Wellbore Stability of Mud-Shale in Ultra-Deep Carboniferous Formation: A Perspective from Mechanochemistry

ABSTRACT The wellbore instability is a vital problem when drilling in the ultra-deep carboniferous shale formation in the southwest block of Tarim Oilfield. And, bits are frequently blocked and stuck. Unfortunately, the problems are hardly to be effectively solved by increasing the drilling fluid density. At present, the mechanism of wellbore collapse in the block is still unclear. In this way, it is unable to establish an effective prediction model for wellbore collapse. Therefore, this paper systematically analyses the formation mineral characteristics, physical and chemical properties and stress state in the block to clarify the reasons for wellbore instability of mud shale formation. Consequently, the wellbore stability model of mud shale is established considering the degree of formation collapse, drilling fluid density, activity and plugging. Then, through quantitative analysis and definition of the relationship between the wellbore collapse degree and the density and performance of drilling fluid, the technical countermeasures for wellbore stability are proposed. The analysis model proposed in this paper provides a new way for quantitative design of drilling fluid density and performance in ultra-deep carboniferous mud shale formations. INTRODUCTION Based on statistics, the confirmed recoverable reserves of deep oil and gas reach up to 7.29×1010 t oil equivalent, constituting 49.07% of the world's total recoverable reserves. As much as 70% of China's unconventional oil and gas resources are stored in deep formations, leading to a growing number of ultra-deep wells drilled(Chen et al., 2009; Y. Lu et al., 2011; Zhang et al., 2022). The Tarim Basin in China is a significant region for its vast oil and gas reserves. The Tangu Depression, particularly the Madong buried hill belt in the southwest block of the Tarim Basin, is recognised for its abundant oil and gas resources. However, the Madong structural belt underwent multiple tectonic movements, resulting in extremely complex structural conditions in the basin. The reservoirs in this area are buried deep, reaching depths of more than 6000m on average and are subjected to high temperatures and ultra-high pressure. As a result, this area faces complex wellbore stability issues during exploration and development. In the initial stages of development, due to an unclear understanding of the lithological characteristics and structural stress in the area, complex problems such as well wall collapse, drag, and pipe sticking were common during the drilling process(Zhang & Chen, 2022).