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,
2-D seismic and well data were used to discover and develop gas pools.
In the late 1980s the conversion of a depleted pool to storage utilized
this same data set, augmented by more “observation” well data to define
the pool’s boundaries.
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 reservoir
engineer uses it to construct the reservoir model for production and
drainage studies.
- The geologist
uses it to assign target depths to wells, to map gas migration paths,
to pinpoint fault locations and measure fault throws for “thief” assessment.
The geologist also uses it to site observation wells outside the pool
to monitor the spill point(s).
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.
Gas-Water Contact
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.
What happened?
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.
Summary
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.