DESIGN AND EVALUATION OF WELLBORE STRENGTHENING MATERIALS FOR FRACTURED RESERVOIRS

Fractured formations are commonly encountered while drilling carbonate reservoirs. Drilling of fractured formations usually results in lost circulation. In some cases, target zones cannot be reached due severe to lost circulation incidents. Fractures encountered before reaching target zones can be sealed off by using wellbore strengthening materials (WSM). WSM are drilling fluid additives which are specially sized and designed particulates. They are generally used to strengthen the wellbore and to prevent the occurrence of losses by propping and sealing the fractures while drilling. In this study, experiments were carried out to determine optimum concentration and particle size distribution (PSD) of WSM to seal off fractures with certain sizes. A non-damaging water-based polymer drill-in fluid with acid soluble marble particles having different concentrations and PSD were used in this study. Plugging efficiency of different blends were tested using modified Permeability Plugging Apparatus by pushing WSM-imbedded drill-in fluid through fracture widths of 400, 800 and 1200 mu m manufactured on metal disks. Different WSM blends were compared with each other with regards to sealing time required to reach target pressure of 2000 psi and volume lost until the seal is achieved. During the study, tests were run to investigate the effect of different PSD, concentration, and fracture width on sealing efficiency. Results were interpreted to determine optimum WSM formulations for different fracture sizes by taking into account continuous particle addition strategy while drilling together with downhole tools limitations. Optimum WSM blends were determined to seal off the mentioned fracture widths successfully.