Abstract :
[fr] Ce travail est motivé par les problèmes posés par la stabilité des puits forés dans des formations argileuses avec des boues de forage à base d'eau. En effet, les roches argileuses ou argilites (shales en anglais) ont la propriété d'absorber de l'eau, entraînant ainsi la déstabilisation des puits, soit par gonflement de certaines espèces minérales, soit par annulation de la pression de soutènement de la paroi par suite de la modification de la pression de pore. La déstabilisation peut être purement mécanique avec une plastification entraînant un cavage du trou, ou dépendre essentiellement des interactions physicochimiques entre le fluide de forage et l'argile. Le but de ce travail est donc de mettre expérimentalement en évidence les mécanismes susceptibles de jouer un rôle dans les phénomènes évoqués, et de tenter de quantifier l'importance de ces mécanismes pour en tenir compte dans les modèles de calcul. Le système expérimental que nous utilisons est basé sur une cellule triaxiale perméable aux rayons X, et donc destinée à un fonctionnement sous scanner (tomographie). Ce système, conçu et construit à l'Institut Français du Pétrole (IFP), est nouveau et nous avons contribué dans le cadre de ce travail à sa mise au point. La démarche consiste à mettre un échantillon d'argile sous confinement au contact avec un fluide de composition déterminée et à voir si les composants de ce fluide migrent dans l'argile ou non. Bien entendu, la tomographie ne permet que d'avoir des densités globales avec des résolutions de loin supérieures à la dimension du pore d'un matériau argileux sous confinement. Il est donc évident que les indications fournies par cette méthode doivent être complétées avec d'autres types de méthodes pour arriver à une étude sélective de la migration des éléments en solution. Pour ce qui concerne le matériau, notre choix s'est porté sur l'argile de Boom en Belgique, d'une part pour sa disponibilité et, d'autre part, pour la grande quantité d'informations disponibles sur ce matériau.
[en] General ConsiderationsThis work deals with problems encountered regarding the stability of wells drilled in the clay material formations with water based muds. In fact, clays or shales have a property of taking water, thus causing the instability of wells either because of the swelling of some mineral species, or because the supporting pressure is suppressed by modification of the pore pressure. The aim here is to experimentally emphasize the principal mechanisms driving the phenomenon of instability, and to try to quantify the importance of these mechanisms in order to include them in calculation models. The behaviour of a shale put in contact of a water based fluid depends on its initial water activity and on the composition of the fluid. According to the situation, the shale will take or expel water, with a consequence of swelling or shrinkage. Three factors play an important role in the water activity of shales :- The electrostatic interaction which is related to the cation exchange. This mechanism consists in a cation passing from the solution to the surface of a layer and an interlayer cation of the clay doing the opposite path. Clay materials are characterized by the Cation Exchange Capacity or CEC;- The salt concentration which is related to the osmotic phenomenon. If a shale, and especially a montmorillonite, is put in contact with a pure solvent, in addition of the ion hydration, the solvent will be taken by the shale in order to dilute the high ionic concentration of that shale. This last mechanism is macroscopically expressed by a difference of osmotic pressure between the external solvent and the pore fluid of the shale. If we consider the case in which the solvent is pure water the osmotic pressure is expressed by equation (2) in which awi is the water activity of the shale and vw the partial molar volume of water (we suppose here that this vw has the same value in the shale and in the solution, an assumption which is valid in the case of diluted solution);- The saturation degree which is related to the suction pressure (or capillary pressure). We have divided the behaviour of shales in two fundamental groups of mechanisms although they are physically related, these are the deformation mechanism and the transport mechanisms. The mechanism of poroelastic deformationThe Biot theory for the behaviour of saturated porous material can be generalized in the case of a chemically active material like a shale by the equations (4) and (5) which apply to an elementary system formed by a solvent and r - 1 other species contained in the pore fluid. In that equations epsilon ij is the deformation, mr the mass of the rth species for a reference volume, sigma ij is the stress and µr is the chemical potential of the rth species. The cross-coefficients can be determined by specific experiments. In fact, the experimental systems give the possibility of choosing some specific boundary conditions as the confining state of the material, the chemical composition of the pore fluid, or of the fluid put in contact with the shale; and by