The effects of heterogeneities on the economics of thin gas column reservoirs

After decades of successful exploration and exploitation of gas fields in the Netherlands, the Dutch E&P industry has accumulated a substantial portfolio of gas fields that are considered economically unviable, the so-called ‘stranded fields’. There are various reasons for a gas field to be classified as economically unviable, one of them being the small size of the field. With the current challenges the E&P sector is facing it is valuable to re-evaluate the criteria often used in determining a fields economic viability. One of those criteria is the column height encountered in the well, which when it becomes too small can be very sensitive to water inflow. Gaining further understanding of the relation between gas column height and gas production can be valuable in determining the potential of a gas field with a limited gas column. Consequently, the focus of this study is twofold: 1) finding the minimum gas column height required for producing a sufficient amount of gas (0.1 – 0.2 BCM) and 2) investigating the effect of specific important reservoir heterogeneities (i.e. high-permeability streaks and clay layers) on the production results (total gas production and production time). These two research topics were approached through two separate methods: 1) a sensitivity study carried out using two types of models, a simple box model and a complex reservoir model and 2) an analysis of gas wells, and the corresponding gas reservoirs, that encounter small gas columns. Both research methods are focused on Rotliegend gas reservoirs. The results from this study show that the minimum gas column range for yielding economic gas production was found to be 20 – 40 m, depending on the field’s characteristics and configurations. Furthermore, in the models used for this study the presence of a highpermeability streak may either cause an increase or decrease in the total gas production, however they are found to significantly reduce the production time which is beneficial towards the economic analysis of a potential project. Additionally, they are found to be highly beneficial in gas reservoirs with a low average permeability. The clay layer on the other hand, significantly increases the total gas production of the models, although this is accompanied by an increase in the total production time. A potential downside of the clay layer is the restricted flow of gas which is particularly problematic in horizontal reservoirs with a significant portion of the gas column situated below the clay layer. Lastly, a possibly positive correlation between transition zone and the total gas production from thin gas columns reservoirs was found in the well analysis, on which further investigation is recommended in order to be able to draw more meaningful conclusions.Applied Earth Science