Interaction Between Shale and Water-Based Drilling Fluids: Laboratory Exposure Tests Give New Insight Into Mechanisms and Field Consequences of KCl Contents

Abstract

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

Maintaining the stability of deviated and horizontal wells: Effects of mechanical, chemical and thermal phenomena on well designs

“…The strength of Arco shale is enhanced after exposure to lowconcentration KCl solutions, and then decreased with increasing concentration. Similar phenomena were observed by Horsrud et al (1998).…”

Brittleness of shales: Relevance to borehole collapse and hydraulic fracturing

“…More significant however was the significantly enhanced plasticity (reduced B 1 ) that occurred during exposure of this shale to KCl brine. This observation is explained by ionic exchange within the smectite, which contributes to irreversible shrinkage of the rock (Horsrud et al, 1998).…”

Drilling Fluid Technology: Performances and Environmental Considerations

“…Several studies on shale-fluid interactions confirm that various causes are at the origin of borehole instability: water adsorption, osmotic swelling and cation exchange. Different approaches to WBM design have been suggested depending on given shale formations (Darley, 1969;Chenevert, 1970;Roehl and Hackett, 1982;Beihoffer et al, 1988;Zamora et al, 1990;Hale and Mody, 1992;Bol et al, 1992;Cook et al 1993;Mody and Hale, 1993;Bailey et al, 1994;Simpson et al 1994;Durand et al 1995;Horsud et al, 1998;Pernot, 1999). Other recent studies focused on shale-fluid interactions Schlemmer et al, 2002;Van Oort, 2003).…”

“…They observed that, after increasing the upstream pressure, the outlet pressure increases due to a higher pressure in pore caused by the hydraulic flow. Horsud et al (1998) have also studied the phenomenon of swelling pressure in clays and concluded that osmosis does not play a role, but that pressure (or suction) is the main parameter that controls the development of the swelling. Pernot (1999) quantified the effect of the swelling pressure of a variety of fluids in contact with several types of clays and concluded that the methylglucoside type 'Gumbo' stops clay swelling.…”