Teasing natural gas from Cook Inlet
Difficult seismic, more subtle plays and discontinuous sands all add to the challenges in finding and producing gas from the basin
Over the years Alaska’s Cook Inlet basin has proved to be a prolific source of gas.
But with oil and gas fields found decades ago becoming depleted and the easy finds on major geologic structures all discovered, people now need to seek new gas in more elusive prospects, tucked away in the basin’s challenging geology, as well as by continued development of existing fields, Julie Houle, resource evaluation section chief in Alaska’s Division of Oil and Gas told the House Special Committee on Energy March 17.
“As exploration geologists we think there is more gas to be found in Cook Inlet, and we think it’s going to be found both in existing fields and in new exploration play types,” Houle said.
Those easy finds made years ago on the big structures resulted from 2-D seismic surveys — the division would like to see companies now shooting some 3-D seismic to pursue new exploration concepts in the basin, she said.
The division thinks that there is new gas to be found in what are known as stratigraphic traps, situations where the way in which the rock strata were laid down has resulted in a reservoir rock becoming juxtaposed with an impervious seal rock.
“It will take someone in the Cook Inlet with money and a lot of fortitude and courage to keep going, looking for these subtle stratigraphic traps,” Houle said.
Dome-shaped structures, to the side of main folds in the rock strata, are other possible new exploration targets, she said.
Difficult seismicBut seismic data from the Cook Inlet basin are notoriously difficult to interpret. The river-lain sands that form gas reservoirs in the basin tend to be too thin to resolve in the images depicted in seismic cross sections of the subsurface, while the coal seams that pervade many of the rock sequences also tend to distort those seismic images.
The reservoir sands also tend to be discontinuous, forming thin lenses rather than continuous strata. Consequently, when drilling through a prospective structure an individual sand that may contain viable pay can easily be missed. And a gas discovery in one well requires a series of delineation wells to locate all of the various sand bodies that might produce gas.
At the same time, testing of the gas pressure may be necessary to determine whether a reservoir sand at one place connects with a similar reservoir sand nearby.
“All sands don’t hit all wells and you really need close control in order to figure out what is in communication,” Houle said. “… You could go back in, in a field, and drill in between wells and you could actually get more gas.”
In addition, a sand that has been producing gas for some time tends to fill with water if the well that taps that sand is shut-in.
“That’s why once you drill a well in Cook Inlet you want to keep it producing,” Houle said.