An Experimental Investigation on the Stability and Reservoir Hydrocarbon Capacity of Shales
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 121451, ’An Experimental Investigation on the Impact of Capillary Pressure, Diffusion Osmosis, and Chemical Osmosis on the Stability and Reservoir Hydrocarbon Capacity of Shales,’ by Talal M. AL-Bazali, U. of Kuwait; Jianguo Zhang, SPE, BP plc; and Martin E. Chenevert, SPE, and Mukul M. Sharma, SPE, U. of Texas at Austin, originally prepared for the 2009 SPE Offshore Europe Oil and Gas Conference and Exhibition, Aberdeen, 8-11 September. The paper has not been peer reviewed.

Wellbore instability in shales is the most challenging and costly issue in drilling operations. Wellbore instability in shales can be attributed to many factors, some of which have been well studied and documented. However, the physicochemical- and mechanical-properties alterations in shales that eventually lead to wellbore failure largely have been ignored. Water and ions movement in and out of shales plays a major role in the alteration of the physicochemical and mechanical properties of shales, thus leading to wellbore-instability problems and possible hole collapse.

Introduction

The unfavorable interaction between shale and drilling fluids is considered to be the primary cause of many wellbore-instability problems. Such interactions are very complicated and include mechanical, chemical, physical, hydraulic, thermal, and electrical phenomena. The overall effect of these interactions mainly is related to the movement of water and ions into or out of shales. The physicochemical and mechanical properties of shale around the wellbore, such as permeability, strength, pore pressure, and elastic modulus, can be altered greatly by such movement.

It is well-accepted that the adsorption of water results in shale-strength and elastic-modulus decrease, in swelling, and in pore pressure increase. These changes around the wellbore may cause wellbore instability problems during drilling and completion operations. The magnitude of water movement and the effect of absorbed water on shale properties are influenced by the presence of ions in the solution. Ionic diffusion also may result in the movement of ions into the shale formation, resulting in chemical alteration.

Water and ions can move in and out of shales by many complicated mechanisms. These mechanisms include, but are not limited to, chemical osmosis, diffusion osmosis, capillary suction, and convective flow. In the full-length paper, the focus is on the diffusive flow of water and ions. Specifically, the effect of both chemical osmosis and diffusion osmosis on shale alteration was investigated. The experimental setup and procedure were designed to minimize the effect of convective and capillary flow.

Shales play a major role in petroleum exploration and production because they commonly are considered to be both source rocks and seals. Their ability to exhibit good sealing characteristics arises from their small, water-wet pores. The small pore throats in shales are responsible for generating high capillary pressures, which exclude hydrocarbons.