We discussed phase decomposition and its applications in the February installment of Geophysical Corner. It is an interesting technique that can decompose a composite seismic signal into different phase components, and which in turn can help with the characterization of thin target sandstone or carbonate reservoirs. Here we extend that discussion to the application of phase decomposition as a reservoir management tool, with the odd phase component (sum of plus 90 degrees and minus 90 degrees phase components) showing better correlation with the wells that control the injection and withdrawal of a natural gas storage reservoir in Denmark.
Determining Properties of Natural Gas Storage Reservoirs
Natural gas storage serves as a good buffer in the supply-and-demand cycles and ensures reliable and responsive delivery of natural gas during times of peak demand. Natural gas storage is usually carried out in depleted oil and gas reservoirs, aquifers and salt-cavern formations, with the depleted oil and gas formations close to consumption sites being more common. In other cases, natural aquifers have been converted to natural gas storage reservoirs. Besides the physical characteristics (porosity, permeability, capacity and retention capability) of a reservoir, the deliverability rate and injection rate are other considerations for natural gas storage reservoirs.
In Denmark, there are two underground gas storage facilities that serve as a buffer for supply of gas from the North Sea. The first is the salt caverns in Jylland, and the other is a deep aquifer at a depth of 1,500 meters near Stenlille (figure 1), located approximately 30 kilometers southeast of the Havnsø CO₂ storage prospect.
Storage of a gas into a geological formation is not a straightforward process and requires its complete description, which is usually collected from core, well logs, seismic and other types of data. A secure geological container is required for the gas to be stored in the target formation successfully. Capacity, containment and injectivity are essential properties of a container that need to be understood. While capacity of a formation mainly depends on porosity, permeability plays an important role for injectivity. Likewise, containment of a formation is a function of fracture systems under the present-day stress conditions, or enhanced injection overpressures that might be generated. All these properties can be determined from seismic data in the form of reservoir characterization of the target interval. The seismic data used for this purpose is a stacked 3-D seismic volume from Denmark shot over a natural gas storage structure.
Natural gas has been injected and stored at Stenlille since 1989, where the reservoir occurs within a domal subsurface structure and is covered by the tight Fjerritslev Formation caprock (figure 2). The Upper Triassic Gassum Formation forms the natural gas storage reservoir and consists of interbedded sandstones and mudstones. Overlying the Gassum Formation is the 300-meter-thick Lower Jurassic sandstone Fjerritslev Formation, which consists of marine mudstones and shales, and is the regional caprock. The storage capacity of the Stenlille structure has been estimated to be three billion cubic meters, but due to reservoir heterogeneity, the gas is stored in several separate zones. Below the Gassum Formation are the impermeable mudstones of the Vinding, Oddesund and other older formations, notably the Zechstein Formation at approximately 2,800 meters below the surface, where the salt movements resulted in the formation of the Stenlille structure.
At the Stenlille facility, the natural gas is stored in an anticline structure defined by six sandstone reservoir zones in the Gassum Formation. The Gassum sandstone reservoir is approximately 140 meters thick, but only the upper 40 meters are used for storage in order to prevent gas migration through the spill-point. The upper 40 meters are divided into five gas storage zones segregated by thin shale beds (figure 3) which operate as two separated units, namely zones 1-3 operating as one integrated unit, and zone 5, which is located below. The estimated total gas storage volume is 3 billion cubic meters within the four-way closure, covering an area of approximately 14 square kilometers.
