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.