No abstract

Dry gas injection has been widely practiced across the oil and gas industry. Under this approach, lean gas is injected into the producing formation to attain amongst many others the three key physical responses: (1) Reduction in reservoir pressure depletion (2) Re-vaporization of in-situ condensate dropout (3) Swelling of reservoir oil. All of these reactions could yield to improved hydrocarbon recovery. However, not all of the responses are fully described by standard blackoil simulation. In such instances, compositional modeling is required to provide for accurate estimate of field recovery but at the cost of time and human resources. Through a field case study on the Vestflanken reservoirs, this paper will reveal the strategy used to evaluate and justify the necessity of compositional modeling in a dry gas injection project. Blackoil and compositional models were constructed with the use of Eclipse 100/300 and the results were compared. The study has concluded that (1) Pressure depletion was equally well described in blackoil and compositional simulations (2) In the event of short injection time during which the injected gas stream was unproduced, re-vaporization of dropout condensate would not be a dominant recovery process. Blackoil simulation approximates the results of compositional model (3) In saturated reservoir conditions, oil swelling is insignificant. In addition, the study has also recommended on a workflow that enable the engineers to estimate the maximum impact of condensate re-vaporization on field recovery in the absence of compositional model. By the use of vapor incremental rate function in blackoil simulator, the uncertainty range in re-vaporization rate, which also equates to the key discrepancy between blackoil and compositional simulations on re-vaporization behavior could be quickly established. The magnitude of discrepancy serves to justify the necessity of compositional modeling work thereafter. An accurate reservoir simulation is important for large investment like dry gas injection. General impression amongst engineers were such that compositional modeling is imminent but this paper has demonstrated a case scenario in which blackoil provides just as good a representation of the fluid behavior as the compositional simulation. It serves as a good analogue and reference for all reservoirs and projects of similar nature.