multivariate production systems optimization - Stanford University

5.1 Optimization of Phase Separation
The optimization of phase separation practices is important because by varying the blend of
surface oil and gas produced per reservoir volume, an operator may markedly affect the
total value of the mix. The total value of the mix is affected by the fractional volumes
produced of each phase as well as the quality of each phase. The fractional volume of each
phase is important because traditionally the market has paid a significant premium for
hydrocarbons in the liquid phase. At times, the vapor phase was considered to be of no
economic value, prompting operators to flare the gas rather than attempt to find a market for
it. In addition to the gross volume produced of the oil and gas phases, the total value is
affected by the quality of the phases. A gas price is adjusted for the BTU content while the
price paid for crude varies with the API gravity of the oil.
An important concept in surface phase separation is that the fractional liquid
recovery will always be enhanced by adding more separators between the wellhead and the
stock tank. By increasing the number of separators, the liberation process between the
wellhead and the stock tank is essentially transformed from a flash liberation process to a
differential liberation process, thus improving the fractional liquid recovery. In a stage
separation process, the light hydrocarbons molecules that flash are removed at relatively
high pressure, keeping the partial pressure of the intermediate hydrocarbons lower at each
stage. As the number of stages approaches infinity, the lighter molecules are removed as
soon as they are formed and the partial pressure of the intermediate components is
maximized at each stage.
An equally important concept is that for a finite number of separators, there is an
optimal combination of discrete separator pressures that will maximize the fractional liquids
recovery. At the optimal combination of separator pressures, the API gravity of the crude
will be maximized and the gas-oil ratio will minimized (see Figure 5.3).
To mimic the behavior of surface facilities, this study elected to model the
performance of a two-stage separation process. A two-stage separation process involves
flashing the well stream initially at the separator operating conditions and then flashing the
resulting liquid stream at stock tank conditions. Assuming the temperatures of the two
stages to be constant and by setting the stock tank pressure to atmospheric pressure, the
separator pressure can be optimized to yield the most beneficial mix of oil and gas fractional
recoveries.
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