Strength varying laws of rock soaked with supercritical CO 2 drilling fluid and the prediction of safe mud weight window

Drilling with supercritical CO 2 (SC‐CO 2 ) is an important new technology in petroleum engineering. The technology has many unique advantages for increasing the penetration rate and reducing formation damage, but it is a disadvantage for the wellbore stability. Therefore, an experimental technique has been developed to determine the safe mud weight window for drilling with SC‐CO 2 . More specifically, a special experimental apparatus has been designed to mimic the in situ temperature and pressure conditions during drilling operations. Different experiments were conducted using core samples soaked in SC‐CO 2 and water‐based and oil‐based drilling fluids, respectively. Once the experimental measurements regressed, an empirical correlation, including time, temperature and pressure factors, was obtained to describe the compressive strength of the SC‐CO 2 ‐soaked core samples. Further modifications were then made to the formulations used to calculate the densities of the drilling fluids at the corresponding burst and collapse pressure, respectively. In addition, the effects of dynamic cohesion, internal friction angle, and tensile strength on the wellbore stability were determined. Compared to the conventional water‐based and oil‐based drilling fluids, the SC‐CO 2 drilling fluid performs better with respect to wellbore stability and has a wider safe mud weight window. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd.