Underbalanced drilling (UBD) has been widely used as a means of reducing formation damage during drilling. The benefit of reduced formation damage results from the fact that the borehole pressure is intentionally maintained to be lower than formation pore pressure during drilling the pay zone.

UBD operations require additional surface equipment such as separators and reserve tanks to process and store the produced reservoir fluids. Selection of separators and reserve tanks for UBD involves using a method to predict reservoir fluid inflow rate and volume. Such a method is not available in the literature for UBD horizontal wells. This study filled the gap.

In this study, we derived an equation for pressure distribution in the borehole by rigorously coupling formation inflow and borehole hydraulics. A simple function for fluid influx rate distribution in the borehole was then developed. Fluid influx rate is predicted as a function of horizontal borehole length, which is time-dependent. The predicted fluid influx rate at the total depth is used for sizing separators. The total influx volume is predicted by integrating the influx rate over the total time for horizontal drilling. The predicted total influx volume is used for determining the required capacity of reserve tanks. A work procedure was developed for generating fluid inflow rate and influx volume profiles. Application of the procedure is illustrated in solving an example problem. This paper presents drilling engineers a mathematical tool that can be used for sizing separators and reserve tanks during horizontal UBD planning.

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