Can We Ignore Asphaltene in a Gas Injection Project for Light-Oils?

Abstract

Abstract When light-oil reservoirs are considered for miscible gas injection processes (with CO2 or hydrocarbon gases), more often the potential of asphaltene precipitation is either overlooked or not anticipated for two seemingly compelling reasons: first, the asphaltene content in most light-oil reservoirs where gas injection is implemented could be minute, and second, the reservoir has had no prior experience of asphaltene precipitation during its primary production phase. As a consequence, the potential of asphaltene precipitation occurring during gas injection does not get the careful attention it deserves. Operators are often confounded and taken by surprise when they experience a manifestation of serious asphaltene precipitation soon after the gas injection starts. Once the asphaltene precipitation occurs, it causes severe permeability reduction and wettability alteration in the reservoir, and plugging in the wellbore and surface facilities. Remedial measures, which are expensive and disruptive, usually include chemical treatments and workover operations are often disruptive and expensive. Therefore, one must investigate the possibility of asphaltene precipitation prior to implementing a gas injection scheme. Introduction Extensive field and laboratory data confirm that the lighter the oil, the lower is the asphaltene solubility; that is, asphaltenes will precipitate more easily from light oil than from heavy oil, even though the heavier oil may have much higher asphaltene content. For example, the Venezuelan Boscan crude with 17.2-wt% asphaltene was produced nearly trouble-free, whereas Hass-Messaoud crude in Algeria with only 0.15 wt% asphaltene has numerous production problems due to asphaltene problems. Moreover, asphaltene precipitation process is further exacerbated by gas injection, and coincidently, light-oil reservoirs are more often candidates for gas injection processes. For example, when CO2 is injected, a pH change may take place destabilizing the asphaltenes held in the native oil. Likewise, injection of a hydrocarbon gas could lower the C:H ratio causing precipitation of asphaltenes from the reservoir oil. Field data confirm that asphaltene precipitation and deposition in a gas injection process could have detrimental consequences in surface & sub-surface facilities and in the reservoir. Deposition within the reservoir causes reduction in the fluid permeabilities and may alter formation wettability (Cuiec, 1984). In Rainbow Keg River area in Alberta, Canada, no prior asphaltene problem was encountered until the implementation of HC miscible flood. Severe asphaltene depositional problems started to manifest soon after the HC gas injection, requiring costly well-completions, frequent shutdowns for workover, chemical treatments with xylene and pigging of surface lines (Nagel et al., 1990). Ninety-seven per cent of precipitates were asphaltenes. A theoretical model was developed to predict asphaltene precipitation and to optimize operating conditions either to avoid or minimize asphaltene problems. In a tertiary CO2 flood in Midale in Canada in a naturally fractured reservoir, asphaltene and wax deposition in the wellbore became a serious operational problem once CO2 injection started. Analyses of laboratory and field data confirmed that gradual diffusion of CO2 into the oil, which was being produced at a low rate of about 100 BPD, had led to the precipitation of asphaltenes from the parent oil (Beliveau and Payne, 1991). Again, as in most other reported field cases, this problem was not envisioned a priori as Midale had no prior history of asphaltene/wax deposition in the preceding phases of primary production or waterflood. Asphaltene deposition also reported to have occurred in other CO2 floods, such as CO2 floods in Little Creek Field, Mississippi and in West Texas. In this paper, circumstances and reasons that lead to serious asphaltene precipitation in a gas injection process will be reviewed together and remedial measures that need to be considered when implementing gas processes in a light-oil reservoir containing asphaltenes.