Membrane Efficiency of Shales Interacting With Water-Based and Oil-Based Muds
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100735, "Factors Controlling the Membrane Efficiency of Shales When Interacting With Water- Based and Oil-Based Muds," by Talal M. Al-Bazali, Kuwait U.; Jianguo Zhang, SPE, Baker Atlas; and Martin E. Chenevert, SPE, and Mukul M. Sharma, SPE, U. of Texas at Austin, prepared for the 2006 SPE International Oil & Gas Conference and Exhibition in China, Beijing, 5–7 December.
The full-length paper presents a comprehensive set of experimental data for the membrane efficiency of four shales when interacting with different water-based muds (WBMs) and oil-based muds (OBMs). Pressure-transmission tests were used to measure the membrane efficiency using three different cations and two different anions at different concentrations (water activities). Results suggest that the membrane efficiency of shales is directly proportional to the ratio of the cation-exchange capacity to the permeability of shales.

Introduction
Osmosis has long been recognized as a means to extract water from a shale when the water activity of the shale is higher than that of the drilling fluid. In the absence of a hydraulic-pressure gradient, the movement of mud filtrate into shale is governed mainly by the chemical-potential difference between the pore fluid and the mud, and this results in the osmotic transport of water. However, recently it was shown that the osmotic potential generated between shale and drilling fluid is influenced greatly by the flow of ions into or out of the shale because of ionic-concentration imbalances. Therefore, the actual osmotic effect often is less than the osmotic potential. This has spurred much interest to quantify the effect of ionic flow on the osmotic potential, and that in turn has led to introducing the concept of shale membrane efficiency. Membrane efficiency describes the ability of a shale to hinder ion movement when interacting with drilling fluids. If the shale completely stops ionic flow, the shale is said to act as a perfect semipermeable membrane, with a membrane efficiency of unity. If the shale lets ions flow freely, the shale is said to act as a nonselective membrane, with a zero membrane efficiency. In this work, the membrane efficiency of shale, when interacting with WBMs and OBMs, has been estimated by use of pressure-transmission tests. The dependence of the membrane efficiency of shale onion type and concentration in the drilling fluid was explored using different cations and anions over a range of concentrations. The influence of shale permeability and cation-exchange capacity on the membrane efficiency also were investigated using four different shales.

Test Description
The membrane-efficiency test made use of a pressure-transmission technique where shale samples were subjected to both hydraulic and osmotic gradients during exposure to drilling fluids. The pressure drop (resultant osmotic pressure) across the shale sample was measured in response to both hydraulic- and osmotic-pressure gradients. The pressure drop then was converted to a membrane efficiency that was defined as the ratio of the pressure drop across the shale sample to the osmotic potential resulting from the differences in water activities.