Electrochemical aspects of wellbore stability: Ionic transport through confined shales

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Int. J. Rock Mech. Min. Sci. Vol. 35, No. 4/5, pp. 548-549, Paper No. 098, 1998 © 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0148-9062(98)00114-4 ISBN: 0080433332 ISSN: 0148-9062/98 $19.00 + 0.00

Electrochemical Aspects of Wellbore Stability: Ionic Transport through Confined Shales R. F. T. LOMBAt M. M. SHARMAt M. E. CHENEVERTt Paper No. 098§ Full paper on enclosed CD-ROM Shale-related problems associated with the use of water-based drilling fluids can represent a large percentage of the cost of the well. These problems have long been identified as a result of the disturbances experienced by the rock due to interactions with the drilling fluid. Shales are highly water-sensitive formations that react when contacted by water-based fluids. The differences in chemical activities of the pore fluid and the drilling mud are responsible for the establishment of a non-equilibrium state. The low permeability of shales and the presence of charged surfaces on the constituent clays contribute to make the phenomenon very complex. The overall process still lacks complete understanding despite the numerous efforts that have been dedicated to the subject. The water sensitivity of shales was overcome in the past by the use of "balanced activity" oil-based emulsion drilling fluids. This kind of fluid prevents the water and ions from being transported into the shales. The oil and emulsifiers act as barriers to the movement to the ions, and the water droplets which contain salt creates an osmotic cell which maintains the system in an equilibrium state. The osmotic pressures that are developed counterbalance any applied hydraulic pressure and contribute to the stabilization of the wellbore and to minimize drilling problems. However, the use of oil-based muds became progressively less frequent due to environmental concerns. Most water-based fluids are environmentally safe but they lack the inhibitive properties of oil-based muds. The absence of an impermeable restraint to the movement of ions in water-based fluids enables the interactions with the pore fluid and with the charged clay surfaces. The mobility of the ions in the shale, however, greatly differ from the behavior in dilute bulk solutions. The negative charges on the clay surfaces tend to accelerate the positive ions and to retard the negative ions. This confers a membrane character to the shale. Therefore, shales may be classified as ion-exchange membranes with fixed sites and narrow pores. The presence of charged groups and pore spaces confers permselectivity (ability to exclude co-ions) and/or semipermeability (osmotic pressure development) to the membrane. Because of the presence of fixed charges, the membrane tends to exclude the co-ions by electrostatic repulsion. The extent to which shales behave as non-ideal semi-permeable membranes shall be herein investigated. This paper presents a brief overview of the electrochemical interactions related to wellbore stability problems and an experimental study on the diffusion of ions and water through a North Sea shale sample subjected to elevated stress states. An experimental apparatus was designed to operate under confining pressures to 6000 psi and to measure the diffusion rates of ions and water through shales using tritium, chloride-36, and calcium-45 radioactive tracers. The chemical potential difference of the tested species was the driving force for the flow of ions. No hydraulic differential pressure was applied to the system neither was any electric potential developed across the sample. The diffusion of tracers was monitored with time under different stress conditions. A diffusion test under atmospheric pressure was also carried out. The effect of the confining pressure on the diffusion of the tracers and on shale permeability was evaluated. Results show that indeed diffusion of both water and ions is further restricted through shales by the increase in confining pressure, however, in all cases, even at stress levels equal to depth of burial, ions are still free to tCurrently with PETROBRAS, Petroleo Brasileiro S.A., Exploration & Production Department, Av. Republica do Chile, 65-10° andar, Rio de Janeiro, Brazil. tCenter for Petroleum and Geosystems Engineering, The University of Texas at Austin, CPGE Building, Room 2.502, Austin, TX 78712, USA. §Conference Reference: USA-829-2 548

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move. Therefore, complete restnctIOn of ionic flow from water base muds is not possible at this time. This suggests that fluids, which might have other compositions and characteristics, be further studied such that the restriction of flow of water and ions can be achieved with water base muds. The implications of the work are that elevated downhole stress levels do have a significant effect on both the ionic and the aqueous movement of compounds into shales, however, the movement of water and ions still exist, only at a reduced rate. The results also show that the water moves at a much more rapid rate than the ions, under all levels of stress applied. Key words-shale, ionic transport, ionic diffusion, wellbore stability, drilling fluids, drilling muds, radioactive tracers, membrane, membrane efficiency, high pressure