Asphaltene precipitation and deposition can occur in some heavy oil reservoirs during the primary depletion or gas-lift process, when a pressure drop from the subsurface reservoir formations to the surface treating system is rather large. These precipitated asphaltenes may be deposited onto various locations along the production system so that it can be partially or completely plugged. Therefore, it is necessary to determine the possibility and severity of asphaltene precipitation at the early stage of heavy oil production. On the other hand, a number of chemical, thermal, and mechanical methods have been developed to reduce or eliminate asphaltene precipitation and deposition. The most commonly used means is through chemical treatment by applying an asphaltene precipitation inhibitor (API), which is a specially designed and synthesized chemical compound to control or inhibit asphaltene precipitation.
In this paper, both visual and filtration methods are used to determine the onset pressures of asphaltene precipitation and redissolution of a recombined live heavy crude oil during an isothermal pressure-reduction process at different temperatures and gas-oil ratios (GORs). The experimental results show that a higher temperature can lead to a lower onset pressure of asphaltene precipitation and a less amount of precipitated asphaltenes, i.e., delayed and reduced asphaltene precipitation. This is because at a higher temperature, the solubility of the resins in the live heavy crude oil is higher, which helps to stabilize asphaltenes in the oil. It is also found that in contrast, the live heavy crude oil with a higher GOR has earlier and stronger asphaltene precipitation during the isothermal pressure-reduction process. In this case, the composition of the live heavy crude oil with a higher GOR is lighter and the solubility of asphaltenes in such a live crude oil is lower. Thus, asphaltenes become more unstable and likely to aggregate and precipitate. Finally, the filtration method is also applied to evaluate the effectivenesses of two APIs (API-A and API-B) by comparing their abilities to stabilize asphaltenes in the API-treated live heavy crude oils during the isothermal pressure-reduction process. API-A is proven to be capable of substantially lowering the onset pressure of asphaltene precipitation and the pressure-drop increase in the filtration process. Furthermore, API-B is found to be more effective to inhibit asphaltene precipitation than API-A as no noticeable pressure-drop increase during the isothermal pressure-reduction test is measured for the live heavy crude oil treated with API-B.