CO2-Assisted Gravity Drainage EOR: Numerical Simulation and Scaling Models Study
Doctor of Philosophy, 2010
University of Adelaide
Increasing demand of the oil and gas have given rise to surge in drilling and exploration activities to recover oil from other unexplored oil-bearing formations (such as offshore) as well as in the efforts to improve and/or modify the existing methods of the enhanced oil recovery to recover the residual oil left-behind by the applied EOR method. Nearly one-third volume of the original oil in place (OOIP) is left-behind by the current EOR technologies. Estimated 2 trillion barrels of this volume is lucrative to cater the energy needs of the respective countries. Gas injection EOR method is a major contending process in exploitation of this resource, and its application is on the rise since last decade. Continuous gas injection (CGI) and water-alternating gas (WAG) injection are the most notable and commonly field-implemented horizontal displacement type gas injection EOR processes. The limitations of CGI are the severe gravity segregation and poor sweep efficiencies. Although the reservoir sweep efficiencies are improved with the WAG, review of 59 field projects suggest that they yield only maximum of 10% incremental oil recoveries due to the detrimental effects of increased water saturation to diminish gas injectivity, reducing oil mobility, decreased oil relative permeability and oil bypassing due to gravity segregation. Conversely, vertical downward oil-displacement gas driven gravity drainage EOR methods uses the gravity forces to its advantage for enhancing the oil recovery. Gravity drainage EOR methods have been applied to dipping and reef type reservoirs in the field projects and reported to yield high incremental oil recoveries.
In this study, the CO2-assisted gravity drainage EOR method is investigated in the non-dipping reservoir through the 3D reservoir simulations and scaling and the sensitivity analysis. Both the compositional and pseudomiscible black-oil numerical reservoir simulations are conducted in the 50 and 35 oAPI gravity oil-reservoirs respectively. Main objectives of this research are to (i) develop a better production strategy for the oil recovery optimization (ii) investigate the options to optimize oil recovery in the CO2-assisted gravity drainage EOR process (numerical simulation studies) (iii) to develop a set of scaled models sufficient to completely scale the CO2-assisted gravity drainage EOR process through the scaling and sensitivity studies.
Original contributions of this research are (i) First comprehensive demonstration of the CO2-assisted gravity drainage EOR method application in 50 oAPI gravity oil-reservoir, (ii) Development and verification of a new hypothesis of the horizontal gas floodfront in the top-down CO2-assisted gravity drainage EOR process, (iii) Development of a general process selection map for the preliminary choice between the immiscible and miscible process, (iv) Grid size effect studies: Changes in both the x and y grid-dimensions has no impact on the CO2-assisted gravity drainage oil recovery, (v) Grid thickness effect studies: Thin layers, even in the upper layers, facilitates the optimum CO2-assisted gravity drainage oil recovery (vi) Heterogeneity in permeability effect: Presence of heterogeneity in permeability (kv / kv = 0.001) improves the CO2-assisted gravity drainage oil recovery performance (95.5% incremental oil recovery) thereby reducing the number of pore volumes and the operational time. It has been found that recovery further improves when the molecular diffusion effects are taken into account, (vii) Heterogeneity in porosity: Porosity values increasing downwards, such as in the overturned faults, promotes the CO2-assisted gravity drainage mechanism to yield better oil recovery performance, (viii) Clear identification of the overall mechanisms and the supporting micro-mechanisms through the parametric analysis of the reservoir simulation results, (ix) Development of a new correlation (combination number, NJadhawar and Sarma) that encompasses the traditional process affecting multiphase operational parameters in the form of the dimensionless groups. It is further validated using the field projects including the data from the Oseberg field, Norway. Excellent logarithmic correlation match is obtained between the new combination number, NJadhawar and Sarma, and the oil recoveries from both the immiscible and miscible reservoir simulations as well as the field projects. New combination number, NJadhawar and Sarma, is a useful tool to predict CO2-assisted gravity drainage oil recoveries, and (x) Development of a set of the additional scaled models sufficient to completely scale the CO2-assisted gravity drainage EOR process are proposed and validated.