Abstract

Recovery of heavy crude oil by steam soak has to be complemented by injection of heated diluent at the later stage of a production cycle. The amount of diluent for individual wells and the outlet temperature of the light crude diluent heater are critical for the economic efficiency of a heavy oil gathering and transferring system. A constrained non-linear programming model was developed and solved by Powell method with penalty functions to optimize the operating parameters. A software OPTOP was developed and used in the heavy oil gathering and transferring system of a heavy oil field, Shanjiasi field. Optimized operation achieved a saving of 15–25% of total energy cost and 15–20% of light crude diluents.

Introduction

Shanjiasi heavy oil (13.5 °API) of Shengli Petroleum Administration has been recovered by steam soak for its high viscosity (about 10,000 MPa.s at 50 °C) since 1985. The temperature of the produced fluid decreases and its Viscosity increases sharply with time, resulting in high wellhead back pressure, sucker rod pump motor overloading, abnormal well performance and even premature ending of a steam soak cycle. A light crude from a nearby oilfield is injected down the well annulus as a diluent to reduce the viscosity of the produced fluid at the later stage of the production cycle. The amount of diluent for each well is an important factor affecting the energy cost and diluent consumption of a gathering and transferring system. The tests of diluent response were usually conducted in individual wells in the past. It is difficult to generalize a reasonable amount by testing in individual wells since crude oil and diluent properties vary greatly from field to fleld and the amount of diluent is related to the specific conditions of the surface pipeline network and the wellbores. At present the amount of diluent in production is basically determined by experience.

The authors made an analysis by taking the gathering pipeline network and the wellbores as a whole system. A constrained non-linear programming model was developed with the total energy cost as the objective function. The optimal amount of diluent and outlet temperature of the light crude heater were solved by Powell method combined with penalty functions.

System analysis

In steam injection recovery, the production rate GI, wellhead temperature Tk and water cut Fi of a heavy oil well change with time, as illustrated in Figure 1. The gathering and transferring system is in a dynamic state Although G1, Tk and Fi change sharply at the initial stage of a production cycle, dilution is not necessary since the temperature of the produced fluid is higher and the oil well is in good working order. Dilution is only necessary at the later stage of a production cycle, when production parameters change much more slowly. Such a dynamic problem with a slow change in variables can be reduced to a static one in order to simplify the mathematical model.

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