Study on the Mechanism of Wellbore Stability and Drilling Fluid Technology Countermeasures in Ultra-Deep Composite Salt Formations in the Tarim Basin

Abstract

ABSTRACT: In the Tarim Oilfield, drilling encountered the Cambrian strata's composite salt layer, resulting in significant wellbore narrowing and collapse. The lithology of the composite salt layer primarily consists of salt and gypsum interbed, exhibiting complex and variable features. Given the recurring drilling incidents, it is imperative to investigate wellbore instability mechanisms within these composite salt layers and adjust related drilling fluid techniques. This paper analyzed the engineering properties of composite salt rock, carried out experiments on physical and chemical properties and rock mechanical strength, and elucidated the control mechanism of wellbore stability of composite salt rock; experiments were carried out on the dissolution rate and creep characteristics of salt rock, and a model for characterizing the density and chlorine content of drilling fluids for maintaining the wellbore stability was set up. Model analyses and on-site verifications were carried out. The study results show that the composite salt layer belongs to the wellbore stability mechanism controlled by the combination of dissolution and creep in water; the appropriate drilling fluid density and chloride ion concentration can prevent the dissolution and creep of the composite salt layer. By utilizing the appropriate density of undersaturated brine drilling fluid and fine-tuning the chloride ion concentration, the salt rock's dissolution rate and creep rate in the wellbore can be effectively balanced, and the wellbore stability can be improved. 1. INTRODUCTION Wellbore instability poses a significant global technical challenge in drilling engineering [1]. Salt and gypsum layers are common in both onshore and offshore deepwater oil and gas drilling [2-3]. During drilling operations, especially in the case of composite salt layers, the variable lithology and complex mechanical, physical, and chemical properties exacerbate the issue of wellbore instability, leading to incidents such as narrowing, enlargement, collapse, and sticking [4-5]. Consequently, both domestic and international scholars have conducted extensive research on collapse prevention drilling fluids for salt and gypsum layers, leading to the development of relatively mature saturated and undersaturated brine drilling fluid systems [6]. These advancements have contributed to alleviating the complexities encountered in downhole operations. However, due to limitations in research approaches and the focus on specific regions, as well as the inherent complexity and variability in lithology and structure of salt and gypsum layers across different areas [7], the issue of wellbore instability in composite salt layers remains unresolved.