Preliminary study on response characteristics and feedback mechanism of core discing under deep in situ stress environment

Abstract

In-situ stress data are fundamental for deep engineering purposes. Traditional methods of measuring in-situ stress are inefficient and lack standardization. The theoretical framework is predominantly built on numerous assumptions, making it challenging to assess its accuracy. High in-situ stress at significant depths often leads to the formation of various core discing shapes. A more scientific, cost-effective, convenient, and rapid method for calculating in-situ stress involves the inversion of stress through core discing characteristics. To achieve this, understanding the behavior of core discing under deep in-situ stress conditions is essential, as well as defining the intensity coefficient of core discing in two-dimensional settings. By focusing on discing cores from varying depths in the Songke-2 well, their macro-morphology is captured using a three-dimensional data acquisition system. Additionally, the mineral composition and microstructure of the cores are analyzed through the X-ray diffraction and lithofacies thin section tests. Concurrently, the mechanical mechanism governing core discing behavior under different stress conditions are studied using the discrete element method. The analysis reveals five distinct morphological types of discing cores in the Songke-2 well: broken, thin-disc, thick-disc, irregular, and semi-disc. The fracture patterns on the end face of the discs includes staggered step, plane, lamp, and petal formations. These morphological attributes are found to be closely linked to burial depth, mineral hardness, and failure mode. Core discing behavior is primarily controlled by tensile stress, with crack initiation typically occurring at the core's root and propagating outward in a concave or planar manner. The horizontal principal stress emerges as the primary factor influencing core discing behavior, leading to concave core fractures and wide-band tensile cracks, while the vertical principal stress serves as a secondary factor, generally inhibiting discing behavior. During drilling operations, the onset of discing coincides with a systematic energy shift, marked by a gradual decrease in total strain energy and a corresponding increase in dissipation energy. The instantaneous release of total strain energy is identified as the primary cause of core discing. These research findings are expected to offer some novel insights and valuable references for the inversion of in-situ stress based on core discing analysis.

Keywords

• core discing
• songke-2 well
• particle discrete element
• macro-meso characteristics
• failure mode