Selection of OBM salinity through effective osmotic pressure evaluation in carbonera shale for colombian foothills

Wellbore instability in shales is attributed to many factors. Two of them are mechanical effects and physico-chemical effects. Drilling and drilling fluid cause physico-chemical interaction and the flux of water and ions that may alter the shale stress state through pore pressure and shale strength. This paper presents the analysis of the chemical osmosis phenomenon between drilling fluids and shale formations to evaluate the chemical parameters necessary for modeling the aqueous flux. These parameters are the drilling fluid activity (Adf), shale activity (Ash) and membrane efficiency (ME). This work also characterizes the shales for drilling purposes and describes an integrated methodology to obtain the magnitude of the chemical parameters. Furthermore, it is stated how the generation of effective osmotic pressure between the formation and drilling fluid define the water flux direction. Finally, the application of the results of the chemical analysis to Carbonera shale is presented. The design of laboratory tests for two mud formulations, Mud A and Mud B, and the field application is also showed. The Mud A is a balanced activity mud and the Mud B is a high salt concentration mud which may produce water flux out of the shale formation (dehydration) during drilling, in some sections of the wellbore, increasing the formation strength. The results presented in this paper will help to reduce the risks associated with wellbore instability during the drilling of shale formations and thereby lowering the overall non-productive time and reducing drilling costs.