Field development strategies face a myriad of variables that impact long-term capital allocation decisions. For maturing fields, enhanced oil recovery (EOR) methods have the potential to build optionality into said strategies by offering a lever to optimize full-field development beyond primary depletion. In 2018, ConocoPhillips launched a pilot program to test the feasibility of miscible gas injection EOR in the Eagle Ford as a viable option for improving oil recovery. The objectives of this paper are to (1) summarize the experimental design of the field trials and (2) describe reservoir mechanisms observed in response to gas injection EOR operations. Produced fluids were collected before and after gas injection and analyzed using time-lapse geochemistry (TLG) to determine the degree of mixing between the injectant and in situ hydrocarbons. The TLG program consisted of high-resolution gas chromatography for oils and isotopic compositional analyses for gases. A geomechanical analysis was conducted using reservoir mechanical properties and in situ stresses to compare against modeled flowing bottomhole pressures (FBHP) to estimate degree of fracture redilation. Results from this study are summarized as follows: (1) TLG analyses confirmed that injected gas mixed with reservoir oil in the fractures and the matrix, therein reducing oil viscosity, swelling oil in place, and mobilizing residual oil. (2) Geomechanical analyses indicated that maximum FBHP exceeded fracture closure pressure and suggested that fractures redilated during gas injection. Developing EOR cycle designs that maximize both mechanisms is key to optimizing long-term incremental EUR uplift.
Although unconventional reservoirs (UR) have accounted for >50% of US oil production over the last decade, the high decline rate of horizontal shale wells and low recovery factors emphasize the need to test advanced methods for unlocking resources incremental to primary production. As such, recent years have seen significant investment by government agencies, academic institutes, and oil and gas operators to tailor enhanced oil recovery (EOR) methods for UR (e.g., Thakur 2019). These can be summarized into three broad categories (e.g., Alfarge et al., 2017):
1. Mixing chemical surfactants with hydraulic fracturing fluids to induce wettability alterations of intermediate- to oil-wet reservoirs and enable imbibition and displacement of oil from the reservoir matrix into the hydraulic fractures.
2. Smart water flooding to alter shale wettability and reduce oil-water interfacial tension upon contact with low-salinity water to enhance oil displacement.
3. Miscible gas injection via cyclic injection and production, or Huff ‘n’ Puff (HnP), using injectants such as CO2, N2, or enriched natural gases where injection pressures exceed minimum miscibility pressures (MMP) with reservoir oils to induce oil swelling, reduce oil viscosity, vaporize oil components, and provide reservoir pressure support.