Combatting In-Situ Overbalance Stress Reduction by Alteration of Osmotic Concentration: A Successful Approach to Wellbore Stability

Abstract The study presented aims to address the impact of osmotic pressure on mud weight window optimization through alteration of Osmotic concentration. The method uses the science behind enhanced reverse osmosis to understand the difference between in-situ overbalance and apparently maintained overbalance stresses around the wellbore exerted against formation which causes compressive failure of rock matrix/wellbore instability especially in the drilling wells having elevated bottom hole temperature in conventional OBD (Overbalance Drilling) technology. Under the conditions of drilling fluid having higher salt concentration than formation fluid, osmotic pressure is generated across the wellbore mud cake and formation interfaces, which acts against the hydrostatic overbalance stress being acted on formation as the osmotic pressure is dependent on the molar concentration of solute in mud as well as wellbore temperature. Under such conditions, the osmotic pressure counters sufficiently to lower the effective in-situ overbalance stress against the measured pore pressure causing subsequent downhole instability. The phenomenon of osmosis takes more promising shape as the bottomhole temperature increases. The presented study carefully considers all the dependent factors and calculates the osmotic pressure as a colligative property under varied drilling fluid osmotic concentration at a particular borehole temperature to design a precisely optimized mud weight window for a safe and stable well.