Chemical-Mechanical Modeling of Wellbore Instability in Shales
Shales make up over 75% of drilled formations and cause over 90% of wellbore instability problems. The drilling of shale can result in a variety of problems ranging from washout to complete collapse of the hole. These problems are severe and for the industry have been estimated to be a conservative $ 500 million/year problem. The majority of wellbore instability simulators fail to predict a safe mud weight range to be used in a specific wellbore due to several factors. Two of these critical factors are i) the rock is modeled as an elasto-plastic material so that the stresses induced by flow into or out of the formation, the so-called poroelastic effects, are neglected, and ii) the chemical interaction between shales and the drilling fluid, the well-known term called swelling pressure by the petroleum industry, is not taken into account. The model developed in this dissertation is based on the poroelasticity theory and introduces the chemical effects into the wellbore stability model using the swelling pressure term based on the activity difference between the drilling fluid and the shale. The major conclusion of the model is the importance of combining both the mechanical and chemical aspects of the drilling fluid/shale interaction to optimize borehole stability. Also, an experimental effort was made to understand the effect of total stress and temperature on the shale water activity, one important chemical input of the simulator. An experimental set-up, called Downhole Activity Cell (D.A.C.), was built, and four well-preserved troublesome shales cored from oil wells were tested. The results clearly show that temperature and stress field have a positive influence on this chemical variable, which controls the flow behavior between shale and drilling fluid. The effect is so dramatic that it is possible to conclude that all tests performed at ambient conditions can give misleading results, and the water activity alteration should be taken into account in future implementations of wellbore instability simulators applied to shales.