An objective approach to the selection of an optimized medium for single phase gas & multi-phase fluid for underbalanced drilling (UBD)

As the reduction of formation damage, the urge to discover bypassed pay zones, and increased reserves becoming a major challenge for the Oil & Gas Industry, Underbalanced drilling has become a widespread solution to many operators. While the advantages of UBD applications are many, predominantly avoiding lost circulation, increasing rate of penetration & improving production evaluation, economic reasons are by far & large, the main driver for utilizing UBD techniques.;Though not as common as overbalanced drilling, underbalanced drilling is achieved when the pressure exerted on the well is less than or equal to that of the reservoir. It is performed with a light-weight drilling mud that applies less pressure than formation pressure.;Under-Balanced Drilling Dynamics There are three primary types of fluids used in underbalanced drilling operations. The first is Gaseous (compressible), which comprises of Air, Nitrogen & Natural Gas. Second, is the two-phase or lightened drilling fluids, which consist of either foam-type fluids or aerated drilling mud. The third is the single phase liquid which utilized conventional drilling fluids to provide underbalanced conditions. The fluid type is dictated by the boundary conditions of the drilling system. Typically, the boundary conditions are defined by bottom-hole flowing pressure, formation fracture pressure, borehole collapse pressure and formation pore pressure.;Scope of Work One of the major limitations of UBD is down-hole fires. The circulating fluid needs to be a nonflammable mixture down-hole to avoid it. Hence, replacing Air with Nitrogen or Natural Gas is often considered the solution. Naturally, those solutions come at cost & the selection of the most suitable gas type depends heavily on the economics.;The efforts herein have been specifically focused on evaluating the technical advantages & economics of the Single Phase Gas & the two-phase. Volumes required for the alternative methods of fluid to perform the UBD conditions, their respective limitations, flow performance & cost have been rigorously examined & interpreted.;For single phase, the Nitrogen flow versus Natural gas flow have been compared both technically & economically to find which one presents a more practical solution to the industry. Alternatively, for the two-phase drilling fluids, economics of gasification using two different gases, Nitrogen versus Natural gas were studied.;Technical & Economic Evaluation The operational feasibility, limitations, risk assessment, equipment setup, flow simulations for single flow & multi-phase flow using the world-leading Neotec software have been done & examined. Different flow simulations have been run on both vertical & horizontal well trajectories to assess the dynamic impact of the well profile on UBD techniques. Volume & flow rate of gas &/or liquid needed to be pumped in the hole for both cases of single phase gas & multi-phase drilling fluids were introduced & an overall cost of the whole operations as a whole calculated & compared.;Conclusion (1) To identify an optimized, practical & safe solution for single-phase gas flow; utilizing Nitrogen or Natural Gas (2) To identify the cheapest medium that can be used for single-phase flow. (3) To compare the flow rates, efficiency & volumes required for certain well profiles for both Nitrogen & Natural Gas. (4) To identify an optimized & safe solution for two-phase flow; utilizing Foam or aerated drilling mud. (5) To identify the cheapest medium that can be used for two-phase flow.;To provide an optimum solution & elaborate selection scheme for the Oil & Gas Industry to facilitate decision making when employing UBD techniques with single phase gas flow &/or two-phase drilling fluids.