Enhanced Oil Recovery

After primary and secondary recovery, but before the reservoir is completely exhausted, operators may use tertiary recovery, also known as enhanced oil recovery (EOR). Today EOR may be applied at any stage of reservoir development. These technologies frequently use a secondary injection well to repressurize the producing formation or otherwise loosen oil for extraction.. As such, Class II regulations cover enhanced oil recovery techniques. Enhanced oil recovery includes three broad categories of techniques: gas injection, chemical injection and thermal injection.

Gas Injection

Injection of carbon dioxide, natural gas or nitrogen into a producing formation reduces the tension between oil and water in the reservoir and reduces the viscosity of the hydrocarbon. Additional controlled water injection pushes the hydrocarbon toward the production well.

Chemical Injection

Solutions of chemicals injected into producing formations can be used alone or in tandem to increase the flow of hydrocarbons to the surface. For example, alkaline solutions neutralize some natural acids found in oil and produce soap which reduces the tension between oil and the surrounding rock. Secondly, polymers to increase the viscosity of water improve the effectiveness of a water flood.

Thermal Injection

Heating oil while still in the formation can reduce its viscosity and increase its permeability. Thermal injection may also vaporize oil, allowing it to condense as an improved oil, easier to raise to the surface. Steam flooding and fire flooding are the two main strategies.

Cyclic Steam Stimulation

This process uses three phases to loosen heavy oil from its producing formation. During the first phase, operators inject high pressure steam into the producing formation. Then, operators leave the well undisturbed to allow heat to distribute throughout the formation. Finally, hot oil is pumped from the well. This process may be repeated several times for the same well. Within the formation, cyclic steam stimulation (CSS) produces a region of depleted oil sand. Further rounds of CSS enlarge the size of the depleted region in the same shape.

Steam-assisted Gravity Drainage

Steam-assisted gravity drainage (SAGD) emerged as a solution for producing oil from bitumen deposits for which CSS was ineffective. This method requires two parallel horizontal wells. High temperature steam injected into the top well warms the heavy crude or bitumen and the lower well collects the lower-viscosity oil for transport to the surface.

Class II EPA UIC Wells. During SAGD, steam pumped from the upper pipe increases the viscosity of heavy oil and bitumen allowing it to flow into the lower pipe.

Solar Steam

Solar EOR is an emerging form of steam flooding which uses solar energy to heat the water for steam. This eliminates the need for combustion on site to heat water and therefore reduces the overall carbon emissions of an operation.

Fire Flooding

Similar to steam injection, fire flooding uses heat to move viscous oil underground. In this case, combustion is used directly within the producing formation to reduce oil viscosity and turn reservoir water into steam. The fire effectively pushes oil toward production wells.

Research Spotlight

Cyclic Injection (Huff N Puff) For Unconventional Shales

Huff N Puff is an enhanced oil recovery method in which one well alternates between fluid injection (huff), shut-in (soaking) and production (puff).  Pilot studies, laboratory experiments and computational reservoir modeling indicate significant amounts of oil from unconventional shales could be recovered using the Huff N Puff process1.  It is potentially an important EOR process for unconventional shales where current oil production techniques are estimated to recover just 5-10% of original oil in place oil and where the drop off in production may be as much as 70% in one year2.

Various fluids including water, carbon dioxide, nitrogen, recycled gas and propane have been studied for use in Huff N Puff recovery process.  In laboratory experiments and computational modeling work at The University of Texas1 recycled gas, propane and water were studied as injective fluids.

Schematic Diagram of Experimental Set-up in Isbell, 2016

The laboratory project compared recycled gas and propane for improved oil recovery. The diagram above outlines the equipment used for the laboratory experiments.

A total of five oil recovery experiments were conducted: three with recycled gas and two with propane.  The table below indicates that the Huff N Puff process using recycled gas and propane increases oil recovery.  However, in the laboratory experiments propane significantly outperformed recycled gas experiments.1

Numeric reservoir modeling in Isbell, 2016

Numerical reservoir modeling was conducted to replicate the lab-scale experiments mentioned above and to study complex recovery mechanisms. Injection fluids included recycled gas, propane and water. In the model propane significantly outperformed recycled gas and water and demonstrate that Huff N Puff may be an effective process for improving oil recovery in unconventional shales.

Incremental oil recovery for one Huff N Puff treatment using three different injection fluids in Isbell, 2016

Several operators have already initiated or are planning Huff N Puff field programs in the Eagle Ford, Permian, Bakken, the DJ and SCOOP.

Stories from the Field

Data from Texas Railroad Commission files identified several cases of increased production after Huff N Puff in the Eagle Ford.3

11 wells that reached a combined peak production rate in December 2011 of about 90,000 bbl a month. By August 2017, these wells were pumping out only 5,000 bbl. After gas injections began, the group produced 40,000 bbl a month—an average increase from about 15 B/D to 117 B/D per well.

14 wells that peaked at 330,000 bbl a month in 2013, then dropped to 10,000 bbl. Post injection, output increased to 170,000 bbl a month.

Citations
  1. Isbell, J.T, 2016. Using huff n’ puff with a recycled hydrocarbon gas as a means for enhancing oil recovery in a liquid-rich shale reservoir. Masters Thesis. The University of Texas at Austin.
  2. Hoffman, B.T. (2018). Huff-N-Puff gas injection pilot projects in the Eagle Ford. SPE Canada Unconventional Resources Conference, 13-14 March, Calgary, Alberta, Canada. Society of Petroleum Engineers.
  3. Jacobs, T. (2019). Shale EOR Delivers, So Why Won’t the Sector Go Big?. Journal of Petroleum Technology71(05), 37-41.

Images: “GlassPoint EOR Kern County CA” by GlassPoint Solar licensed under CC BY SA 3.0 ; “Class II EPA UIC” by Top Energy Training, courtesy Coleman Tharpe; “GlassPoint EOR Kern County CA” by GlassPoint Solar licensed under CC BY SA 3.0; “Schematic-Diagram-Fig.-3.11” by Isbell, 2016; “Numeric-summary – Isbell” by Isbell, 2016; “Huff-n-puff-Fig.-5.17” by Isbell, 2016