Experimental work and numerical modelling have been applied to study mechanisms related to time dependent borehole stability during drilling in shales. Fully coupled numerical modelling showed that consolidation effects may act on a time scale of days, due to the low permeabilities of shale. Creep testing of a North Sea Tertiary shale showed that above 80% of the failure stress, the observed strain rates indicated that delayed borehole failure could occur within a period of 15 days. When exposing outcrop shale with close to zero swelling clay content to non-native fluids, substantial changes in mechanical properties were observed. Elevated temperature (80 C) also seemed to cause a weakening both in static and dynamic properties. Introduction Stability problems during drilling in shales account for a significant amount of lost time and extra cost during drilling. Operational experience indicates that stability problems tend to increase with time after drilling, i.e. the longer a shale section stays open, the greater the risk of experiencing stability problems. Our qualitative and quantitative under- standing of the mechanisms responsible for this behaviour is still inadequate. This paper discusses some of the mechanisms involved in time dependent behaviour of shale, illustrated with examples from laboratory testing and numerical modelling. MECHANISMS RESPONSIBLE FOR TIE DEPENDENT BEHAVIOUR OF SHALE Four different mechanisms that may lead to delayed borehole failure will be discussed. There are two intrinsic mechanisms which control the stress-strain-time behaviour of a saturated rock which is exposed to a change in load:Hydrodynamic consolidation, characterized permeability, rock frame stiffness, fluid stiffness and porosity (re. Biot theory of consolidation).Creep, characterized by stress and time dependent strains at stress levels below the failure stress. In addition, we will discuss two extrinsic mechanisms:Shale-fluid interaction, characterized by a change in shale behaviour after extensive periods of contact with a non-native fluid (i.e. fluid that is not in chemical equilibrium with the formation).Temperature effects, i.e. mud temperature variations which may alter the stresses and which may also affect the mechanical properties of the formation. Other effects of a more indirect nature will not be discussed here, like time dependent boundary conditions in the borehole, such as changes related to mud pressure (e.g. surge, swab). EXPERIMENTAL SET-UP In order to characterize the mechanical behaviour of shale cores, specialized methods and procedures for triaxial testing have been applied. Fig. 1 shows the internal instrumentation in the triaxial cell, facilitating measurement of both static and dynamic response of the test sample. The shale samples (1.5" diameter and 3" length) by are installed in the triaxial cell, and the following general procedure is applied for triaxial testing:Loading to a predetermined level of consolidation (normally hydrostatic) with a set pore pressure.Consolidation of the sample under drained conditions, i.e. equilibration of pore pressure. P. 259

A mechanistic -empirical model for geosynthetic base-reinforced flexible pavements is proposed. The model uses traditional components of an existing unreinforced mechanistic -empirical model developed in the USA through NCHRP Project 1-37A. These components include a finite element response model, material models for the asphalt concrete, unbound aggregate base and subgrade and damage models for asphalt concrete fatigue cracking and permanent deformation in the pavement cross-section layers. New components for the reinforcement are introduced and include structural elements for the reinforcement, a material model for the reinforcement, a model for reinforcement -aggregate shear interaction, additional response modelling steps that account for the influence of the reinforcement on lateral confinement of the base aggregate during construction and subsequent traffic loading, and a modified permanent deformation damage model used for aggregate within the influence zone of the reinforcement. This paper describes the basic components of the model with a focus on the ability of the model to predict permanent deformation, which is compared to results from test sections. This comparison shows favourable agreement that is on the level seen with existing unreinforced mechanistic -empirical models and a large improvement over previously proposed models for reinforced pavements.

Gas migration into well annuli with resultant high pressures is a common problem in oil and gas production wells. On a North Sea major gas producer high pressure has been observed in the outermost annulus of several wells and anomalous pressures were observed below the gravity platform base. Gas may migrate along the well-path and into the well annulus to create pressure problems and was further suspected to continue to the surface. The source of such gas is often linked to shallow gas pockets or small reservoirs and the mechanism whereby the gas migrates into the annulus and creates excess pressure problems has often been associated with poor cementing or micro-cracking in the cement. The pressure and associated gas volumes are routinely dealt with by bleeding off the gas and/or water, but this action does not necessarily solve the problem. A detailed study was initiated to investigate if the gas migration was a hazard to well integrity and foundation stability. Work to identify the source of the gas and mechanisms involved on this North Sea platform has provided a new insight into the understanding of gas migration in sediments and overburden. A gas source exists even if no gas pockets are present, and the migration into annuli is independent of the presence and quality of cement. The gas migration as a potential hazard for the platform foundation and a possible link with a natural gas flux in the seabed and the formation of pockmarks has been checked. The time scale for gas migration is important to judge the hazard. A model is presented in which water insoluble H2 gas plays an important role. This paper gives a brief overview of the problems associated with gas migration, pressure build up in the wells and the foundation, and its relation to the formation of pockmarks in the soft clayey seabed of the Norwegian Trench. BACKGROUND AND DESCRIPTION OF PROBLEM The Troll A Platform is a huge concrete gravity base structure located in the Norwegian trench at a water depth of 305m, see Figure 1. The platform was installed in 1995 and in the following two years 40 wells were installed, 39 gas production wells and 1 monitoring well. A typical well design is shown in Figure 2. The platform is the North Sea largest gas producer with an average yearly production in the order of 26 GSm3 (923 billion cu.ft). After a few years of production it was realized that pressure bleed-off activity from outer annuli was high, for some wells 10-20 bleed-offs every month. It is standard practice to set safe threshold values for pressure in well annuli and if this value is reached, pressures are reduced by bleeding off gas. The threshold for outer annulus was 6 bar, and this was raised to 15 bar after an evaluation of wellhead seals in 2001. This action reduced the problem of very high bleed-off activity to more normal conditions, typically once a month for an average well.

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Based on a containment transport model developed for hydrogeological purposes, a numerical method for the analysis of intrusion of potassium ions into a shale has been developed. The scheme has been applied in the back-analysis of KCl-brine exposed specimens of smectite-rich Tertiary Paleocene shale from the North Sea. The specimens, exposed under effective confining stresses in a triaxial cell at 80 C, shrank during the KCl-exposure. Two tests with different KCl concentrations (5wt% and 20wt%) have been back-analysed. The ion transport is modelled by diffusion Using a finite difference scheme. In the back-analysis it was assumed that the observed shrinkage is due to the ion exchange when the bound Na+-ions are exchanged with K+-ions from the exposure fluid. The agreement between simulated and measured shrinkage rate was good, indicating that the assumed relation between ion transport, ion exchange and shale shrinkage is a valid mechanism. The simulations showed tensile stresses near the specimen boundaries, where the shale first shrinks. A downhole situation was therefore analysed with a linear elastic model, and the development of tensile stresses with time was investigated. This linear elastic analysis showed that in the vicinity of the borehole wall, large tensile stresses develop as the front of the K+ ions progresses into the shale. Such tensile stresses may lead to the development of cracks and fissures, which in turn increases the surface area of the brine-exposed shale. An accelerating mechanism of cracking may thus develop, increasing the potential of destabilising the shale. For each practical case there is thus an upper limit to the KCl-concentration which should be used in smectite-rich shale. Up to this limit, stability is improved due to a reduced stress concentration. Above this limit, stability problems will increase with increasing KCl-concentration. P. 273