Underbalanced drilling (UBD) is one of the most widely used technologies preferred in depleted and/or low pressured formations. In order to achieve underbalanced conditions, drilling fluids are usually gasified. Major drilling fluids preferred during UBD are pure gas, gas-liquid mixtures, foams, etc. This study is focused on gas-liquid mixtures. As the gas is introduced, the behavior of the drilling fluid becomes hard to explain due to many reasons. First of all, gas is compressible and physical properties of gas are very sensitive to changes in pressure and temperature. Second, a multiphase flow phenomenon arises. When there is multiphase flow, flow patterns should be considered. It is known that, there is a difficulty to predict hydraulic behavior of gas-liquid mixtures due to this flow pattern dependence. During a drilling operation, one of the parameters that should be well considered is hole cleaning. Hole cleaning is a challenging task even for a single phase drilling fluid. Moreover, there is still a lack of information how the cuttings are transported when gas-liquid mixtures are used as drilling fluids. Flow rate optimization during UBD operations for liquid and gas phases are usually conducted based on formation pressures only. However, only considering this criteria alone as the optimization objective is misleading and may cause serious problems during the drilling operation. In this study, gas and liquid flow rates during UBD operations are conducted not only based on formation pressures, but also based on effective hole cleaning. It is assumed that liquid phase is the major contributor for cuttings transport, and gas phase is only influencing the bottom hole pressure. A mechanistic model is introduced for estimating the hydraulic behavior of gas-liquid mixture drilling fluids under different flow patterns. Based on the bottom hole pressure and effective hole cleaning point of view, an algorithm is proposed for estimation the optimum required flow rates for liquid and gas phases based on the introduced mechanistic model. Also, the model predicts the required backpressure that has to be applied.
Underbalanced drilling operations are increasingly applied, especially when new wells are required at depleted reservoirs in order to conduct enhanced oil recovery techniques or secondary recovery applications. Usually, gas-liquid mixture fluids are preferred for applying underbalanced techniques. However, one of the main problems for applying underbalanced drilling techniques is te selection of proper liquid and gas volumetric flow rates. Generally, these rates are selected based on the formation pressure, i.e., at static conditions, the hydrostatic pressure of the mixture is selected to handle the formation pressure. But, in such a case, at dynamic conditions, bottomhole pressure may exceed formation pressure, and the advantages of underbalanced conditions are missed. If the dynamic conditions are considered for selecting the liquid-gas rates, this time at static conditions, bottomhole pressure may be too low when compared with formation pressure, and formation fluids may intrude into the wellbore. Therefore, it is a better option to keep the bottomhole pressure constant and equal to formation pressure for all times, i.e., static and dynamic conditions.