Unconventional oil and gas reserves and production have significantly changed the energy game in North America – and for the most part this turnabout in domestic E&P has come from shale zones, which have been long recognized as source rocks for other reservoirs.
Now that many hydrocarbon-rich shales are the targets of the drill bit, geoscientists are working diligently to develop new approaches and technologies to better understand and produce them.
Horizontal drilling and multi-stage hydraulic fracturing have been key to economic development of these rocks.
These are the newsmakers that the public reads about.
There’s a raft of “behind the scenes” advanced technology applications that long precede the drilling stage. After all, these zones tend to be considerably more esoteric than the ordinary sandstone reservoir – or even the unconventional tight sandstone target.
Competent assessment of unconventional prospects demands integration of geology, geophysics, geomechanics, petrophysics and engineering, according to AAPG member Scott Singleton, ResSCAN technical manager GeoVentures group at ION Geophysical Corp.
He succinctly summarized what each of these disciplines brings to the table:
“All of these data types are essential to piecing together a complete reservoir assessment,” Singleton emphasized.
He also noted that considerable attention has been directed to modifying the traditional conventional geophysical reservoir characterization workflow, to offer useful outputs to integrated asset teams in unconventional resource plays. These teams typically are comprised of both reservoir and drilling engineers.
“With this serving as the impetus, geophysicists are consolidating their efforts in four principal areas,” Singleton noted.
Singleton said they have adopted this philosophy in their unconventional reservoir characterization workflow, where geophysics alone is insufficient to delineate true rock properties, the same as with conventional reservoir characterization.
Using the workflow they formulated, he and his colleagues conducted a study focused on a Devonian-age Marcellus shale prospect in Pennsylvania.
Singleton pointed out this effort was undertaken while he was at RSI.
The study results demonstrate that petrophysics, rock physics, geophysics and geology can successfully be integrated with reservoir and production engineering to characterize shale reservoirs.
Digging down, each data type yields information typically not provided by the others.
“The project objective was to determine production drivers at the wellbore using all available data and then to extrapolate this set of criteria away from the wellbore using only seismic data and its derivatives,” Singleton said.
“The results showed that rock brittleness and also pre-existing fractures can impact well production,” he noted. “Additionally, they showed that a comprehensive suite of fracture characterization methods, such as anisotropy and principal stresses, are necessary to effectively determine if a pre-existing fracture zone will reopen or stay closed when being subjected to hydraulic fracturing in this area.”
He asserted that a more robust method might be to incorporate reservoir quality data in the production prediction, e.g. gas-in-place, porosity and thickness.
Over the course of the Marcellus study, Singleton had an “aha!” moment – involving an area far removed geographically from Pennsylvania.
“In addition to the Bakken, the production statistics available show that the Marcellus and the (Cretaceous-age) Eagle Ford in south Texas are the hottest shale basins in the United States,” he said. “Even a peripheral observation of these two basins indicates there’s a bunch of similarities.
“My immediate thought was, ‘hmm, I wonder if the analytical techniques I developed for the Marcellus are applicable to the Eagle Ford?’” Singleton said.
“I’ve seen others do exactly the same thing on the Eagle Ford that I did on the Marcellus,” he continued. “We’ve shared techniques, so I know we’re both going down the same path – I’ve seen their results, and they’ve seen mine.”
Singleton assembled a list of comparisons and contrasts between the Marcellus and the Eagle Ford (EF):
♦ Similarity: Depositional Environment
♦ Difference: Lithology
♦ Difference: Structure
♦ Difference: Fracture Characterization
♦ Similarity: Reservoir Characterization