Investigation of gas hydrate formation during sea water injection based on undersaturated oil core flooding experiments

Abstract

The Wisting Field is located at the Barents Sea, Norway. It is a shallow reservoir encountered at 71.9 bar and 17.8°C. The bubble point pressure is 68.6 bar at reservoir temperature, resulting in an undersaturated oil reservoir. The risk of gas hydrates formation is an important strategic knowledge since waterflooding with cold seawater (2 - 5°C) is being considered for the development of the field. The main concern regarding the formation of gas hydrates is that it can negatively affect the waterflooding process by reducing the intrinsic permeability of the reservoir rock. For this reason, the objective of this work was to assess the possibility of gas hydrate formation during the injection of cold water by performing coreflooding experiments and numerical simulations. Three different coreflood experiments were designed to determine hydrate formation conditions during cold water injection. The first setup used a 1.5 inches diameter and 2 inches in length core to assess static gas hydrate conditions with formation water and seawater. The detection of hydrates was performed with ultrasonic measurements. The second experiment was a coreflood that used a core sample of 3 inches in diameter and 10 inches in length instrumented with geophysical sensors to detect fluid phase transitions to a solid phase. The last setup was a coreflood with a 1.5 inches in diameter and 12 inches in length core sample. The experimental procedure consisted of detecting gas hydrate formation by an increase in the differential pressure measured across the core with high accuracy. Numerical simulations of the coreflooding experiments allowed to scale up the laboratory experimental observations to a field-scale waterflooding process. The model captured the permeability reduction as a function of hydrate saturation and was used to predict the performance of a waterflooding process in a five-spot vertical-well pattern. The simulations were performed using the STARS reservoir simulator from Computer Modelling Group (CMG) The results of this research showed that gas hydrates can form from dissolved gas inside porous media and are a risk during the waterflooding process. Fluid flow had a significant impact on triggering the hydrate formation reaction. The static coreflood test required a subcooling of 3°C larger than the coreflood experiments with fluid flow for gas hydrate formation. Results also showed that gas hydrates reduced the permeability of the reservoir.