measuring couples of experimental values it is possible to set some correlations in which the desired coefficients can be identified. The transport mechanismsMody and Hale (1993) have classified the mechanisms characterizing the transport of fluids in a clay material, and between the mechanisms presented, three are of particular importance for our study :- The hydraulic flow for which the driving force is the hydraulic pressure difference. The flow rate depends on the permeability of the porous medium. - The osmotic transport (diffusion) in which the driving force is the chemical potential of water. This mechanism is related to the semi-permeable membrane and during the diffusion process only the molecules of water move into or out of the shale. - The open communication (diffusion) for which the driving force is the chemical potential of ions and water. Here ions and water diffuse into or out of the shale. The general law describing the movement of solvent and ionic species in a shale is presented in equation (7) for which qr represents the mass flux of the rth species, µs the chemical potential of the s species, V the gradient operator, and L a tensor containing the coefficients to be determined experimentally. The physical meaning of the coefficients depends on the transport mechanism considered. For the three fundamental mechanisms presented above, the transport law is expressed respectively by the equations (8) for hydraulic flow with K being the permeability of the shale, (9) for the osmotic transport with Lw representing the moisture adsorption coefficient, and (10) for the more general open communication. To build the diffusivity laws we need some conservation principles as the mass conservation which is expressed by equation (11) and the electroneutrality represented by (12). Combining the law of mass conservation (11) with the hydraulic transport law (8) leads to a diffusivity law characterized by a consolidation coefficient Co which can be deduced from permeability measurements. In the same manner, we obtain a diffusivity coefficient Cd by combining the mass conservation law (11) with the osmotic transport law (9). For the general open communication, the electroneutrality law (12) may be used with transport law (10) to build a more complex diffusivity law represented by the equation (13) (Sherwood, 1993). For the particular situation in which the pore fluid contains two species r and s in a one dimensional system (x direction), we have developed the so above presented laws. For the purpose of simplification we have considered in the chemical potential only the concentration of species r and s. The transport law leads in that case to the equations (17) and (18) expressing the mass flux of species r and s. Putting this equations in the mass conservation law (11) leads to the diffusivity law (20), and if we neglect the influence the concentration of the species s on the diffusion of the species r, assumption which is generally admitted (Put et al, 1987, 1991), we obtain a diffusivity law (21) known as Fick's law. In that equation Lr is the apparent diffusion coefficient which depends on the accessible porosity, on the density and on other factors as expressed in equations (22) and (23). The clay material and the experimental systemThe Boom clay materialWe have chosen the experimentally well known Boom clay material from Belgium in order to study the mechanisms of ions diffusion and/or osmotic transport. This is a gray clay rich in pyrite concretions and septaria, almost homogeneous in the middle part of the geological formation and more silty in lower band and in the upper part with rhythmic changes in silt content. This material from marine depositional environment contains 60% of clay minerals. The 100 m thick geological formation has a burial depth of about 200 m at Mol where a nuclear research center is installed Centre d'études de I'énergie nucléaire (CEN-Mol). Many known characteristics of the Boom clay are given in paragraph 3. 1 : mineralogical composition, geochemical parameters, petrophysical and hydraulic parameters, geomechanical parameters. Two cores were provided graciously by the CEN-Mol and appendices 1 to 4 show some scanner images. It clearly seems when observing these images that the material is heterogeneous on the X-rays point of view, and we think that the more absorbent regions are pyrite concretions. This material is very sensitive to the atmospheric conditions and reacts with oxygen to give a sodium sulphate pore water type whereas the original pore water is of sodium bicarbonate type. To minimize the influence of atmospheric conditions, the time delay has to be minimized between opening of the core and loading of the sample in the experimental system. Four samples were prepared, two of dimensions 40 x 80 mm for a triaxial system and two of dimensions 40 x 22 mm for oedometric testing. Some physical measurements show that the water content is 19. 