Condensate banking is one of the main challenges facing operators of gas condensate reservoirs. Different techniques have been used over the years to improve well deliverability in condensate reservoirs, including gas recycling, gas injection, matrix acidization and hydraulic fracturing. This study investigates design options for effective CO2 injection strategies to reduce liquid blockage and improve productivity in tight reservoirs.
We use dynamic compositional models to simulate condensate formation under realistic operating conditions. Initial sensitivity studies are performed using single well models, to design simulation cases that capture the condensate banking problem accurately for selected permeability scenarios. Then full field models are constructed to evaluate the potential of CO2 injection schemes: near-wellbore huff and puff, and field scale continuous injection. Different field development options are investigated, considering the effects of well placement, injected gas volumes, quality of injected gas and frequency of huff and puff cycles.
The results of this work show that CO2 huff and puff processes can be highly effective for mitigating condensate drop-out in tight reservoirs, whereas there is likely to be no significant advantage for huff and puff in comparison to continuous CO2 injection in moderately permeable reservoirs. Well spacing and injected volumes can impact recovery significantly: we suggest analytic methods, based on well drainage areas, to estimate suitable values for these parameters. As the production scheme will involve recycling CO2, we consider the effect of impurities in the injection stream: our study indicates that it is not essential to use pure CO2 but sensitivity studies should be performed to check the effects of injected gas composition on recovery. We also demonstrate that recovery from huff and puff operations can be improved by using variable-length cycles, and propose a method to adjust the injection periods dynamically.
In this paper, we have identified important parameters that can affect the performance of CO2 huff and puff processes in tight gas condensate fields. The methodology described here provides a guideline for simulation studies of CO2 injection, which will help reservoir engineers to optimize field development and operations.