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Figure Captions

Figure 1. Subsea structure map to top of reservoir sandstone. The map was created by multiplying the time structure map, as interpreted from the 3-D data set, by the velocity gradient map. Color transition from white to green represents the pool’s original gas/water contact. Grid distance is 400 feet on a side.

Figure 2. Vertical seismic section parallel to the path of a "horizontal"well. Looking at many sections like this, and at time slices and flattened time slices, is invaluable when planning a well path. The amplitude anomaly continuity gives a "feel" for a variability of internal reservoir stratigraphy or structure.

Figure 3. Cross section through a Mist gas pool after primary production. Note the position of the “new” gas-water contact. Discovery well on left; injection/withdrawal well on right. Vertical exaggeration 1.7-to-1.

Figure 4. Two-D seismic line acquired prior to primary production. Note strong trough (red) amplitude anomaly at top of Clark and Wilson Sandstone (C&W), and strong, flat peak (blue) amplitude anomaly at the gas-water contact (GW). (Figure published with permission of BP.)

Figure 5. Vertical seismic section from 3-D survey parallel to 2-D seismic section of Figure 4. The top of the Clark and Wilson Sandstone (C&W) has a strong trough (red) amplitude anomaly. There is a “reflict” amplitude anomaly near the downdip edge of the original gas-water contact (GW). There is no amplitude anomaly associated with the “new” gas-water contact (NG/W).

Figure 6. The resistivity and neutron density logs of the mist storage pool development clearly identify the “new” gas-water contact. The sonic log, however, continues to respond to the original gas-water contact.

Mist Storage Facility

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.

Mist Gas Field

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.

Gas Storage

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 boundaries of the pool. 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 downdip 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 Jack, 1998, for 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.

References

Jack, Ian, 1997, Time-Lapse Seismic in Reservoir Management: Distinguished Instructor Series 1, SEG, Tulsa, Oklahoma.

Rutherford, S.R., and R.H. Williams, 1989, Amplitude-versus-offset variations in gas sands: Geophysics, v. 54, p. 680-688.

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