Study on Wellbore Stability of the Intrusions in Manshen Block, Tarim Basin

A set of intrusions is developed in the deep Ordovician in Western China. However, the complex coexistence of losses, collapse and jamming during drilling seriously restricts the exploration and development of deep oil and gas resources. Aiming to solve the problem, this article first conducted geophysical and engineering characteristics analysis on the drilled wells that encountered the intrusions in the field, then carried out the physical and mechanical experiment with the intrusions and performed numerical simulation of the intrusions, to clarify the wellbore instability mechanism and controlling countermeasures of the intrusions in the block. The results of the experiment show that the existence of microfractures significantly reduces the overall strength of the intrusions, and provides a channel for drilling fluid intrusions. The results of the numerical simulation show the existence of the weak plane effect of microcracks plays a main role in controlling the wellbore instability of the intrusions. For the intrusions with high degree of breakage, within the safe drilling fluid density range, the impact of density changes has little influence on the wellbore instability, while the plugging property has great influence on the wellbore stability. The research achievements are helpful to guide similar strata to improve wellbore instability. 1. INTRODUCTION As global energy consumption continues to increase, achieving efficient development of deep and ultra-deep resources has become an important goal to continue conventional oil and gas resources and increase oil and gas production. However, according to statistics, the phenomenon of wellbore instability widely exists in the exploration and development of deep and ultra-deep resources in major oil and gas fields around the world. It is currently the most important complex problem plaguing the drilling engineering community (Chen et al., 2008; Jin and Chen, 2012). The Manshen block studied in this article is located in the hinterland of the Taklimakan Desert and in the Aman transition zone of the northern depression of the Tarim Basin. It has abundant oil and gas reserves and huge development prospects. The strata below the Ordovician in this block are affected by diabase intrusions. During the drilling process, losses and collapse are prone to occur, resulting in frequent downhole obstructions, which seriously restricts safe and efficient drilling.