In the mid-1990s several of the partnership states participated in a project to update the US DOE Tertiary Oil Recovery and Information System (TORIS). To do this they inventoried the largest fields in each state to account for 70% of proven reserves. Among other data, they calculated the original oil in place (OOIP) and recoverable oil in place (ROIP) for these fields. The results show that more than 13 billion bbl of oil remain in produced reservoirs in Kentucky, Ohio, Pennsylvania and West Virginia. While Michigan did not participate in the TORIS project, researchers there are eager to prepare such an inventory. William Harrison III and colleagues at Western Michigan University believe Michigan has at least 2.3 billion bbl of ROIP.
While the researchers gather and analyze this data for a new inventory, others are busy gathering data from current and past secondary and EOR projects in the region. They want to see what has worked and what has failed, and why.
Currently, only one miscible CO2 -EOR project is underway within all of the Northern Appalachian and Michigan basins. Within northern Michigan a unique set of circumstances has led to CO2 flooding of Silurian Niagaran Reef fields using CO2 recovered by a gas processing plant from natural gas produced from the shallower Devonian Antrim Shale reservoirs. Associated CO2 (CO2 content is 15% to 30%) is currently removed from Antrim natural gas at six centralized gas-processing plants in northern Michigan. The majority of this processed CO2 is currently vented to the atmosphere. However, Core Energy LLC is taking CO2 from the emissions stream of one plant and is capable of handling up to 11 MMcf/day in its compression and pipeline facilities.
Using this CO2, Core Energy has successfully flooded two Niagaran reef fields (the Dover 33 and Dover 36). With an estimated 30% primary recovery factor of OOIP, actual CO2 /EOR incremental oil recovery was 11% and 7%, respectively, in these two fields, with combined recovery at 42% and 37%. The operators predict that tertiary recovery may eventually approach primary recovery. They have now expanded their operations to additional reef fields.
More than 270 million bbl of oil has been produced from nearly 2,000 wells in more than 1,000 northern reef trend fields from reservoirs at depths of 5,000 ft to 7,000 ft (1,525 m to 2,135 m). Ultimately, by use of waterfloods and CO2/EOR it is estimated that an additional 90 million bbl of oil to more than 250 million bbl of oil may be producible from this trend. The only other current known use of CO2 for EOR in the region is a fairly novel project in Lee County, eastern Kentucky. Here, the Big Andy Pool is a shallow oil field operated by Bretagne G.P. that is undergoing cyclic nitrogen repressurization with immiscible CO2 injection for EOR. The field has produced more than 2.3 million bbl of oil from more than 600 producing wells since its discovery in 1961. The Silurian Keefer Sandstone ranges from 65 ft to 75 ft (19.8 m to 22.9 m) in thickness at depths below 1,200 ft (366 m), has about 16% porosity and exhibits permeability from less than 1 md to greater than 100 md. Ambient air is compressed and dried, and a membrane unit is used to produce a stream of 95% nitrogen and 5% oxygen, which is injected into the annulus of producing wells on a selected lease. When reservoir pressure builds to a threshold, nitrogen injection ceases, and wells are selected for CO2 injection. CO2 is injected at 0°F (-17.7°C) to a reservoir pressure of about 150 psi. Each producing well is treated with 20 tons of CO2 about once a year. Recovery efficiency is around 1,500 cf of CO2 per bbl of oil. At the end of each injection phase, the wells are shut in for a soak period (approximately 30 days), after which they are put back into production. This cycle alternates among the leases operated by Bretagne, with multiple wells in each of the phases of the cycle at any one time. As of 2000, average production per well had increased from about 0.5 bo/d to 1.3 bo/d. Overall, production continues to increase with the refinement of the cyclic process, new wells and the addition of CO2 injection (Figure 2).
More work to be done
When comparing secondary recovery projects among states in this region, a surprising discrepancy jumps out. While most of the states in the region have pursued secondary recovery fairly assiduously, data shows Ohio to lag far behind in secondary recovery production and attempts. This is surprising because Ohio typically leads the Appalachian states in the amount of oil produced and wells drilled. More than 1,000 oil and gas fields have been discovered and well more than 1 billion bbl of oil produced in the state, yet state records show that only 13 fields have had secondary recovery attempted. Production from secondary recovery in Ohio is typically less than 0.5% of total production as compared to as much as 50% in neighboring states.
A large percentage of wells drilled and hydrocarbons produced in Ohio in recent decades has been from the lower Silurian "Clinton" sandstone, at typical producing depths of about 3,500 ft to 5,000 ft (1,068 m to 1,525 m). This formation is usually fairly tight, and the few waterflood attempts that have been made in this formation have not been very successful. Thus, this explains a portion of why little secondary recovery has been performed in the state. However, there are other formations such as the Cambrian Rose Run Sand, Trempealeau Dolomite and Devonian Berea Sand that should lend themselves well to EOR operations.
Researchers are encouraged by results obtained from low-permeability fields in the west that did not respond well to early waterflood attempts yet have responded very well to CO2 flooding. Thus, researchers at the Ohio Geological Survey are eager to model a CO2 flood of a Clinton field, then perhaps perform a pilot injection project. Should this prove successful, there are hundreds of millions of barrels of oil that may be recoverable from Clinton Sandstone reservoirs alone.