Stimulation by Defoaming Increases Thermal Oil Production

Abstract

Abstract Foamed oil emulsions can exhibit significant increases in apparent viscosity due to the entrainment of gas bubbles in the oil. Crude asphaltic components act as natural foaming and emulsifying agents. Their influence causes a large portion of the dissolved gas, which would otherwise be liberated as free gas, to remain entrapped in the water-in-oil emulsion as small bubbles. Such in-situ generation of foamy oil emulsions in the vicinity of the wellbore can have a profound effect on well productivity. This type of formation damage has been found to be fairly common in low gravity oil reservoirs where steamflooding or steam stimulation is used to enhance oil recovery. A novel well stimulation technique, applying a blend of defoaming and demulsifying surface active agents with a bullhead squeezing technique, has proven to be successful in the Cymric and McKittrick fields, California. The cost of incremental oil gained is less than $1.00 per barrel. Introduction Foams have been widely used in petroleum engineering applications for drilling, well stimulation, and sweep efficiency improvement in steam and CO2 enhanced oil recovery processes. In these applications, the foam reduces fluid mobility because of its high apparent viscosity and pore blocking ability. Similarly, indigenous foam resulting from liberation of dissolved gas from oil as it is being produced manifests itself as a kind of formation damage, i.e., a combined foam and emulsion block. All crude oils foam to some degree upon being degassed. Foams from light crudes usually disperse quickly in a few seconds. In contrast, heavy oil foams can be very stable and may persist for several hours. This is observed in wellhead samples from steamflood wells in the San Joaquin Valley, California. Figure 1 highlights heavy oil steamflood fields in the San Joaquin Valley. Heavy oil is produced in the form of a foamy emulsion, having the appearance and consistency of chocolate mousse. Such foamy behavior has also been observed in the heavy oil wells of Alberta and Saskatchewan producing under solution gas drive. Stable oil foams can cause problems in separation equipment, such as liquid carry over into gas lines and serious asphaltene plugging problems in separators and oil and gas lines. For example, production problems have been reported in gas processing equipment in the Jusepin plant in El Furrial field, Venezuela (22 to 33 API crude, containing 1 to 13% asphaltenes) and in the Sullom Voe Terminal in Shetland, U.K., processing Ninian and Brent crudes (37 API). Heavy oils contain relatively little dissolved gas, but their foamy emulsions can sometimes reduce pump efficiency. For this reason, a downhole defoamer treatment apparatus using a hollow sucker rod was first used in 1970. Up to now, formation damage caused by foam blockage has received little attention as a potential production problem. The intent of this paper is to identify the occurrence of foamy oil emulsions in heavy oil reservoirs and possible consequences and impact on oil recovery. Attention is especially drawn to the asphaltic fractions of the oil and their role in bubble formation. Experimental verification of in-situ foam formation under steamflooding conditions is presented. Also discussed is an inexpensive stimulation technique which can be used to enhance heavy oil production in thermal wells. Causes of Formation Damage It is well established that viscous emulsions of oil and water in the formation near the wellbore can drastically reduce the productivity of oil or gas wells. However, relatively little is known about oil-continuous foams or oil-continuous emulsions containing both water droplets and gas bubbles. P. 461