Testing and Evaluation of Shale Stability for Zubair Shale Formation

Abstract

Zubair Formation consists of approximately 55% shale, which causes almost 70% of wellbore problems due to incompatibilities of drilling fluids and shale formations. The most common and effective solution to shale instability is through the design and selection of drilling fluids. Understanding the interaction between drilling fluids and shale has been a challenge due to the complexity of both physical and chemical variations in shale formations. This paper presents some of the important laboratory and wellsite testing techniques that are often used by mud engineers for characterizing and remediating drilling fluids and shale interaction. Well-preserved shale samples were analyzed to describe the special characterization of the Zubair shale. Moreover, the traditional laboratory methods such as capillary suction time test, hot rolling dispersion test, bulk hardness test, and the linear swelling test were used to evaluate the stability of shale in the presence of test fluids. Our laboratory test results show that the Zubair shale is composed mainly of brittle mineral (quartz and calcite) with average content 51.46% and 43.54% of the clay mineral. In addition, the cation exchange capacity analysis and capillary suction time test indicated that Zubair shale has low to moderate reactivity with drilling fluids. This paper will present the preliminary process of analysis and understanding of shale structural failures due to shale/fluid interactions. Consequently, it can be used to control and minimize shale instability by improving the selection of chemical additives for clay inhibition.