Field Results of Solvent Stimulation in a Low-Gravity-Oil Reservoir (1972)

In general, additives play an important role in the successful solvent stimulation of low-gravity crude oil reservoirs. However, neither the nature nor the concentration of the additives used in these tests in California's Santa Maria basin influenced the response nearly so much as the timing of the stimulation cycle. Introduction Production of low-gravity crude oil today assumes an Production of low-gravity crude oil today assumes an important role, particularly in California, Production of low-gravity crude (API gravity less than 20 degrees) as a percentage of the state total has been about 50 percent since 1967. Furthermore, studies by the U. S. percent since 1967. Furthermore, studies by the U. S. Bureau of Mines show that more than 100 billion bbl of low-gravity crude oil is known to be present in approximately 2,000 petroleum reservoirs in this country. This oil has additional advantages: (1) we know where it is, so exploration costs are negligible, and (2) it is generally near existing producing facilities. Currently, the most common technique for dealing with heavy oil is steam stimulation. During 1970, of the total oil wells completed in California, 1,219, or 77 percent, were drilled in heavy-oil fields for steam stimulation. Solvent stimulation is another well known technique for increasing the production of low-gravity crude. It involves the use of low-viscosity hydrocarbon solvents in the wellbore, in liners, or in the formation. Stimulation can be achieved by (1) removing both organic and inorganic damage, (2) demulsifying, and (3) lowering the viscosity. In an earlier paper it was shown that solvent stimulation of low-gravity crude reservoirs had been successfully applied in several areas. The principal stimulation mechanisms were the removal of organic material deposited from the crude and demulsification. The authors showed that specific chemical additives can be found that materially improve the ability of the solvent to remove deposits, such as asphaltenes. They also showed that it might be possible to effectively inhibit such deposits within porous media by establishing a water-wet surface. Viscosity reduction by solvents did not appear to be the primary stimulation mechanism, because of the difficulty of obtaining adequate subsurface mixing of solvent and crude. As a result of that earlier paper, a series of field tests has now been run in the Santa Maria basin. This is an area where solvent stimulation has been successfully applied for many years, and where, it is believed, the stimulation mechanism is the removal of deposited asphaltenes. This paper presents the results of those field tests and supporting laboratory investigations. The results confirm the earlier conclusions concerning damage by asphaltene deposits, the effectiveness of asphaltene stopping agents and water-wetting surfactants, and the need for improved solvent deposit contact. The results also show the importance of job timing. History of the Area The field to be discussed is in the Santa Maria basin, Calif. It was discovered in 1938 and is presently in late stages of depletion. Production is from the Monterey fractured shales at about 3,500 ft. Typical completion intervals are on the order of 1,000 ft through slotted liners. Oil gravity is about 12 degrees API, and bottom-hole temperature is approximately 100 degrees F. Successful solvent stimulation was started in the field in 1956. From 1956 through 1967 considerable additional oil was produced as a result of 164 jobs. From 1968 to the present an additional 23 solvent stimulation jobs were performed; they became the basis for this paper. Fig. 1 shows the results of a series of stimulation jobs performed on one well in this field during the period from 1956 through 1971. JPT P. 1213