3%, and that the porosity and the degree of saturation are respectively 38 and 83% in the destressed state (the material is saturated in the in situ state). The result supplied by the oedometric test is shown on the figure 1 where the value of Sc, the consolidation stress, can be considered as the proof that the material is normally consolidated. We have also estimated the consolidation index lambda = - 0. 0398 and the swelling index k = - 0. 0067. The experimental systemThe main experimental system is a triaxial cell designed and built by the Institut français du pétrole (IFP) on which we have worked for fine setting. The specificity of this cell is that it is permeable to X-rays, so that it can be used on the scanner system of IFP. The technical specifications of the triaxial system are an axial maximum pressure of 60 MPa, a confining maximum pressure of 20 MPa, an inlet maximum pressure of 10 MPa, and an outlet maximum pressure of 5 MPa. For purpose of feasibility, we have tested a Fontainebleau sandstone of 5% porosity. The test consisted firstly in applying an isotropic confining pressure of 5 MPa, saturating the specimen with demineralized water, and injecting a CaCI2 2M solution. Figure 3 shows the progression of CaCI2 in the sandstone specimen as determined with the scanner system. We can see that the front of CaCI2 is clearly identified in three chosen sections of the specimen. It can also be observed that there is a gradient between the two limits of the specimen and we think that this is due to non perfect saturation of the material. Based on the curves supplied by figure 3, we estimate the rate of progression of the CaCI2 front in the specimen. The results so obtained are summarized in table 1 where vm is the average rate computed between the actual section and the inlet limit, and vi the rate between two contiguous sections. Assuming that the hydraulic flow is the principal transport mechanism, we have calculated the rate of flow based on the permeability of the material and the known flow of 25 cc/h. The comparison shows that the results obtained are very similar, the rate calculated on the permeability base being 0. 105 mm/s while the vi gives a value of 0. 11 mm/s in the central part of the specimen. In order to plan tests on the clay material (which take more time than on sandstones) we have decided to make some simulations of ions diffusion and pressure, propagation in the Boom clay. Data available from diffusion measurements done at the CEN-Mol are presented on the figure 5 where we observe the decrease of the diffusion conductivity êtaRD (êta is the accessible porosity, R the retardation factor and D the apparent diffusion constant) under increased confining pressure. Solving the diffusion equation (21) with the boundary conditions presented on figure 4 leads to the Crank equation (26) giving the quantity Q diffused in the outlet reservoir after a given time. In this equation S represents the surface of the right section of the cylindrical specimen, and Ce is the constant concentration of the migrating species in the inlet reservoir. Using equation (26) with data supplied on figure 5 gives the diffusion simulations presented on figures 6 and 7 for respectively 4. 41 and 6. 86 MPa of confining pressure. It is clear on these figures that water (tritium) diffuses more rapidly than the other components and the difference in rate of diffusion increases with the confinement. At 6. 86 MPa the water reaches the outlet reservoir after 270 hours whereas the iodide reaches it after 720 hours. This observation shows that during 450 hours, the shale specimen could be considered as behaving as a semi-permeable membrane. In the same way, for the purpose of understanding the pressure propagation in the specimen of figure 4, we have used the Jaeger-Carslaw relation (27) to compute the increase of pressure in the outlet reservoir during the course of time. In that equation Co is the consolidation coefficient while po represents the constant pressure of the inlet reservoir. Figure 8 shows the results obtained with the inlet pressures of 1, 5 and 10 MPa. The sodium chloride migration test in the Boom clayExperimental conditionsThe aim of this experiment is to understand the diffusion of water and/or ions in the clay material in different confining conditions. To do this we have to set two parameters, that are the confining pressure which we choose to fit the in situ value (7 MPa), and the concentration in the fluid to be put in contact of the clay specimen. The original pore water of the Boom clay is sodium bicarbonate type with a 0. 