Influence of Clays on Borehole Stability : a Literature Survey Part One: Occurence of Drilling Problems. Physico-Chemical Description of Clays and of Their Interaction with Fluids

Influence des argiles sur la stabilité des parois de puits : revue bibliographique. Première partie : les problèmes rencontrés lors du forage dans les argiles. Description physico-chimique des argiles et de leur interaction avec les fluides

¹ Elf Aquitaine Production
² Institut Français du Pétrole
³ TOTAL

Abstract

This survey of literature was undertaken by ARTEP- the French Research Association for Oil Exploration and Production Techniques- at the beginning of STAR (=STabilité des ARgiles), a project on the influence of clays on borehole stability. Knowledge of theories and laboratory experiments was indeed felt very necessary to help understanding, and thus becoming able to prevent, quite damaging phenomena. During the time spent on this project, ideas and interpretations of all participants underwent some evolution due to the comparison between theories and experiment, and new procedures and interpretations are being proposed elsewhere. The survey is divided in four sections :The first section recalls the specific problems caused by the occurrence of shales during drilling operations for oil or gas : about 90 % of the problems, for about 70% of the drilled formations. The behaviour of the shales leads to a classification in four different classes : dispersive, swelling, heaving and brittle. They are spread all over the world, not only on the Gulf Coast of the USA and in the North Sea, where they have been more extensively studied, but also in former USSR, in Asia and Africa. Due to deposit conditions and diagenetic history, they occur at different depths, with different properties : reactive shales, at shallow depths, under-, over- or normally-compacted formations, and reservoir caps. Besides their mineralogical and textural properties, due to the large proportion of clays, they have damaging properties as the low permeability (10 to the power of (-6) to 10 to the power of (-12) D). The variety of reactions with water extends from a complete dispersion in mud to cavings or swelling of the borehole, with cuttings ranging from less than 1 mm to more than several cm. This has led to use of various empirical solutions to protect the borehole, with mainly mechanical or chemical objectives. However, it is felt that the general solutions can arise only from a synergistic effort of both rock mechanics and physico-chemistry, hence the STAR program. The second section is devoted to a survey of physico-chemical reactions between clays and water. It begins with some definitions of the clays as solids, from the rock to the atomic level. All clays are characterised by their small size, in the range of 1 µm, and thus, their high surface area : from one to several hundred M²/g. Existence of a layer charge, and of compensating cations is the key of the behaviour of the swelling clays (i. e. smectites) versus water, pure or with cations, which deserve particular attention. Description of the various kinds of water associated, more or less energetically, with clays, as a function of relative humidity RH, or water activity a index (w), helps to distinguish their effect on porosity and texture. Hydration and dehydration behaviour, with a particular hysteresis, is described, in solid-gas systems as well as in solid-liquid, closer to field conditions. Two main domains are distinguished : crystalline swelling, up to a water content of circa 50 vol. -%, inducing swelling pressures in the range of several thousand atmospheres, and osmotic swelling, for higher water contents, inducing pressures in the range of several atmospheres. Influence of nature and amount of cations is examined, mainly for calcium, which induces limited swelling, and for potassium which is both less hydratable and proner to irreversible fixation on clay. Little work has already been performed on the influence of temperature and pressure. Mechanisms of water and/or ionic species transport are reviewed: diffusion and osmosis, without applied pressure, and effect of pressure. Behaviour on compaction, always showing hysteresis, depends on the nature of the clays, and of the cations, but also on the composition of the solution. It is described using suction pressures or mechanical stresses, which induce different properties of the final solid. Experiments in soil literature, generally performed in the presence of a gas phase, cannot be readily compared to the in situ behaviour of the shales, but give insights on the possible artefacts of laboratory experiments. Caution is thus necessary before any application of literature results to real samples, all preliminary conditioning (initial state and composition of the clay and the water, way of hydration/dehydration, or compaction) being able to modify the behaviour of the clay-water system. The third section sets the problem of describing the mechanical behaviour of the rock formation on drilling. This behaviour depends on initial in situ stresses, pore pressure and temperature, and on the constitutive law of the rock, i. e. the relation between stress and strain. As an example, the Cam Clay elasto-plastic law is developed. Then the laboratory experimental sets used to identify mechanical properties are described : triaxial tests, drained or undrained, oedometric tests, and hollow cylinder tests, the first ones being used to calibrate borehole stability, while the latter simulate drilled boreholes. Specific aspects of shales are then recalled : dependence of mechanical properties on the water content, anisotropy and influence of time. Coupling between physico-chemistry and mechanics arises from the lack of chemical equilibrium between the solid and the liquid. This desequilibrium induces a transfer of water and chemical species in solution, modifying the pore pressure, thus the stress on the rock, and leading to chemical reactions, which have been described in section III. Follows a description of stability models, which should be able to predict mud characteristics for the drilling as well as evolution of the borehole with time. Stability models intend to calculate the maximum/minimum mud weight, from a relevant instability criterion, drawn from well data, mechanical data and fluid properties. The choice of the constitutive law is thus important, and elasto-plastic ones seem the more relevant. Taking into account physico-chemistry has been done generally using an osmoticpressure, with the assumption that the shales behave as a semi-permeable membrane. Even if this assumption is too simplistic, it is still used, but more refined models are being studied, which take into account variations in pore pressures and salinities. The fourth section deals with what actually occurs on application to real wellbore. Improvement of mud formulation tries to prevent any problem occurring during drilling. Evolution of formulation is described, from lime, oil, KCI to polymers additions, and to nowadays constraints brought by environment concerns : requirements may be opposite, and thus compromises must be found. Mud monitoring seems a good prospect. In a second part, availability of representative samples, artefacts related to the recovery and storage of samples as well as choice of experimental conditions are reviewed. It is recalled that downhole conditions are rarely taken into account, and problems like drying of the samples, which induces a suction pressure, anisotropy and cohesion of the samples are rarely considered. The conclusion emphasises that if swelling pressures can now be more precisely defined in hydration domains, the effects of physicochemistry on mechanical properties are still to be investigated more thoroughly. Further experiments should be set in conditions closer to downhole ones, and teams must work together to get all the data needed. This will allow proper stability modelling, with coupling of physico-chemistry and mechanics, to become a predictive tool.

