Tracer analysis in flow channel characterization and modelling of gas and CO2 injection EOR in unconventional reservoirs

Abstract

A large amount of unproduced oil remains in unconventional shale reservoirs. In this paper we present the use of water and oil tracers as a key factor in flow channel characterization and in constructing a viable numerical model to forecast oil and gas recovery by depletion drive and enhancing oil recovery (EOR) by hydrocarbon gas or CO2 injection in Niobrara and Codell formations in the Denver-Julesburg (DJ) Basin. First, a dual-porosity compositional model was built based on seismic interpretation results, well logs, and core analysis. Two hydraulic fracture scenarios, (1) uniform dimensions and (2) variable dimensions, were included in the static rock frame of the numerical model. Production performance matching demonstrated that the use of variable length and height for hydraulic fractures led to a more realistic representation of the reservoir performance. On another front, injection of water and oil tracers and flowback analysis provided the means to better quantify the fracture network distribution, flow communication between wells, and hydraulic fracture performance in each well. Finally, two ‘huff-and-puff’ (injection, shut-in, production) cycles of lean gas and CO2 injection into the reservoir were modeled to assess EOR potential of cyclic gas injection. With identical gas injection rates, lean gas produced more oil than CO2; however, CO2-EOR modeling results indicated that a substantial amount of CO2 was stored in the reservoir. The net carbon stored after CO2-EOR was approximately 13% of the injected CO2 and CO2 utilization was 39,000 scf per incremental oil barrel produced which is much larger than the average CO2 utilization of about 12,000 scf per incremental oil barrel produced in conventional reservoirs. However, the produced CO2 from unconventional reservoir EOR operation can be recycled to achieve complete storage of CO2 — rendering CO2-EOR an effective means of decarbonization. Finally, transmissibility analysis of an existing major fault zone in the study area indicated that CO2 did not leak via a major fault in the study area nor via the associated nearby natural fractures in the study area.