To predict and cure borehole instabilities during drilling, it is important to have a profound understanding of the effects that the drilling fluid (in particular water-based muds) may have on shale behaviour. This paper focuses on the consequences of KCl exposure on smectite-rich shales. These effects have been studied by various experimental techniques, both under atmospheric and simulated downhole conditions. This has identified several particular effects as a consequence of potassium intruding the shale. One effect is that the shale shrinks, primarily due to cation exchange. This process increases the permeability and changes the deformability and possibly also the strength of the shale. Simulations of the shrinkage effect around a borehole show that the compressive stresses will decrease with increasing KCl concentration, and this improves the stability. Tensile stresses may eventually be generated in the formation at high concentrations. This enhances the possibility of borehole instability problems and also increases the rate of potentially damaging effects (pore pressure diffusion, ion transport). An important conclusion is that an optimum KCl concentration appears to exist. Shale-fluid interactions were found to depend strongly on the test methods and procedures, calling for a careful review of methods used for optimum drill fluid design. The testing also revealed that the effect of potassium is fundamentally different from other cations upon exposure to shale. P. 215
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
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
A Geomechanical Earth Model (GMEM) is wanted for every field development and should be maintained for the life-time of the field. These models are needed in order to contribute to safe and optimum drilling and production in depleting and complex reservoirs. This strategy is only possible if an automated workflow is developed. Links between the stress simulator Abaqus and the geological software Irap RMS and between Abaqus and the reservoir simulator ECLIPSE are established in order to have; (1) faster and better generation of geomechanical reservoir simulation models, (2) to better account for geomechanical effects in the reservoir simulation and 4D feasibility studies. Abaqus scripting interface is used to link Irap RMS and Abaqus. The link consists of a set of Python scripts that rebuilds the reservoir geometry in the CAD, meshing and visualization program Abaqus/CAE. This is believed to be a unique feature of the developed workflow as opposed to earlier developments that reuse the reservoir grid. In addition, a link between Abaqus and ECLIPSE is developed transferring reservoir pore pressure data, initial porosity and degree of water saturation between ECLIPSE and Abaqus through the file system. Verification and demonstration of capabilities of the developed workflow is done using a faulted North Sea oil and gas field. Introduction Both commercial and research simulators that take the fully coupled nature of three-phase-flow and deformation into account exists today. Stone et al 2000 have extended ECLISPE-300Ô(trademark of Schlumberger) to include geomechanics in a finite difference context, while for instance Li and Zienkiewics 1992 have developed a similar approach using the finite element method. It is then natural to ask if this will make the partly coupled approach described here superfluous. Our experience is however that a partly coupled approach between a conventional reservoir simulator and a stress simulator is the best approach for the near future when advanced geomechanical issues must be taken into account. This is also the industry trend as for instance Schlumberger now is marketing the partly coupled approach between ECLIPSE and the finite element stress simulator VISAGEÔ(trademark of Schlumberger). The partly coupled approach benefits from the latest developments in physics and numerical techniques of both simulators. Computer programs from three vendors are involved in the geomechanical reservoir modelling workflow illustrated in Figure 1:Irap RMSÔ (trademark of Roxar Technologies) for geological modelling,ECLIPSEÔ(trademark of Schlumberger) for reservoir modelling andAbaqusÔ (trademark of Dassault Systèmes) for geomechanical reservoir modeling/stress analysis. A standard procedure is to build the faulted reservoir geometry with the geological tool Irap RMS. After gridding and upscaling a simulation ready model is exported to the reservoir simulator ECLIPSE. A similar coupling between the stress-simulator Abaqus and Irap RMS did not exist starting this project. Parts and assemblies can be imported into Abaqus/CAE (Computer Aided Engineering) from a third-party CAD (Computer Aided Design) system. However Irap RMS does not support the CAD industry standards implying that the reservoir geometry must be generated from scratch within Abaqus/CAE using the geometry creation tools: Solid features, cut features, shell features, wire features, datum geometry and partition tools.
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