Chemical Osmosis, Shale, and Drilling Fluids

Abstract

Summary This paper describes continuing efforts to develop a water-based drilling fluid that will provide the osmotic membrane behavior and wellbore stability of an oil-based drilling fluid. A pore-pressure-transmission technique in use for several years as a tool to measure osmotic behavior has been refined for improved measurement of changes in shale permeability and pore pressure in response to interaction with drilling fluids. Conventional invert-emulsion and water-based drilling fluids containing selected additives were tested with outcrop and preserved shale specimens using an innovative screening method. Observed pressure differences across each shale specimen were compared with the values predicted by osmotic theory. From this comparison, an empirical concept of "membrane efficiency" was developed. Three distinct types of "membranes" are postulated to describe the interaction of various drilling fluids with shales. Type 1 membranes are generally characterized by coupled flows of water and solutes between fluid and shale. Type 2 membranes greatly reduce the near-wellbore permeability of shale and restrict the flow of both water and solutes. Type 3 membranes transport water more selectively, but shale permeability and fluid chemistry may alter performance measurements. Invert-emulsion fluids tend to form efficient, Type 3 membranes; however, under certain conditions, these fluids can yield lower capillary pressures than described previously and invade the interstitial fabric of high permeability shales. Several water-based mud formulations were prepared that achieve approximately one-quarter to one-half the measured osmotic pressure of a typical oil-based mud (OBM). Fluid additives that supplement or reinforce a Type 1 membrane, such as saccharide polymers (especially in combination with calcium, magnesium, or aluminum salts), can induce relatively high efficiencies. As expected, fluids that form a Type 2 membrane, such as silicate and aluminate muds, provide the highest membrane efficiencies. Basic Osmosis Concepts Leakiness governs the effectiveness of osmosis and determines efficiency for a semipermeable membrane, which restricts the passage of solutes while the solvent is relatively unrestrained. Leakiness may more accurately describe a phenomenon for which the term "selectivity" has been applied previously. Membrane efficiency in restraining the passage of solutes is quantified by the reflection coefficient, sigma. The "reflection" analogy comes from an optical model adopted by researchers. The model assumes a semipermeable membrane analogous to a mirror - fully or semisilvered. All solutes of a solution to which a membrane is exposed will be fully or partially "reflected" by the membrane. An ideal semipermeable membrane (i.e., one that allows passage of the solvent only) has a reflection coefficient, s, of 100% or 1. Nonideal membranes, which allow partial passage of solute, have reflection coefficients of less than 1 and are, therefore, referred to as "leaky." Clay-based materials have intrinsic membrane behavior with reflection coefficients between 0 and 1, depending on the fluid contacting the clay surface. A high-permeability sand, on the other hand, does not exhibit semipermeable properties, and its reflection coefficient is essentially zero. For a system at thermal and electrical equilibrium, osmosis across a semipermeable membrane consists of solvent transport (usually water) from higher to lower water activity [i.e., from the side containing a lower concentration of solute (dilute) to the side with higher concentration of solute (concentrated), such as a salt, sugar, or glycol]. This flow of pure solvent is commonly referred to as "chemico-osmosis" or "chemical osmosis." Solvent flow will continue unless or until osmotic pressure is balanced by hydraulic pressure. For an ideal semipermeable membrane, that is the extent of osmosis. For a leaky membrane, however, solute species will also flow and can flow in both directions1–3; furthermore, hydrated species will carry solvent with them, leading to countercurrent flow of water and solutes.4,5 For a shale in contact with a typical salt-based aqueous drilling fluid, water will flow from the shale into the drilling fluid, but opposing, hydrated cations and anions will flow from the drilling fluid into the shale. Additionally, hydrated salt species in the pore network of the shale will tend to flow into the drilling fluid. Further complicating the picture is the resulting exchange of ions on the clay. These "coupled flows" that characterize osmosis complicate predicting membrane efficiency.5 Soil scientists and drilling-fluids researchers commonly observe osmotic pressure development as a developing hydraulic head in an atmospherically pressured environment. Measured osmotic pressure curves typically develop, as presented in Fig. 1. The slope of the pressure development curve of an ideal membrane approaches zero, and the slope of a nonideal membrane becomes negative after a period of equilibration. The clays composing shales are natural membranes made up of combinations of two basic structural units. The silica tetrahedron and alumina octahedron are assembled in sheets. Clay minerals are characterized by differences in the stacking of these sheets and the manner by which the sheets are held together. Differences in the crystal structure of the sheets (isomorphic substitutions) are commonly seen as the replacement of Al3+ for Si4+ in the tetrahedral sheet and of Mg2+ for Al3+ in the octahedral sheet. These substitutions cause clay surfaces to have a net negative surface charge. Electrical neutrality is preserved by attraction of cations, which are held between the layers and at the surface of the platelets. This electrostatic attraction results in a charged clay surface and a concentration of counter-ions that diminishes with distance from the surface. The charged clay surface with the counter-ions in the pore water form the diffuse double layer. The double layer is affected by changes in salinity, pH, temperature, and valence of counter-ions.1 The ability of clays to act as membranes is a consequence of overlapping double layers of adjacent clay platelets. Compaction, as occurs during the formation of shales, results in a higher concentration of cations and a reduced concentration of anions in the double layer with respect to an equilibrium solution. The aqueous environment of narrow pores can be overwhelmed by the merged, opposing double layers. Diffusion of anions through the narrow aqueous film is inhibited because the anions are repelled by the net negative charge of the platelets. Advection (the flow of solutes and heat that accompany the bulk motion of a fluid) is restrained, and the effect is known as the "Donnan Exclusion."4

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