Work in an MIT lab may help energy companies withdraw millions of additional barrels of oil from beneath the sea floor.
Typically, companies recover only 30 percent to 40 percent of the oil in a given reservoir. Since a single reservoir may contain a billion barrels total, increasing that "recovery efficiency" by even a single percentage point would mean a lot of additional oil.
Toward that end, Assistant Professor David Mohrig of earth, atmospheric and planetary sciences and Carlos Pirmez, a research geologist from Shell International Exploration and Production Inc., have been examining one type of geological formation of interest to industry -- channels filled with highly permeable and porous sedimentary deposits that extend deep below the sea floor.
These structures form when sediment-laden currents flow off the continental shelf and into channels on the deep-ocean floor, dropping sand, silt and clay as they go. Over many thousands to millions of years, the channels can become filled with porous sandstone covered by impermeable mud -- a perfect trap for oil and gas that seep up from below.
Over the past 20 years, energy companies have withdrawn significant amounts of oil from such buried channels. But they could extract even more if they understood the channels' internal structure.
"If we could understand how they develop, then we would also understand a great deal about what they're composed of -- the distribution of clay, silt, sand and even gravel that they're built out of," Mohrig said. With a better understanding of porosity and permeability within a channel, companies could more accurately determine how much oil is present, where it is located and how quickly it can be withdrawn.
Researchers have been re-creating the formation of submarine channels in Mohrig's Morphodynamics Laboratory using a 5-meter-square sand table.
The experiments have yielded results that the collaborators call "counterintuitive." On a map, the sinuous submarine channels look like meandering surface rivers. However, they exhibit behaviors that are markedly different and -- to us surface-dwellers -- totally unexpected.
The behaviors stem from differences in density. Water in a river is about a thousand times denser than the fluid it flows through -- air. As a result, a flow tends to remain confined to its riverbed, escaping over the banks only rarely. In contrast, the current running through a submarine channel may be only 10 percent denser than the seawater around it. Thus, the current can spill out of its channel more easily and frequently than a river might.
That difference explains several unexpected findings. For example, at times the bottom of the current sloshes almost all the way up the edge of the channel and then back down again. And at bends, the current may go straight, pouring up and over the bank and dropping its sediment outside the channel -- an outcome with important implications for energy companies as they plan to drill.
Because of their close and continuing involvement in the scientific investigation, the Shell researchers are prepared to put the research findings to practical use. "The experiments that David is doing have never really been done before, so we're learning new things about how channels are put together," Pirmez said. "We're getting new ideas, new concepts that may change the way we think about the subsurface."
The result should be improved predictions, reduced uncertainty and more efficient recovery from these oil-rich submarine formations.
This research was supported by Shell International Exploration and Production Inc. through the MIT Department of Earth, Atmospheric and Planetary Sciences.
