An Evaluation of Large Capacity Processing Units for Ultra Deep Water and High GOR Oil Fields

Abstract

A study of offshore production units with large fluids processing capacity is presented, focusing in ultra deep water scenario (higher than 2,000 m) and the production of light oil, with high Gas Oil Ratio (GOR) and CO2 contamination. This situation fits to some exploratory and production areas offshore Brazil. Taking a regular size FPSO as the comparative base, analysis explored the impacts of different processing capacities on energy demand, weight and footprint requirements. The evaluation considered two gas processing plants: the first is less complex, without CO2 separation from the produced gas, reinjecting the whole produced gas into the reservoir; the second is more complex, with CO2 removal and a more flexible plant, capable of exporting treated gas and reinjecting rich CO2 stream or the whole gas. As the driving force of CO2 separation by membranes is the pressure drop between feed gas and the permeated rich CO2 stream, the second option leads to a lower processing capacity, mainly due to the increase in compression and power demand. The risks and benefits of large production systems are also evaluated from the reservoir drainage and gathering systems point of view. One of the main constraints is the indication that for high GOR oil, considering or not CO2 separation in the processing plant, its capacity would have to be limited due to maximum topside dry weight and deck footprint that could be installed on a converted hull or a new built unit. Additionally, constraints related to maximum power generation must be taken into account. If CO2 is not removed, and the gas is reinjected with no export, a higher processing capacity could be achieved. The design of higher capacity plants shall also consider reservoir aspects, flow assurance through long tie back wells, operability, maintainability and increase in difficulty to purchase high capacity equipment, such as vessels, valves, pumps, compressors and other.