Analysis of Sticking and the Releasing Technology of the Composite Gypsum-Salt Rock in the Tarim Basin
Authors: Hongtao Liu , Chongyuan Zhang , Bao Zhou , Chengxin Yang , Qiang Deng , Yanan Xu , Guozhi Song 
Published: September 2022

ABSTRACT:
The Kuqa FoId-Thrust BeIt in Tarim Basin is verified as the most challenging geological structure for ultra-deep hydrocarbon development in China onshore. The complicated thick gypsum salt layer is commonly found in Kuqa FoId-Thrust BeIt, with the thickness of up to 3000m. It is easy to get stuck during drilling in such formations, and how to deal with sticking is a difficult problem in gypsum salt rock drilling. Taking the sticking treatment process of gypsum salt rock in a well in Kuqa FoId-Thrust BeIt as an example, this paper analyzes the causes of sticking, and discusses the influence of sticking agent and construction process on the sticking treatment. The field operation shows that the drilling fluid density is an important operation parameter in the drilling operation of composite gypsum salt formation. The drilling fluid with low density has insufficient support to the wellbore, which will cause wellbore instability and block falling; On the other hand, the creep speed of salt layer is fast, resulting in serious hole necking. Necking caused by creep and accumulation of falling blocks are extremely easy to cause sticking accidents. The high-density and high-viscosity pre isolation fluid can effectively separate the oil-based drilling fluid from the unblocking agent. The unblocking agent without slurry mixing with the oil -based drilling fluid can dissolve the salt rock at the sticking point, which is the key to successful unblocking. The combination of releasing agent and isolation fluid can successfully solve the sticking problem of gypsum salt layer in the drilling process of oil -based drilling fluid, accumulate valuable experience and provide new ideas for the sticking treatment of gypsum salt layer of oil-based drilling fluid.

1. INTRODUCTION
The gypsum-salt layers in the Tarim Basin are widely distributed, and most of them are compound salt and gypsum formation, which is not beneficial to drilling. Under the combined action of buoyancy, gravity, differential sediment load and compressive stress, the salt body is prone to obvious plastic flow(Jia & Wei, 2002; Teng et al., 2015; Zhou et al., 2021). The composite gypsum-salt rock layer is mixed with mudstone, gypsum mudstone in the salt and gypsum layer. The mudstone has strong hydration properties, which leads to the collapse of the wellbore, shrinks the diameter and drags the adjacent anhydrite and salt rock to collapse, which causes the instability of the wellbore(Guimei, Liangjie, Wenzheng, & Yixin, 2007). The main reason is that the interface between the gypsum layer, the salt layer and the mudstone has the greatest risk of instability, which is likely to cause stuck pipe accidents, which is a thorny problem that hinders the drilling of the composite gypsum-salt layer(Da et al., 2016; Liu et al., 2021). Oil-based drilling fluids have the advantages of high temperature resistance, salt resistance, lubricity, good for wellbore stability and less damage to oil and gas formations. It has been widely used as an important means of drilling composite gypsum salt rock formation in Tarim Oilfield and achieved obvious results (Zhu et al., 2015) However, there are still complex accidents such as borehole instability and sticking during the drilling of composite gypsum-salt rock formations. Scholars have conducted comprehensive research on the safety drilling technology and casing damage in gypsum-salt formations(Jian et al., 2017; Long et al., 2018; Zhao et al., 2019), but there are few studies on the drilling sticking treatment technology in gypsum-salt formations. The treatment of the stuck pipe accident is mainly based on the formation characteristics and the type of stuck pipe, and the treatment methods such as oil soaking, releasing agent, shock, undercut, sleeve milling, pressure reduction and card release are adopted (Rashidi et al., 2020; Zhu et al., 2015). Among them, acid soaking in calcium carbonate or calcium bicarbonate formation is a very effective method for unblocking (Jardine, McCann, & Barber, 1992; Wisnie & Zhu, 1994). The idea of dissolving the rock at the location of sticking to achieve the effect of sticking is worthy of reference for compound gypsum-salt rock unblocking technology.