Calculation of Flowing Bottomhole Pressure Constraint Based on Bubblepoint-Pressure-vs.-Depth Relationship

摘要

Thick reservoirs usually have variations of PVT properties such as bubble point pressure and dissolved gas oil ratio with depth. A considerable effort usually spent to describe such properties to be used in dynamic models either in Black Oil or compositional simulators. However, during predictions many dynamic models set the BHP constraint as a constant value for all producers in the entire reservoir regardless of perforations depth. Usually this constant value is close to the average bubble point pressure or a little higher to ensure single phase (oil) production and not considering changing of bubble point pressure with depth. The above procedure introduces uncertainty regarding well potential, which involves a risk of not presenting accurate well performance in the simulator and would impact oil production and recovery forecasts. The proposed method was developed to calculate following BHP constraint based on bubble point pressure versus depth relationship for each grid cell in the dynamic model, thus reating 4D-array (I, J, K, BHP constraint) for the entire reservoir. The proposed method is fully automated and tested to assign BHP constrains in schedule section of huge dynamic model (2.6 million cells) with 600+ wells. Two prediction cases were tested (each case run twice, one run with constant BHP constraint for all the produces and the other one using variable BHP constraint recommended by the proposed method) to evaluate the impact of BHP constraint on production and oil recovery forecasts. The results of these two runs showed a considerable difference in length of oil production plateau and oil recovery forecast, but negligible differences in gas oil ratio. The proposed method could be used for any dynamic model with vertical and/or horizontal variation of bubble point pressure to calculate an optimum BHP and thus WHP to maximize production rate without the risk of introducing gas phase at reservoir conditions.