Field experience indicates that primary depletion of tight oil formations, using multistage fractured horizontal wells, commonly recovers only 5 to 10% of OOIP. The impact of various EOR techniques on recovering additional oil from these formations is still not fully understood. This paper investigates the applicability of feasible EOR methods and determines their technical and economic success over the natural depletion process under different well and fracture designs. Additionally, the study investigates the minimum reservoir permeability required for success.
To achieve the objectives, both black oil and compositional simulation models were generated for a Western Canadian tight reservoir containing volatile oil. In addition to primary, the EOR recovery processes that were considered include waterflooding, immiscible-N2 and miscible-CO2 gas flooding. Combinations of these techniques, coupled with the effects of various well and fracture design parameters were technically explored, and economically ranked using a comprehensive economic analysis. Furthermore, the optimal case of each process was subjected to sensitivity on matrix permeability to determine the minimum permeability at which these methods can be applicable.
In the EOR scenarios evaluated, the highest cumulative oil produced was associated with the closest well and fracture spacing, and longest fracture half length. With a larger well spacing (in the order of 400 m), the wells were found to be too far apart to offer any benefit from any EOR technique. Additionally, the capital expenditure of tight-oil projects is high and therefore greatly influences the economic success. Several scenarios yielded similar NPV values, however, the IRR performances and CAPEX requirements helped further evaluate and rank the scenarios.
For the reservoir model used, waterflood was found to be uneconomical at the initial permeability levels investigated (around 0.3 md) and required a minimum permeability threshold (1 mD) to become profitable. The primary recovery mechanisms in waterflooding are pressure maintenance and areal sweep, which were more pronounced in the N 2flood. This was the best recovery technique based on NPV. However, the best recovery technique based on oil recovery was the miscible-CO2 flood. It offered an increase in oil recovery factor from 11% to 23% over the best natural depletion case, which was a result of increased oil mobility by dissolution of CO2. At lower permeability values (down to 0.03 mD) immiscible-N2 flood became the most effective method via pressure maintenance within the drainage area. For even tighter reservoirs (under 0.03mD), natural depletion remained the best option for this reservoir.
This paper provides an elaborate workflow for evaluating and optimizing EOR techniques in tight oil formations through an integrated modeling approach. It helps to identify the most technically and economically proficient techniques under different levels of permeability, well spacing and fracture parameters.