Résumé

Cette revue bibliographique a été réalisée dans le cadre du programme STAR (STabilité des ARgiles) entrepris par l'ARTEP (Association de Recherche sur les Techniques d'Exploitation du Pétrole). Elle se divise en quatre sections: La première section rappelle les problèmes spécifiques rencontrés lors du forage des argiles, qui représentent environ 70 % des formations traversées, et sont répandues dans le monde entier, à diverses profondeurs, donc dans des états de compaction et d'évolution diagénétique différents. Leur comportement a conduit à une classification opérationnelle. Des solutions empiriques, destinées à protéger les parois de puits, ont été mises en oeuvre. Toutefois, il apparaît évident que des solutions de caractère général ne peuvent résulter que d'un effort conjoint en mécanique des roches et en physico-chimie : tel est l'objectif du programme STAR. La seconde section est consacrée à la description des réactions physico-chimiques entre les argiles et l'eau. Après avoir décrit les argiles en tant que solides, et plus particulièrement les argiles gonflantesqui ont une charge structurale faible compensée par un cation interfoliaire échangeable, on s'attache au comportement de l'eau qui leur est associée en fonction du degré d'humidité et de la nature et de la concentration des sels dans l'eau. Le gonflement cristallin , limité à une teneur en eau de 50 % environ, et correspondant à des pressions de gonflement de milliers d'atmosphères, est distingué du gonflement osmotique , qui intervient à de plus fortes hydratations, et induit des pressions de l'ordre de la dizaine d'atmosphères au maximum. L'influence spécifique des cations comme le potassium et le calcium est décrite. Des exemples de comportement à la compaction, fonction de la nature des argiles et des cations sont décrits. Il apparaît qu'une description très précise des conditions de départ et du déroulement des phénomènes est indispensable pour l'interprétation, et qu'elle est malheureusement absente dans nombre de publications. La troisième section traite de la description mécanique du comportement des roches argileuses lors du forage. La possibilité de modéliser ce comportement dépend de l'acquisition des paramètres pertinents, et du choix du modèle : le modèle Cam Clay, qui utilise une loi élasto-plastique est donné en exemple. Les tests de laboratoire sont décrits. Dans les argilites, les déséquilibres chimiques entre solide et fluide induisent des transferts d'eau et d'espèces chimiques, modifiant la pression de pore, donc la contrainte à laquelle est soumise la roche, et conduisant à des réactions chimiques décrites dans la deuxième section. Les modèles de stabilité prennent en compte ce couplage par l'usage d'une pression osmotique , avec l'hypothèse simpliste que les argilites se comportent comme des membranes semi-perméables. Des modèles plus affinés sont en cours d'élaboration pour prendre en compte les variations de pression de pore et de salinité. La quatrième section traite des pratiques de terrain, en particulier des améliorations apportées par une formulation de plus en plus élaborée des boues, pour répondre à la fois aux problèmes de forage, et aux exigences de préservation de l'environnement. La question de représentativité des échantillons destinés au laboratoire, et des artefacts liés à la récupération et au stockage est traitée : en particulier les variations de pression capillaire lors du séchage et de la réhydratation imposent des protocoles stricts. Des conditions expérimentales proches des conditions de fond sont rarement employées. En conclusion, si la notion de gonflement, et les conditions dans lesquelles il peut intervenir, apparaissent plus précisément, l'effet sur les propriétés mécaniques reste encore largement à étudier. Une approche multidisciplinaire est indispensable à l'élaboration de modèles de stabilité qui se veulent prédictifs.