This paper presents methods developed to enable the lifting and producing of low-gravity, high-viscosity crude from offshore fields. Relative advantages and disadvantages of several lifting, treating, and transportation methods are discussed. The paper also reviews actual experiences with heavy-oil production offshore southern California, as well as plans for development of newly discovered heavy-oil fields in the Santa Maria basin offshore central California and offshore Sicily in the Mediterranean Sea.
During the recent energy crisis, many refineries were modified to use lower-quality heavy crudes for which there had not been much demand in the past. This conversion changed the market for heavy oil and brought its price closer to that for lighter crudes. As a result of this price change, it became economically attractive to initiate production from offshore heavy-oil fields. Coincidental with this shift in market conditions. a number of heavy-oil fields were discovered offshore California and in the Mediterranean Sea.
Heavy oil usually indicates a crude with a gravity less than 20 degrees API [0.93 g/cm3]. There are, however, few production-related problems with crudes in the range of 17 to 20 degrees API [0.95 to 0.93 g/cm3] therefore, for the purpose of this paper, heavy oil is defined as a crude with a gravity of 16 degrees API 10.96 g/cm3] or less and with viscosities greater than 400 cp at 100 degrees F [400 mPa.s at 37.8 degrees C] and 20 cp at 212 degrees F [20 mPa.s at 100 degrees C]. The crude typically is lacking in the lighter ends and contains heavy metals, particularly vanadium and nickel, and sulfur. Although very viscous, the heavy crudes do behave as Newtonian fluids; i.e., viscosity is a function of temperature only.
Only a few offshore oil fields currently produce heavy crudes. One of these is the Beta field, located offshore southern California, which went on production in 1981. The field is being developed from three drilling platforms and one producing platform that are located in water depths ranging from 160 to 700 ft [50 to 215 m]. Fig. 1 depicts one of the drilling platforms and the production platform. Current production is about 20,000 BOPD [3180 m3/d oil] from several different producing horizons. Gravities from individual wells range from 10 to 20 degrees API [1 to 0.93 g/cm3], depending on the specific production interval. Average gravity is about 16 degrees API [0.96 g/cm3].
Other, smaller, heavy-oil fields in production include the Hueneme field offshore California and the Rospo Mare field off-shore Italy in the Adriatic Sea.
Several large offshore heavy-oil fields are currently in the development stage. These include the Vega field, located offshore Sicily in the Mediterranean. and the Point Pedernalis and San Miguel fields in the Santa Maria basin offshore California.
Gravities and viscosities of the crudes from these fields are shown in Table 1 and Fig. 2.
Little information was available on how to produce and transport the low-gravity, high-viscosity crude from these newly discovered offshore oil fields. Several specific problem areas existed in which onshore experience and practices were of only limited value. including
artificial lifting of heavy crudes from highly deviated wells,
treating heavy crude within the confined space of an offshore platform,
pipeline transportation of the crude to shore through a cold ocean environment, and
system startup or restart after a shutdown.
The key to successful production and transportation of these crudes is the reduction of viscosity through retention or addition of heat or, alternatively. through the addition of a diluent. A diluent can he added either on a temporary basis-i.e., for startup-or on a permanent basis. It can be injected downhole in the well, at the wellhead, or at the pipeline transfer pumps. Heat can be added to the crude during the treating process or at the pipeline transfer pumps.
The nature of the heavy-oil reservoirs and the high specific gravity of the crude necessitate artificial lifting from the wells either at the onset or very early in the life of the field. Methods that have been used offshore include electric pumps, jet pumps, rod pumps, and gas lift.
Electric submersible pumps (ESP's) are the prevalent method for lifting heavy crude from offshore wells. These pumps are used in the Beta field and are also planned for the heavy-oil fields currently being developed offshore central California and in the Mediterranean Sea. Their advantages include the capability of lifting high volumes and their installation in directionally drilled holes. The disadvantages include the lack of production-rate flexibility, a high failure rate, and the need to use a drilling or workover drilling unit to replace the pump. Recent advances in the development of variable-speed controllers and cable-suspended pumps limit or eliminate some of these disadvantages.
Jet pumps have been used successfully for drillstem testing of heavy-crude zones in exploratory wells. One jet pump is currently in operation on a production well in the Beta field and uses waterflood injection water as the power fluid.
One of the few offshore applications of rod pumping units was on Platform Emmy in the Huntington Beach field offshore California. Space requirements, however, make this method impractical for offshore use, and the pumping units have since been replaced with ESP'S.
Gas lift has been used infrequently for lifting heavy crudes. Most heavy crudes have a very low GOR and little gas is available. Gas lift is used successfully, however, on Platform A in the Hondo field offshore California in wells with an average gravity of 17 degrees API [0.95 g/cm3].
A major problem in producing offshore wells with heavy crude is getting the wells started after the initial completion and subsequent restarting after temporary shut-ins. This problem results because the wellbore is cool relative to the producing formation temperature. Until the temperature of the surrounding wellbore is in equilibrium with the temperature of the produced fluid as it comes up the wellbore, the fluid will lose most of its heat and be more difficult to lift.
There are basically three methods for "cold" starting of heavy-oil wells: downhole injection of a diluent, gas-lifting the crude with natural gas or nitrogen, and lifting the crude by pumping. All three methods have been used, and selection of the optimum method will depend to a large extent on location and economics.
Downhole injection of a diluent, such as diesel, usually requires a separate string of tubing that in turn may restrict the diameter of the production tubing. The large amount of diluent needed may require large storage tanks on the platform and an expensive transportation system. This method, however, has been used successfully in initiating production during well tests from exploratory drilling units.
A possibly more economical alternative may be to use a produced-water/surfactant mixture instead of a hydrocarbon diluent. This method, which uses high-powered jet pumps, has been tested in exploratory wells in the Santa Maria basin offshore California.