Analysis of wellbore diameter variation in salt-gypsum formation with the effect of drilling fluid

Abstract

Salt-gypsum rock in Eastern Sichuan Basin is widely developed; however, the creep and dissolution of salt-gypsum rock easily lead to reduced diameter or enlargement of wellbore during drilling process. This phenomenon seriously hinders the safety and efficiency of oil and gas development. In this paper, based on the dissolution test of salt-gypsum rock under the water-based drilling fluid, the salt-gypsum rock dissolution law with the effects of chloride ion concentration and temperature is studied explicitly. By incorporating the creep and dissolution effects of the salt-gypsum rock under drilling fluid, the plate of diameter reduction curve for the salt-gypsum formation is established, and the effects of the Chloride ion concentration and temperature on the reduction rate of wellbore diameter are analyzed. This research demonstrates that, under the dual actions of salt-gypsum rock dissolution and creep, as the chloride ion concentration becomes smaller and the formation temperature is lower, the diameter reduction of salt-gypsum formation turns out to be smaller. In the meanwhile, the safe drilling fluid density can be effectively reduced by optimizing the chloride ion concentration to achieve the low-density drilling, especially for the formation with lower temperature. Furthermore, a novel approach for the design of drilling fluid density in salt-gypsum formation is put forward, which provides a necessary and reliable reference for drilling operation.