Wettability of rock and drilling fluid attracts great attention, especially in the field of micro scale on wellbore stability. The core samples from western China were investigated using X-ray diffraction tests, water activity tests, wetting test, Field Emission Scanning Electron Microscope tests and uniaxial compressive strength tests. The results indicate that wettability has a great influence on micrometer or even nanometer cracks, the stress accumulated by which affects micro fractures extension, whereas traditional theory had ignored usually. According to the mineral composition and wettability of the samples, and the changes of the microscope structure and rock mechanics parameters resulted from the samples contacting with oil-based drilling fluid, a comprehensive fracture mechanics model between rock and drilling fluid based on wettability was proposed. The model indicates that the fractures extension was a combined action result of the opening and sliding mode cracks, and that the stress intensity factors were affected by in-situ stress, net fluid pressure, wettability, and even fracture geometry. Wettability was used as an effective solution to the frequent occurrence of the wellbore instability in fractured formation, even under the density and conventional performances of oil-based drilling fluid within a reasonable range.
Wettability is defined as the tendency of one fluid to spread on or adhere to a solid surface in the presence of other immiscible fluids, which is the fluid's ability to keep contacting with the solid surface, and is caused by the molecular interaction when the fluid and the solid contact with each other [1, 2]. As the research on wellbore stability proceeds gradually in the drilling engineering field, the influence of the wettability on rock attracts great attention. In the 1980s, Anderson, W.G. made a systematic summary of relevant literatures on the wettability, including rock/oil/brine interactions and the effects of core handling on wettability , wettability measurement , the effects of wettability on the electrical properties of porous media , the effects of wettability on capillary pressure , the effects of wettability on relative permeability , and the effects of wettability on waterflooding . At the present time, the study on the wellbore stability has entered the microcosmic field , and the role of the wettability in this scale is highlighted significantly.
Wettability not only controls reservoir fluid distribution, but also greatly affects physical and chemical properties of rock, including capillary force, relative permeability, electrical properties and even strength. Especially before and after drilling formation, wettability exerts a more significant influence on wellbore stability. In most previous literatures, unchanged wettability was considered as a default premise, impact of wettability and its changes on the rock, thereby on wellbore stability was ignored.