42 g/l concentration. We chose to use a sodium chloride solution in the inlet reservoir in order to avoid the cation exchange mechanism, and the concentration of this fluid is so high (46 g/l) that we could expect a movement of water or/and ions between the clay and this fluid. Before putting the salted fluid in contact with the clay, some operations have been made in order to set the specimen in the right conditions. These are the saturation of the circuits and the specimen with an equilibrated water prepared by mixing demineralized water with the Boom clay, the application of the confining pressure of 7 MPa, and the application of a pore pressure of 1 MPa. After four days for setting initial conditions, a first reading on the scanner system was made. The image in appendix 7 shows the specimen on the initial state and the sections which will be studied during the experiment. After this first scanner reading, the equilibrated water in the inlet reservoir was replaced by the salted fluid, and some scanner readings were realized in the course of time. ResultsThe table of appendix 9 gives the results of the density measurements on the scanner system for a period of 54 days. The radiographic heterogeneity of the clay material does not enable the measurement of a concentration gradient between the two ends of the specimen. The unique solution for us was to analyze the variation of the scanner density in some chosen sections of the specimen. We have made this work on five sections : two sections at the entrance, one in the middle part and two others at the exit end of the specimen. In each section, some regions of interest (ROI) have been chosen to take account the heterogeneity of the material. So, for each section we have the bulk density, some values for the high density regions (clear on the images), and some values of the low density regions (dark on the images). Figure 9 shows the variation of the normalized bulk density in the five sections. Some fluctuations at the beginning of the test may be attributed to three principal reasons :- diffusion of NaCI in the clay material,- chemical disturbance of the clay material which is a very reactive system if put in contact with oxygen,- consolidation process which could still be continuing. After 300 hours of test the variations of the bulk densities become more clear and it can be noticed for each section an increase during the course of time. But it is difficult to observe the front of ions diffusion in the specimen as in the case of the Fontainebleau sandstone. In fact, apart from the curve of section 477 which begins to increase only after 300 hours, all the other curves increase from the beginning of the test. The consolidation is possibly very important during the 300 first hours with some influence of the chemical transformations. After stabilization of these two mechanisms, the variations observed are likely to be due essentially to the diffusion mechanism. On figure 10 are presented some curves for selected ROI. In the curves names, the three first digits correspond to the designation of the section, the digit after the underscore represents the number of the ROI in the section, and the last letter indicates if the region is clear (c) or dark (s). We see that the dark regions present an increasing tendency and we think that this is due to the diffusion mechanism; whereas the clear regions tend generally to decrease probably because of chemical transformations. After the experiment the specimen has been cut in three parts to determine the quantity of chloride ion diffused. The X-rays analysis shows a concentration of 1590 ppm at the entrance end, 380 ppm in the middle part, and 180 ppm at the exit end. This gradient in the specimen gives the proof that the diffusion mechanism had not reached a stabilized state when the test was stopped. ConclusionsTo enable the understanding of the physicochemical and mechanical interactions between shale materials and the water based drilling fluids, we have presented in this paper some theoretical considerations. For sake of methodology, we have divided the phenomenons investigated in two fundamental mechanisms : the poroelastic deformation and the transport mechanisms. These theoretical considerations lead to some coefficients to be determined experimentally, and an emphasis was made on the diffusion and the osmotic mechanisms. For our experiments, we chose to work on the well known Boom clay from Belgium, and the principal experimental system consists in a triaxial cell designed to be used on the scanner apparatus. We worked on the setting of this experimental system, and a feasibility study on the Fontainebleau sandstone have shown clearly a front of a CaCI2 solution in the specimen saturated with demineralized water. A further test on a clay specimen confined at 7 MPa and put in contact of a NaCI solution has been run on 54 days. During this test we followed the variation of the scanner density in some chosen sections, and in these sections some regions of interest were selected because of the heterogeneity of the material. Measurements with the scanner system showed a general increase of the density during the course of time, but we did not clearly observed a front of NaCI in the specimen as in the case of the Fontainebleau sandstone. Between the reasons retained to explain this situation we can cite the chemical transformations, the consolidation process at the beginning of the experiment, and the low density of the NaCI solution. For the future, we think that the scanner system could give good results on clay materials if some conditions could be achieved, thess are for example a long consolidation time, a very precise positioning of the triaxial cell on the scanner system, and the use of very dense fluids.
