Testing heavy oil discoveries offshore California has been a major challenge. High oil viscosity and well bore cooling prevented the oil from flowing to the surface naturally while the fractured reservoirs required extensive testing to evaluate. The electric submersible pump (ESP) was found to be an excellent method for testing low gravity oil from an offshore exploration drilling vessel. The ESP's surface display of bottom-hole pressure and high horsepower variable speed pump permitted the reservoir engineer to optimize test procedures.
This paper describes the configuration of the ESP test string, presents the running procedures and highlights the operating results of five ESP tests. Test objectives and pretesting procedures are also discussed.
Oil companies have been attempting to test heavy oil discoveries offshore California since the early 1970's. The typical heavy oil, having 12 0 API gravity or less, cooled as it flowed up the test string and formed a viscous plug before reaching the surface, see Figure 4. In addition, oil samples were usually contaminated with drilling mud and load water from acidizing. Nevertheless, high offshore development costs and complex fractured reservoirs demanded extensive testing of pay intervals to establish commerciality.
Various lifting methods including gas lift with nitrogen and pumping with inverted drilling motors failed to recover the viscous oil in significant quantities. More recently, hydraulic jet pumps run inside the test string on coiled tubing were used to pump 1000 to 1500 BOPD. The small diameter coiled tubing limited power fluid rates and pump horsepower. Another drawback was oil contamination by the power fluid which adversely affected oil rate, gravity and viscosity measurements.
In 1985, Phillips Petroleum Company used the electric submersible pump (ESP) from a floating drilling vessel to production test 9° to 12° API gravity oil at maximum rates of 4000 BOPD. The ESP and the variable pump speed controller proved to be an excellent pumping system for testing heavy oil. The variable speed pump and surface displayed bottom-hole pressure allowed the reservoir engineer to observe real-time reservoir performance and optimize test procedures. It also recovered uncontaminated oil samples for detailed analysis which was crucial in determining the feasibility of offshore processing and to see if oil quality met common carrier pipeline requirements.
ESP tests were designed to maximize pressure draw-downs and oil rates in an attempt to drain the reservoir's prolific fractures and determine the effective permeability. Actual production and processing methods were also evaluated. The oil rate was usually limited by the capacity of the oil and gas separator and by the rate at which closed top atmospheric tanks were filled which if excessive, caused over pressuring from large volumes of vent gas. Pressure draw-downs were limited to the ESP's gas locking pressure which occurred when free gas exceeded 10% to 15% by volume through the pump. The gas locking pressure was estimated from differential gas liberation studies from other wells with similar oil and GORs and from pretest oil samples.