Managing High Differential Pressures in Fractured Carbonate Reservoir by Use of Wellbore Strengthening Material

Abstract

The objective is to understand the impact of applying lost circulation materials preventatively to achieve strengthening of the wellbore when drilling fractured carbonate reservoirs. With a successful application, drilling with higher differential pressure can be enabled so that longer reservoir sections may be drilled, avoid splitting sections, and more remote parts of the reservoir may be reached. Having conducted extensive laboratory tests to evaluate the sealing strength, ease of mixing and circulation, resistance towards degradation, and the abrasivity (erosivity) of selected lost circulation materials, a novel loss prevention material (LPM) product was selected for application into multiple wells. A simple model was developed to predict the wellbore strengthening effect for a given concentration of the product in the fluid system. The product was thereafter applied preventatively for drilling slim-hole reservoir sections in deviated wells on the Norwegian Continental Shelf, where differential pressures were expected to be challenging. An LPM recovery system was used to maintain desired concentrations of LPM in the active system. Data were collected from the operations to provide a robust understanding of the ease of application and the functionality of the selected LPM material. Multiple fractured carbonate reservoirs were drilled under different pore-pressure conditions and with different drilling fluid densities. The results show that wells could be drilled without losses in conditions of around 2,400 psi overbalance, with product concentrations of less than half the maximum tested concentrations. Also, a field record of drilling a long horizontal reservoir section without tool failure was set, under conditions of unexpectedly high pore-pressures. The LPM recovery system proved to be successful with the novel materials, and very high rates of product retention was obtained, proving both the efficiency of the recovery plan and the material's resistance towards degradation. The field application highlights the potential for preventative treatment of the drilling fluid to achieve a strengthening of the wellbore and to integrate this with a simple method for estimating the fracture pressure of the formation at various levels of product concentrations. With this information in hand, both producer and injector wells can be planned for more optimal placements in the reservoir to enhance oil recovery.