Improving wellbore stability of shale by adjusting its wettability

Abstract

The possibility of improving wellbore stability of shale by adjusting its wettability was investigated. First, an optimized surfactants combination (0.2 wt% cetyltrimethylammonium bromide (CTAB) plus 0.1 wt% sodium dodecyl benzene sulfonate (SDBS)) was screened out that could effectively reduce the surface tension of the water-based mud (WBM) or fresh water, and increase the contact angle with shale. Then, regular tests on viscosity, filtration, lubrication, water activity, and special tests on linear swelling, rolling recovery rate, temperature tolerance, salts, and active cuttings tolerance, along with pressure transmission test (PTT) of shale were conducted to evaluate the performance of the optimized surfactants combination. Results show that the WBM's surface tension could be decreased from 45.1 to 20.0 mN/m and the contact angle on the shale could be increased from 34.5 to 63.5°. The optimized surfactants combination could also improve the WBM's performance to inhibit shale hydration. The WBM in the presence of the optimized surfactants combination had good tolerance to a pollutants complex (0.8 wt% CaCl2, 0.8 wt% MgCl2, 5 wt% sodium bentonite, and 10 wt% NaCl) and could withstand a high temperature of 130 °C. Foaming phenomenon could be avoided by adding surfactants in the last step. PTT results show that compared with 4 wt% NaCl, the optimized surfactants combination could effectively retard the pore pressure transmission in shale and reduce its permeability by 98.6%. The addition of CTAB to the WBM or fresh water, hydrolyzed CTA+ cation adsorbs on the surface of shale via an ionic bond, forming a hydrophilic base toward shale and a lipophilic base toward the WBM or water, thus, increasing the contact angle with shale and improve wellbore stability in shale. This paper paves a new way to address wellbore stability issues in troublesome shale formations.