Study on the Effect of Water Activity in Shale on Wellbore Instability

摘要

It is accepted that the water flux in/out of the shale during drilling is the key factor, which controls well-bore instability. This flow can be divided into two components: (1) the hydraulic flow due to the difference between the wellbore and pore hydraulic pressure; (2) the osmotic flow due to the imbalance between activities of the shale and the drilling fluid. The former can be prevented by adjusting the wellbore hydraulic pressure balance in the well hole, while the latter is much more difficult to control . The water activity of shale is a controlling factor in many areas of drilling. It impacts all situations wherein the temperature or the stress state of a shale is altered such as in wellbore stability, drilling rate and hydraulic fracturing. This chemical "potential activity interaction" produces a mechanical failure due to the movement of water in/out of shales. In order to have no shale alteration, it requires that the chemical potential of each component be the same in all phases. This case is seldom. After a lot of studies, the shale activity is shown to be a function of pressure and temperature. Results showed inverse relationship between the platelet distance and the shale water activity. This experimental method proves to be a reliable and efficient way for studying the relationships for the shale water activity, comfining pressure, temperature, and platelet distance.