Preliminary Test Results of Inhibitive Water-Based Muds Used to Mitigate Unconventional Shale Drilling Problems
Authors: Al-Arfaj Mohammed Khalid , Hossain Enamul , Sultan Abdullah , Amanullah Md , Al-Fuwaires Omar , Al-Subai Turki 
Published: March 2015

Abstract
Unconventional resources include oil and gas present in shale, tight sandstone and tight limestone formations. Shale oil can be produced from deposits of shale with estimates of about 5 trillion barrels of oil in place around the world. Because of the high content of clay in the shale formations, water-based drilling fluids tend to cause wellbore instability problems when drilling this type of formations. When it comes in contact with water, clay starts to react, swell and/or disperse leading to shale disintegration and sloughing. As a result of shale sloughing down into the borehole, cleaning efficiency of drilling fluids decreases significantly. Moreover, tight hole problem is expected which may cause drillpipe to get stuck and, as a result, increases non-productive time and well construction cost.
Several types of shale inhibitive drilling fluids were developed using different shale inhibitors and stabilizers. Developing an inhibitive drilling fluid with long-term inhibition can eliminate the need for unnecessary casing and reduce tripping time. Multiple formations including the shale formation can be drilled and cased in one hole section.
This paper summarizes preliminary laboratory testing results for characterizing one shale sample and assessing the interactions with different water-based mud systems. Shale characterization included determination of mineralogical composition using X-ray diffraction and determination of cation exchange capacity (CEC) while shale-mud interactions evaluation included swelling, dispersion and inhibition durability tests.

Introduction
Unconventional resources refer to any petroleum resource that can not be recovered and produced using conventional methods. Examples include: shale gas, tight gas and shale oil. These resources exist in shale and tight sandstone formations with very low permeability. Therefore, hydraulic fracturing is usually necessary to produce at economic rates.
Shales represent a large portion of the unconventional resources in Saudi Arabia. While conventional shale formations are the source rock for oil and gas, unconventional shale formations are the source rock and the reservoir at the same time. The well is drilled and completed into the shale zone. Since conventional water-based mud systems (WBMs) cause shale instability, there is a need to develop inhibitive water-based muds that provide the required long-term shale inhibition especially when drilling development wells horizontally through shale formations to exploit unconventional resources. Although drilling with oil-based drilling fluids can provide better shale inhibition, higher penetration rates and good lubricity, the environmental regulations and the high cost of such fluids make it difficult to justify the decision to drill long horizontal shale sections with these fluids.
Shale formations are characterized based on clay content and composition. Clay minerals are grouped mainly into five groups: kaolinite, smectite, illite, chlorite and vermiculite. Previous experimental work showed that smectite group has the highest reactivity and sensitivity to water compared to the other groups from swelling point of view. Because the degree of reactivity depends on the shale composition and, also, shale composition differs from one area to another, it is necessary to do shale characterization before an inhibitive drilling fluid is recommended for a certain shale formation. Shale characterization includes determining the clay content and reactivity as well as determining the water activity of the shale sample. Chenevert (1970) conducted a laboratory work utilizing the drilled cuttings to measure the water activity of different shales based on the adsorption characteristics. Isotherms for adsorption and desorption were established after drying and placing shale drilled cuttings in a desiccator and allowing them to reach equilibrium. Based on the isotherms and the weight percentage of water in the shale, a scale from 0.1 to 1.0 was suggested for the water activity.