Shale Oil EOR
Today, in the US, there are approximately 42 permitted shale oil EOR projects, on leases totaling 518 wells. 33 of these shale EOR projects, including a total of 429 wells, are situated in the Eagle Ford shale. Eagle Ford shale EOR operators include EOG, ConocoPhillips, EP Energy, Chesapeake, Murphy, Marathon, Petroedge and MD America. EOG Resources is by far the most active shale oil EOR operator to date.
There are 6 permitted shale oil EOR projects in the Permian Basin, 2 in the Williston Basin and 1 in the Scoop/Stack. All but one of these shale oil EOR projects are cyclic natural gas injection ("Huff-n-Puff"). Hess is planning a Williston Basin Bakken shale oil EOR project using foamed natural gas as the injecting fluid.
Incremental oil recoveries for the Eagle Ford shale EOR projects are projected to be 20-70% more than primary depletion EUR. (JPT, 2019).
To date, there are no permitted shale oil EOR projects reported in the DJ or Powder River Basins.There are other operators planning or contemplating shale oil EOR projects in the Eagle Ford and Permian Basins, and shale EOR development is expected to grow as more and more wells decline to near economic limit levels of production.
Cyclic Natural Gas or CO2 Injection Shale EOR
Cyclic injection and production, aka Huff and Puff, in a shale oil reservoir, usually involves the injection of natural gas or CO2, followed by production of the injected gas and recovered oil. Huff and Puff EOR has been conducted in conventional reservoirs for many years. The first Huff and Puff shale oil EOR project was conducted in 2008 in North Dakota, with CO2 as the injectant. It was a very short term test to determine the injectivity of CO2 into the Bakken shale. Six other EOR tests were conducted in the Bakken in 2009-2014.(SPE 180270) However, shale oil Huff and Puff EOR began in earnest in the Eagle Ford shale in late 2012, in a single well. (SPE 189816)
Injection of natural gas or CO2 into deep shale formations such as the Bakken or Eagle Ford requires large, high pressure compressors, and a large supply of natural gas or CO2.
Injection of gas into a shale reservoir can take months - for example, an Eagle Ford well that has produced 1 BCF of gas would need at least 1 BCF injected to repressure it up, and at 15 MMCFD injection rate, would take about 67 days of injection. This is a lot of production downtime and cost for the natural gas, fuel and maintenance of the compression equipment. The compression required in the Eagle Ford is specialized to provide for discharge pressures as high as 6000 psi, and as a result can take up to a year to build and deliver.
Interwell Communication -
Frac Hits and Natural Fractures
Interwell communication can be due to natural fractures or fractures generated by hydraulic fracture stimulation treatments. The presence of these fractures can prevent good pressure buildup during natural gas or CO2 cyclic EOR that is necessary to reach minimum miscibility pressure and enable good oil recovery. The illustration at right is from SPE 195240, describing an Eagle Ford EOR project that experienced significant interwell communication that precluded good pressure buildup without the introduction of offset well pressure barriers. The presence of these interwell fractures, estimated in more than half of all horizontal shale wells, may preclude or limit the implementation of natural gas or CO2 cyclic injection EOR in shale reservoirs.
Cyclic Natural Gas EOR Estimated Oil Recovery
The graph, left, from SPE 189816 (2018), "shows a standardized version of a cumulative oil plot of lease oil production for the 3 pilots where data can be analyzed. The lowest purple line represents a standardized lease production on primary production (no-injection case) based on decline curve extrapolation. The red shaded area indicates projected improved production due to gas injection of 30 to 70% over primary production, which is based upon reported estimates. (Thomas, et al, 2016)"
The graphs at left, from UrTec 624, 2019, show projected increased oil production from 11 leases in the Eagle Ford shale that are under natural gas Huff and Puff EOR.
The results section of the paper states, "The EOR model predicted an average cumulative lease-level oil production increase of ~30%. The base case results in pilot wells earning an average $0.3 million increase in NPV and an additional 75 Mbbl/well of oil above primary recovery."