INFLUENCE OF CLAYS ON BOREHOLE STABILITY - A LITERATURE SURVEY .1. OCCURRENCE OF DRILLING PROBLEMS PHYSICOCHEMICAL DESCRIPTION OF CLAYS ANDOF THEIR INTERACTION WITH FLUIDS

This survey of literature was undertaken by ARTEP - the French Researc h Association for Oil Exploration and Production Techniques - at the b egin ni ng of STAR (=STabilite des ARgiles), a project on the influenc e 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 t ime spent on this project, ideas and interpretations of all participan ts 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 tour sections : The first section recalls the specific problems caused by the occurrence of shales duri ng drilling operations for oil or gas : about 90 % of the problems, fo r about 70% of the drilled formations. The behaviour of the shales lea ds to a classification in four different classes : dispersive, swellin g, heaving and brittle. They are spread all over the world, not only a n 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 diff erent 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 pe rmeability (10(-6) to 10(-12) D). The variety of reactions with water extends from a complete dispersion in mud to cavings or swelling of th e borehole, with cuttings ranging from less than 1 mm to more than sev eral 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 synergist ic effort of both rock mechanics and physico-chemistry, hence the STAR program. The second section is devoted to a survey of physico-chemica l reactions between clays and water. It begins with some definitions o f the clays as solids, from the rock to the atomic level. All clays ar e characterised by their small size, in the range of 1 mu m, and thus, their high surface area: from one to several hundred m(2)/g. Existenc e of a layer charge, and of compensating cations is the key of the beh aviour of the swelling clays (i.e. smectites) versus water, pure or wi th cations, which deserve particular attention. Description of the var ious kinds of water associated, more or less energetically, with days, as a function of relative humidity RH, or water activity aw, helps to distinguish their effect on porosity and texture. Hydration and dehyd ration behaviour, with a particular hysteresis, is described, in solid -gas systems as well as in solid-liquid, closer to field conditions. T wo 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 wate r contents, inducing pressures in the range of several atmospheres. in fluence of nature and amount of cations is examined, mainly for calciu m, which induces limited swelling, and for potassium which is both les s hydratable and proner to irreversible fixation on clay. Little work has already been performed on the influence of temperature and pressur e. Mechanisms of water and/or ionic species transport are reviewed : d iffusion and osmosis, without applied pressure, and effect of pressure . Behaviour on compaction, always showing hysteresis, depends on the n ature of the clays, and of the cations, but also on the composition of the solution. It is described using suction pressures or mechanical s tresses, which induce different properties of the final solid. Experim ents in soil literature, generally performed in the presence of a gas phase, cannot be readily compared to the in situ behaviour of the shal es, but give insights on the possible artefacts of laboratory experime nts. Caution is thus necessary before any application of literature re sults 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 sy stem. 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 c onstitutive law of the rock, i.e, the relation between stress and stra in. As an example, the Cam Clay elasto-plastic law is developed. Then the laboratory experimental sets used to identify mechanical propertie s are described: triaxial tests, drained or undrained, oedometric test s, and hollow cylinder tests, the first ones being used to calibrate b orehole stability, while the latter simulate drilled boreholes. Specif ic aspects of shares are then recalled: dependence of mechanical prope rties on the water content, anisotropy and influence of time. Coupling between physico-chemistry and mechanics arises from the lack of chemi cal equilibrium between the solid and the liquid. This desequilibrium induces a transfer of water and chemical species in solution, modifyin g the pore pressure, thus the stress on the rock, and leading to chemi cal reactions, which have been described in section II. Follows a desc ription of stability models, which should be able to predict mud chara cteristics for the drilling as well as evolution of the borehole with time. Stability models intend to calculate the maximum/minimum mud wei ght, from a relevant instability criterion, drawn from well data, mech anical data and fluid properties. The choice of the constitutive law i s thus important, and elasto-plastic ones seem the more relevant. Taki ng into account physico-chemistry has been done generally using an ''o smotic'' pressure, with the assumption that the shares behave as a sem i-permeable membrane. Even if this assumption is too simplistic, it is still used, but more refined models are being studied, which take int o account variations in pore pressures and salinities. The fourth sect ion deals with what actually occurs on application to real wellbore. I mprovement of mud formulation tries to prevent any problem occurring d uring drilling. Evolution of formulation is described, from