Evaluation Techniques of Wellbore Stability on Complex Formation based on Wettability

Abstract

Wettability is not only controlling factors of 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 has a more important influence on wellbore stability. In most previous literatures, unchanged wettability considered as a default premise, impact of wettability and its changes on the rock, thereby on wellbore stability was ignored. In this paper, an oilfield in western China taken for example, from the viewpoint of wettability, wellbore instability mechanism under the condition of oil-based drilling fluid was studied. During the process, the core samples were investigated using X-ray diffraction (XRD) tests, water activity (Aw) tests, swelling ratio (sw) tests, dynamic wetting test, computerized tomography (CT) tests, Field Emission Scanning Electron Microscope (FE-SEM) tests and uniaxial compressive strength (UCS) tests. The results indicate that wettability has a great influence on micrometer or even nanometer cracks, the stress accumulated by which is one of micro-cracks extension and growth, whereas traditional theory had ignored usually. According to the structure and composition of the cores and wettability changes resulted from the cores contacted with drilling fluid filtrates with different performance parameters, such as water activity et al, a comprehensive characterization model on the properties of rock and drilling fluid based on wettability was proposed to maintain the original strength of rock. The model above considered wettability was used as an effective solution to the problem that wellbore instability occurred frequently even though density and conventional performances of oil based drilling fluid are within a reasonable range of projects. The model will also provide a theoretical basis and technical support based on the wettability for designing performance properties of oil based drilling fluid.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015