Feasibility of Hydrocarbon Gas Injection in Valdemar Field of Danish Central Graben in the North Sea

Valdemar Field was discovered as early as 1977 in the central part of the Danish Central Graben in the North Sea. Significant hydrocarbon volume is reported in the Lower Cretaceous formation which has been developed via pure pressure depletion for the last few decades. Due to the tight nature of the Lower Cretaceous and declining reservoir pressure, the average economic ultimate recovery (EUR) from Valdemar field is significantly lower than what is achievable in the neighboring fields such as Dan, Halfdan or Gorm where water flooding is utilized for pressure support. On top of this, lack of sufficient petrophysical data stand as an important barrier to better understand the certain shortcomings as well as to make reliable predictions for the future performance of the field. In the course of this study, we investigate different development options for the Valdemar Field. We utilize both analytical and numerical methods to study water and/or gas injection opportunities to boost the production and hence EUR in Valdemar. We first compare the efficiency of water and gas injection by employing analytical techniques. Results suggest that better injectivities, availability and short response times may make hydrocarbon gas injection a promising candidate for a further development option. That kind of a development scheme would, however, require a significant capital expenditure such as building a new well head platform and installation of a high pressure gas injection compressor. We present a detailed phase behavior study on a recently obtained Lower Cretaceous oil sample. Both static and dynamic PVT tests, such as the swelling test and multi-contact experiments, were conducted and a reliable compositional model was built. We then construct a 3-D box model to study gas injection via long horizontal wells in a line drive mode. Model honors petropyhsical information obtained from the available well log data. It is further tuned to history match the production and pressure data, which is also in line with the previously developed full-field model. Results suggest that additional oil can be recovered via hydrocarbon gas injection in Valdemar field. We conduct a set of sensitivity studies to optimize the gas injection process. We show that the key parameter is, by far, the gas injection rate. Higher injection rates result in higher pressure drops and hence development of a large miscible zone, which significantly increase the oil production. This would, in return, require larger compression power and more gas recycling capacity