Oil chemistry and its impact on heavy oil solution gas drive

Abstract

A series of laboratory depletion experiments are conducted at reservoir temperatures with three different live crude oils, as well as mineral oils. These tests examine the role of oil composition on the heavy-oil solution gas drive process. The morphology of gas bubbles, pore pressure, critical gas saturation and oil recovery is compared. Core-level depletion indicates that oil composition plays a role in determining metastability of dispersed gas bubbles in foamy oil. Generally, systems that exhibit effluent gas bubbles on the order of the size of pores tend to recover more oil, all other factors being equal. The concentration of organic acid and base groups as well as asphaltene content of crude oil is measured to characterize each crude-oil sample and develop an understanding of the influence of oil chemistry. Results suggest that significant asphaltene content as well as substantial acid number (the amount of potassium hydroxide in milligrams that is needed to neutralize the acid groups in one gram of crude oil) and base number (the amount of potassium hydroxide in milligrams needed to neutralize the acid titrant for one gram of crude oil) are indicators of whether oil is foamy. The partitioning of acid and base groups between the asphaltene fraction and deasphalted oil is also studied. Organic acid and base groups are clearly present in the asphaltene fraction. The role of such functional groups on oil foam stability is investigated by measuring the lifetime of single foam films formed from crude oil and asphaltene solutions. Transparent micromodels etched with a sandstone pore network and containing gas dispersed within the oil are also used to investigate the correlation of acid number, base number, and asphaltene content on gas-bubble coalescence. The results show that a high concentration of asphaltene that exhibits acid and base functional groups tends to increase foamability and film lifetime of gas/crude-oil dispersions. The deasphalted fraction is not foamy despite possessing significant acid and base number. We conclude that acid and base groups within asphaltene, and their interaction at the gas–oil interface, are a source of interfacial stability.