Study on Density and Chloride Ion Concentration of Undersaturated Brine Drilling Fluid in Ultra-Deep Gypsum-Salt Rock Formation with Weak Interlayers
Authors: Chengxin Y. , Ji L. , Feng Z. , Xuliang Z. , Huiliang L. , Yunhu L. 
Published: September 2023

ABSTRACT
The ultra-deep gypsum-salt rock formation with weak interlayers often has two complicated conditions: blockage and lost circulation, and the window of safe drilling fluid density is narrow. Only by increasing the density of drilling fluid cannot solve the problem of well diameter reduction caused by gypsum-salt rock creep. It is significant to study how to solve the blockage problem by using reasonable chloride ion concentration of undersaturated brine drilling fluid to dissolve gypsum-salt rock sidewall. This study carried out creep and dissolution experiments on the gypsum-salt rock. According to the creep and dissolution law, the borehole diameter variation model of gypsum-salt rock considering dissolution and creep was established. This model is used to analyze the borehole stability of the Middle Cambrian strata in a well in Tarim Basin, and the predicted results agree the actual situation. This study can guide safe drilling in ultra-deep gypsum-salt rock formations with weak interlayers.

INTRODUCTION
The Middle Cambrian strata (more than 6000 m) in Tarim Basin developed large sections of giant thick gypsum-salt rock formation with weak interlayers. In the drilling of ultra-deep gypsum-salt rock formations with weak interlayers, the low density of drilling fluid in the borehole will lead to the risk of blockage, and the high density of drilling fluid will increase the risk of lost circulation, which brings challenges to drilling operations[1].
The creep of gypsum-salt rock has been studied in depth. Scholars analyzed the influence of temperature and stress on the creep of gypsum-salt rock through creep experiments [2-4] and established various constitutive models to describe the creep of gypsum-salt rock [5-7]. However, for gypsum-salt rock formations with weak interlayers, lost circulation may occur when drilling with high-density drilling fluid. Severe lost circulation will lead to a sudden drop of bottom hole pressure, and the gypsum-salt rock will creep rapidly, causing the borehole diameter to shrink, bringing the risk of sticking. The problem of gypsum-salt rock sidewall creep and shrink diameter can be solved by adjusting the concentration of chloride ion to control the dissolution rate of gypsum-salt rock when drilling with undersaturated brine drilling fluid so that the gypsum-salt rock sidewall can be supported effectively with low drilling fluid density [8-10]. In this study, the gypsum-salt rock creep experiments were carried out, and the steady-state creep rate of the gypsum-salt rock was calculated by using Heard creep model. The gypsum-salt rock dissolution experiments were carried out, and the linear dissolution rate of gypsum-salt rock was obtained by using the quadratic polynomial fitting method. The relationship between the density and chloride ion concentration of undersaturated brine drilling fluid was deduced according to the steady-state creep rate and linear dissolution rate.