Produced Water Mobility Inhibition Polymer Flooding
Jay C. Reynolds, Applied Mobility, LLC, Oil City, Louisiana
Numerous reservoirs in the US are prone to early transition to high water production and produce at their economic limits in spite of often having 75-80% or more of their OOIP remaining in these developed and de-risked fields.
It is the shallow reservoirs that were discovered first and mis-managed in the early days which are now in the hands of the Mom and Pops, who are notoriously late technology adopters. This is where the big stranded reserves are in the US. The best combination for this process is homogenous geology, relatively low gravity oil, close well spacing and a strong, active, bottom water drive. That combination makes for early water coning and high percentages of stranded reserves in an active bottom water drive reservoir.
A oil cut (WOR) of 1,000/1 is typical for the Nacatoch B Sand in northwest Louisiana; a terrible Adverse Mobility Ratio. In the Nacatoch B, the oil wells are essentially water wells that make oil as a contaminant once the water cones in. About 10,000 of these wells were drilled, a significant number during three separate periods of intense promotion because these wells had good flush production and frequently paid out in a couple of months before the water came in.
The reservoir is acting exactly as physics dictates. This oil is 19-21 gravity and it takes pumping the well down about 150’ to provide a sufficient pressure drop to mobilize oil to the well bore and that is impossible without changing the downhole physics at work. Nacatoch oil is about 250 centipoise viscosity while our water is 1 centipoise with permeability as as high as 3,000 millidarcies. As a consequence, pumping these wells down is impossible because the water channels will expand to accommodate any given pump capacity.
These factors, and the large stranded reserves, led to the develop an inexpensive polymer treatment for water control and enhanced oil production for reservoirs with a low permeability contrast such as those of the Caddo Pine Island Field’s massive blanket sand, the Nacatoch B Reservoir.
A dry polyacrylamide polymer of special design is mixed on the fly and injected into the water bearing portion of the sand with a Mobile Gel Unit. You could think of it as inflating a balloon underground and as long as you are injecting more than you are withdrawing the area affected will continue to expand. That makes this process site specific, you can keep the ‘polymer balloon’ and the oil on your leasehold instead of mobilizing the oil horizontally, potentially off of your leasehold as with a traditional displacement type polymer flood. The produced oil and polymerized water is separated in the usual way and the polymerized water, having value now, is recycled. Bottom line is turning your worst enemy, water, into your best friend.
Think of this as a polymer flood that operates vertically instead of horizontally - that lets oil move in the direction nature wants it to go, vertically. Injection continues until the polymerized water surrounds nearby producing wells. That lets the operator pump those wells down because the wells no longer have access to low viscosity native water. This relieves enough hydrostatic pressure in the well bore to let the reservoir energy mobilize the more viscous oil to our well bores at higher rates.
This technique lets an operator keep the oil on their lease while qualifying as Tertiary Enhanced Oil Recovery on a voluntary leasehold unitization basis in many states.
Without mobility control the reservoir can only be shown about a 20 psi pressure drop no matter what capacity pump is run. A 20 psi pressure drop will move all of the water you can possibly pump through a high permeability sand but transports very little oil. With produced water mobility control the well can now be pumped down. Mixing polymer into the water dramatically improves the mobility ratio and lets us pump the well down to take advantage of the reservoir pressure.
To accomplish polymer placement in the desired portion of the reservoir, we continuously hydrate, blend and inject polymer at our target viscosity. Viscosity is targeted such that the polymer blend preferentially flows into the water productive regions of the sand while not displacing the oil horizontally.
This development began by asking, ‘What would the cut be if the water and oil were the same viscosity?”
“Change the nature of water and the physics downhole changes and a new equilibrium state with respect to how oil and water move relative to one another is established. Darcie’s Law tells us that only three things determine the rate of fluid movement through our sand; pressure, viscosity and permeability. Which of those is easiest and cheapest to change on a large scale? The viscosity of water.
Unlike many EOR methods that rely on changing the characteristics of the oil, where the benefit is lost when the oil is produced, the polymerized water is recycled and what used to be our waste product, water, becomes an asset.
James Sutphen of SNF added, “This has been a very good collaboration thus far. Jay has come up with a game changer for a market that was not risk tolerant. He knew from his perspective as an oil producer the game had to be changed or else geology and depletion would put him out of business. There is a limit to how much fluid you can produce and separate and stay in operation.”
Jay Reynolds (318) 208-1137, firstname.lastname@example.org