Asphaltene Precipitation in a Saturated Gas-Cap Reservoir

Abstract

Abstract Asphaltene deposits were obtained from three high gas/oil ratio (GOR) wells in an offshore field in the Arabian Gulf. The reservoir is saturated and there is a gas-cap present in the field. In many wells the gas cones into the producing wells and causes precipitation of asphaltenes from the crude by stripping. These asphaltenes deposit in the wellbore and have the potential to create plugging problems. This paper describes the results of an investigative study that was initiated to determine the precipitation mechanism and ways to alleviate the deposition problem. Asphaltene precipitation experiments were conducted at reservoir conditions in a special pressure-volume-temperature (PVT) apparatus. The effect of GOR on asphaltene precipitation was determined by titrating the reservoir oil with gas-cap gas. Bulk deposition tests were also performed at different GORs with reservoir fluids. The results demonstrate that the onset of asphaltene precipitation occurs at relatively low GOR values. The amount of asphaltene precipitated at the onset is small. Asphaltene precipitation and deposition increase with increasing GORs. Asphaltene deposition envelopes are provided for the reservoir oil as a function of pressure and temperature. Recommendations were made to alleviate the problem by controlling the maximum producing GORs to less than 1000 scf/stb. Recipes for solvent treatment including asphaltene dispersants are also described in the paper and are currently being considered for implementation in the field.