviour of the engineered clay-barriers proposed for application in the Canadian disposal system concept. The strategy being used to address the key issues in modelling to minimize the model error and to maximize the usefulness of the simulation model, based on testing procedures, is reviewed. Finally, a concept of the validation of codes/models that describe the unsaturated behaviour of engineered clay barriers, is described.