Study on the Wellbore Shrinkage During Drilling Operation in Frozen Soil

ABSTRACT: During drilling operations in cold regions, the frozen soil obtains strong creep characteristics, which will result in wellbore shrinkage. An unreasonable drilling fluid density may cause engineering accidents, such as blocking and stuck drills. To solve this problem, a drilling fluid density calculation model based on the power law model of frozen soil creep was established in this paper for controlling the wellbore shrinkage in frozen soil. The factors affecting the wellbore shrinkage rate in frozen soil were analyzed. A map of the wellbore shrinkage and drilling fluid density was also established. The results show that the wellbore shrinkage rate is affected not only by the physical properties of the frozen soil but also factors such as the temperature, formation depth, drilling fluid density, wellbore size, and well opening time. The density of the drilling fluid required for safe drilling in frozen soil increases as the formation depth increases, and the magnitude gradually decreases. As the temperature decreases, the drilling fluid density at the same depth gradually decreases, and the magnitude also gradually decreases. The results provide an important basis for determining the drilling fluid density and the selection of drilling methods in cold regions. 1. INTRODUCTION As the oil and gas energy situation becomes increasingly severe, unconventional oil and gas is gradually becoming the focus in the petroleum industry. In 2008, the US Bureau of Geodetic Survey conducted exploration in the Arctic and confirmed that the Arctic region is rich in oil and gas resources (Bird et al., 2008). The petroleum industry began to pay attention to the polar regions (Li et al., 2019; Vernikovsky et al., 2013; Sekretov, 2001). Compared with the conventional stratum, there are ice particles in the pores of the frozen soil. The change of temperature and pressure will cause the ice to melt, seepage and secondary solidification, which greatly changes the structure and mechanical properties of the frozen soil and makes the frozen soil own strong creep property. The unique mechanical properties of frozen soil, especially its creep characteristic, would bring a significant impact on drilling operations (Yu et al., 2013; Yang et al., 2010), such as wellbore shrinkage, stuck drilling, and casing could be crushed after cementing. These seriously hinder the exploration and development of oil and gas resources. Therefore, research on the wellbore shrinkage of frozen soil is of great significance for the development of petroleum in cold regions.