A Rapid, Rig ite Deployable Electrochemical Test for Evaluating the Membrane Potential of Shales

摘要

The membrane efficiency of shales is usually measured in the laboratory using pressure transmission techniques that can be very time consuming and require shale core samples that may not be available. These tests also require special high pressure equipment and cannot be conducted at the rig-site. This paper presents a quick and relatively easy method for obtaining the membrane efficiency of shale cuttings (or cores) through electrochemical potential measurements. The electrochemical test measures the voltage drop across shales that are in contact with fluids of different salinities. The measured voltage drop is used to calculate the shale's ion selectivity, which reflects the shale's ability to restrict ion flow (membrane potential). Data are presented to show the influence of ion type and concentration, shale permeability and CEC on the ion selectivity. It was found that the shale membrane efficiency is well correlated with the membrane potential. The membrane potential is shown to be proportional to the ratio of the cation exchange capacity and permeability of shales. A higher cation exchange capacity to permeability (CEC/k) ratio correlates very well with higher ion selectivity. The rig site measured ion selectivity can be correlated with the spontaneous potential (SP) curves in the well. The rapid determination of shale membrane efficiency using shale drill cuttings allows chemical interactions between shales and water-based fluids to be taken into account in wellbore stability calculations without the need for shale core samples.