Evaluation of Gas Assisted Gravity Drainage (GAGD) Using CO2 in a Limestone Formation through Compositional Simulation

Abstract

Abstract Gas Assisted Gravity Drainage (GAGD) is a relatively new gas flooding Enhanced Oil Recovery (EOR) technique developed at Louisiana State University (LSU) to increase oil recovery by application of miscible and immiscible gas injection processes (using CO2, N2, and hydrocarbon or flue gases). This paper describes the evaluation of application of this technique to a limestone formation in North Western Pakistan using fully compositional commercial reservoir simulator. A sector model was taken out of a full field history matched simulation model for subject gas condensate/oil field in North Western part of Pakistan. A seven component fluid model was developed and calibrated against available PVT experimental data using a commercial PVT modeling package and multiple contact miscibility tests were simulated to estimate Minimum Miscibility Pressure (MMP). Phase behavior calculations indicated minimum miscibility pressure (MMP) to be ~ 5000 psia using 100 % CO2 as an injected fluid. Following this, subject reservoir performance was evaluated by placing wells in different configurations. Oil recovery forecasts were projected for different development scenarios including base case primary pressure depletion, Continuous Gas Injection (CGI), Water Alternating Gas (WAG) injection and Gas Assisted Gravity Drainage (GAGD). Simulations indicate that GAGD is a viable development strategy for said reservoir. However, as soon as, injected gas encounters low pressure near well – bore region across producer, it breaks out through path of least resistance and after breakthrough small production rates are possible, hence increasing post breakthrough project operating expenditure (OPEX). Therefore, key is to delay gas breakthrough for as long as possible. If well placement is properly optimized, GAGD results in a recovery factor of ~ 53 % for said reservoir in comparison to 26 % for base case development scenario. Effect of vertical permeability anisotropy (kv/kh ratio) is also investigated and indicates that even for a very low kv/kh ratio of 0.1, ~ 50 % recovery factor can be achieved for said reservoir. It is recommended to give due consideration to well placement before commencement of a GAGD project. Before applying this technique on a full field scale, it is recommended to launch a small – scale pilot project for field testing to get more realistic recovery predictions. This work screens and makes case for application of gas EOR in fractured limestone formation resulting in an increment of ~ 27 % in recovery factor in an area where enhanced development techniques are not traditionally practiced.