Entrance Pressure of Oil-Based Mud Into Shale
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 116364, "Entrance Pressure of Oil-Based Mud Into Shale: Effect of Shale, Water Activity, and Mud Properties," by Andres Oleas, SPE, Collins E. Osuji, SPE, Martin E. Chenevert, SPE, and Mukul M. Sharma, SPE, University of Texas at Austin, originally prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21-24 September. The paper has not been peer reviewed.
Oil-based muds (OBMs) have been developed to combat drilling problems often caused by shale hydration. Therefore, it is of utmost importance to understand the interaction of OBMs as they contact shales. The full-length paper deals with the movement of the oil phase of the OBM, as described by its hydraulic "entrance pressure." Although the oil filtrate of the OBM does not hydrate the shale, it can penetrate and flow into the shale at a certain entrance pressure. Such flow into shale increases the pore pressure of the shale, which can cause wellbore failure.

Introduction
Shales are low-permeability sedimentary rocks with small pore radii that have medium-to-high clay content, in addition to other minerals such as quartz, feldspar, and calcite. The distinguishing features of shale are its clay content and low permeability, which results in poor connectivity through narrow pore throats. Shales also are fairly porous and normally are saturated with formation water, with several factors affecting their properties, such as burial depth, water activity, and the amount and type of minerals present.
Considering the fact that shales account for 70 to 75% of the formations drilled around the world, it is important to understand and minimize shale-related problems while drilling. Drilling performance has demonstrated the effectiveness of OBMs in combating drilling problems caused by shale hydration, differential-pressure sticking, corrosion, and high formation temperatures.
OBMs are water-in-oil emulsions that contain water, emulsifiers, organophilic clay, and a weighting material. The water phase is usually a calcium chloride (CaCl2) salt solution, with a water activity (aw) that resembles the aw of the formation. This eliminates water transfer to or from the water-sensitive zones and, thereby, maintains a stable wellbore. The water in the oil is stabilized with a primary emulsifier (often a fatty-acid salt), while the weighting material and the drilled solids are made oil-wet and are dispersed in the mud with a secondary emulsifier. It is thought that both emulsifiers have dual roles, with the primary emulsifier also acting to some extent as a wetting agent and the secondary emulsifier acting as a true emulsifier.
Ions that are added to a water-based mud (WBM) reduce the aw of the fluid, and consequently, water movement into the shale is reduced because of osmotic effects. This effect is not long lasting because the hydrated ions are not very restricted and they invade the low-salinity shale. However, for OBMs, an efficient membrane exists around each water droplet, and very little (if any) ion transfer occurs.