By H. JACK MEYER
Editor’s note: H. Jack Meyer is a geologist in the Gas Storage Development Department at NW Natural and specializes in storage reservoir development. He also is a speaker for AAPG’s VGP program.
The Geophysical Corner is a regular column in the EXPLORER, produced cooperatively by the AAPG Geophysical Integration and SEG Interpretation committees, and edited by R. Randy Ray. This month’s column is titled “3-D Seismic and Underground Gas Storage, or ‘Will My Gas Be There When I Want It?’”
3-D Helps See Through the Mist
Precision Is Now the Norm for Pool Development
Click on image to enlarge.
The storage field geologist, while worrying about such things as spill points and thief zones, is primarily concerned with “location, location, location.”
Is the pool where it is supposed to be?
Do the leases cover it and all possible escape routes?
Can I get a well into the reservoir where it needs to be?
The recent expansion of the Mist Storage Facility in northwest Oregon demonstrated that a well-designed 3-D seismic survey can yield an accurate geological framework from which these issues and more can be addressed.
The Mist Gas Field is located about 60 miles northwest of Portland, Ore., in the Coast Range Mountains near the town of Mist. The field is structurally very complex and consists of individual gas pools located in discrete fault blocks that range in size from 20 acres to 120 acres.
The productive interval, Clark and Wilson Sandstone of the upper Eocene Cowlitz Formation, is found at depths ranging from 1,200-2,700 feet. The marine deltaic reservoir sandstone is highly porous and permeable and has AVO characteristics similar to a class 3 gas sand of Rutherford and Williams (Geophysics, 1989).
The gas shows as a strong bright reflector because of increased amplitude with offset.
An accurate reservoir model is a prerequisite to successful gas storage
development. In the Mist Gas Field,
A subsurface geologic map of the depleted pool was constructed that fit the reservoir model developed from production.
From the mid-1990s on, the deregulated gas market has placed prime value on deliverability. A high volume horizontal well, which can replace several vertical wells, is the “new” tool that enables the Mist Storage Field to respond to the changing market. This fundamental shift in field operation requires that the geologic mapping be accurate enough to ensure that a horizontal well encounters the reservoir and stays inside it, as well as being detailed enough to guide and constrain reservoir modeling.
There is also a more critical reason for a crystal clear image of a storage reservoir; product security. There is a history in the storage industry of stored gas migrating to places out of control of the operator. Large “buffer” areas generally surround a storage field. An accurate geologic structure map of the reservoir is paramount.
At Mist, this meant acquiring 3-D seismic data over a 3.9-square-mile area of the field.
Specifications, Definition and Design
The shallow depth of the reservoir, high frequency content of the 2-D data and numerous steeply dipping fault surfaces dictated that a 40-foot bin size was required to clearly image the target.
Groves of 150-foot tall Douglas Firs, thick forested undergrowth, and steep topography (many slopes >100 percent) complicated data acquisition, not to mention data processing. Dynamite in shallow holes augured with heli-portable drills was the energy source.
Figure 1 is the subsea structure map of the top of the reservoir sand derived from the 3-D data surrounding a gas pool that was converted to storage. It is the key product from the 3-D seismic survey:
The accuracy of fault location and throw provided by the 3-D image allows the geologist and reservoir engineer to model a fault and its impact on reservoir transmissivity and water migration.
Figure 2 is a vertical seismic section parallel to the path of an injection/withdrawal well. The ability to visualize the well path is one of the powerful tools of a 3-D data set.
The depth to the gas-water contact is a critical piece of data for storage pool development when horizontal wells are to be used as injection/withdrawal wells. The objective is to cut as much of the reservoir rock as possible to defeat any permeability barriers while stopping comfortably short of the water leg.
Figure 3 is a cross section through a depleted pool that illustrates the dynamic nature of the aquifer. During primary production, water encroached into the reservoir several tens of feet and defined a “new” gas-water contact. While the water invaded the reservoir from the bottom up, the “new” gas-water contact is not necessarily flat across the entire reservoir. At Mist, variations resulting from changes in internal stratigraphy or faulting may be of a magnitude that would affect the performance of a horizontal well.
Figure 4 is the 2-D seismic line shot through the pool prior to production. The line shows a strong trough amplitude anomaly (red) at the top of the reservoir sandstone. It also shows a strong and flat peak anomaly (blue) that tunes as it approaches the down dip edge of the reservoir. The flat peak event represents the gas-water contact.
Seismic data clearly imaged the gas-water contact, and with a good velocity model this interface can be converted to depth.
Figure 5 is a parallel line from the 3-D seismic survey, shot a number of years after primary production. The top of the reservoir sandstone has a negative (red) amplitude response. However, the once visible gas-water contact has disappeared.
One plausible explanation is that the “physics” of the reservoir changed. Production reduced the reservoir pressure, and water encroachment changed the density and gas saturation at the original interface and throughout the “encroached” interval.
The resistivity and neutron density logs of Mist storage pool development wells (figure 6) clearly identify the “new” gas-water contact (in most instances, the encroached zone is also identifiable on the neutron density log). The sonic log, however, continues to respond to the original gas-water contact. The residual low gas saturation associated with the original gas-water contact is still an acoustic contrast, but the change in density as a result of water encroachment has decreased the reflectivity.
At the “new” gas-water contact there is a density contrast but only a small acoustic response. In addition, there has been an increase in the bulk density of the highly porous reservoir rock as it compacted in response to pressure reduction.
The physical changes within and to the reservoir may be combining to mask both the “new” and the “old” gas-water contacts. (See Ian Jack’s 1998 SEG publication Time-Lapse Seismic in Reservoir Management for a discussion of rock physics and 4-D seismic where similar effects are observed in other gas fields over time.) Thus the 3-D seismic data set could not be used to model the “new” gas-water contact across the reservoir. The engineering reservoir model had to be relied on for estimates of vertical water movement and for predicting the position of the newly established gas-water contact.
In summary, the application of 3-D seismic technology to the expansion of the Mist Storage Field provided maps with the geologic accuracy necessary to enable the economic utilization of horizontal well technology.
Because of this advanced imaging technology, precision placement of wells is now the norm for storage pool development.