Attempts at Reservoir Characterization
Deterministic poststack impedance inversion (see the May 2015 Geophysical Corner) as well as a neural network approach was used to predict porosity volume, as well as to understand the facies variation within the target zone. Probabilistic impedance inversion was also carried out for predicting the gas probability volume. Discontinuity attributes such as multispectral coherence (see the July 2018 Geophysical Corner), multispectral curvature (see the November and December 2007 installments of Geophysical Corner), fault likelihood (see the August 2016 Geophysical Corner), were carried out on post-stack seismic data for understanding fault/fracture system to mitigate the gas leakage problem and have been discussed elsewhere. A novel technique called “phase decomposition” was also run on the seismic data. It can improve reservoir characterization by decomposing a composite seismic signal into different phase components. The technique is particularly useful in those areas where thin-bed interference causes the phase of the input seismic response to differ from the phase of the embedded wavelet in the data. For a zero-phase wavelet in the data and thin low-impedance layers below tuning thickness, the waveform phase response generated after carrying out phase decomposition is found to be minus-90 degrees, which stands out anomalously. On the contrary, a corresponding high-impedance thin layer exhibits a similar plus-90-degree phase waveform response. By generating a synthetic response with use of well data and a zero-phase wavelet, such observations for thin reservoir layers can be understood with confidence and correlated with real seismic data. Phase decomposition can help immensely in direct interpretation of seismic data in terms of reservoir and non-reservoir zones, among other applications. For complete details please see the February Geophysical Corner. Here we discuss a comparison of results from some of these applications and how they help understand the data for injection/withdrawal of natural gas in the target interval.
There are 20 wells drilled over the Stenlille 3-D area, of which fourteen are operational gas injection and withdrawal wells, and six are observation wells (ST-3, 4, 5, 6, 10, and 15) for monitoring pressure in the aquifer around the reservoir and in the caprock (figure 1). Of the 14 operational wells for zone 1-3, wells ST-2, 7, 9, and 11 are the main injectors and ST-12 is the main producer, though in recent years ST-11 and ST-12 have been the main injection/withdrawal wells, respectively. All wells are designed as injection/withdrawal wells. Gas injection began in 1989 in zone 1-3 and was registered during the first few years of the gas storage establishment in ST-4 observation well. Then from June 1995, gas was subsequently injected into zone 5.
In figure 4a, the seismic stratal slice shows the gas anomaly for zone 1-3 in blue with the wells ST-01, 02, 04 and 19 touching it, implying they are associated with storage or retrieval of gas in the reservoir. Multispectral coherence (see the July 2018 Geophysical Corner) has been overlaid in black, using transparency. The equivalent display in figure 4b is from the odd phase (combination of minus-90-degree and plus-90-degree phase components) component volume, where the blue anomaly is seen as more compact, exhibiting better variation as compared with the other two displays. Besides, the faults marked with purple block arrows appear to be acting as fluid barriers, with the odd phase component following them closely. Another encouraging observation is about the extent of gas migration southwest in the direction of the light blue arrow, which appears to be more convincing on figure 4b than 4a. The equivalent display shown in figure 4c is from the gas probability volume was generated using probabilistic impedance inversion using stacked input seismic data. We notice that the anomaly corresponding to gas probability is spread out and does not exhibit a more constrained fluid distribution as the seismic odd phase component (figure 4b). Notice that the odd phase map predicts less gas around ST-2 and in the area southeast of the most prominent fault lineament in the northeast-to-southwest direction. This is an interesting observation because the injection history from ST-2 in figure 6a shows that approximately 130 cubic meters of gas were injected into this well prior to the seismic survey. This well also seems to be located in isolation from the other wells on the southern side of the prominent fault barrier. The odd phase map still shows a blue anomaly close to ST-2, though much less outstanding than in the gas probability map.
Also, the gas distribution from the odd phase map appears to be more convincing with regard to the gas fluid movement northwest, where it nicely conforms with the multispectral coherence attribute and looks more like what might be suspected as the shape of a growing gas plume (more like a circle/ellipse shape in a homogenous sandstone reservoir, as is seen more clearly in zone 5 map in figure 5, whereas the gas probability map predicts more of a fingering/channeling fluid distribution that conforms not so well with the northwest fault barrier.
In a reservoir management context, small deviations in the spatial extent of a hydrocarbon anomaly as seen on different map displays could be valuable information in order to improve the understanding of fluid movement within the reservoir. Thus, what we surmise from these observations is that the odd phase component can provide detailed information, which can be more accurate than what is seen on the input seismic amplitudes. A very good correlation of the horizontal wells operational for this zone is seen, with the toe of each well falling in the blue zone. Well S-19 is only slightly deviated and not be treated as a horizontal well.
The equivalent stratal slice comparison display at the level of zone 5 shown in figure 5 again shows the gas anomaly to be better defined on the odd component (figure 5b) display than the seismic amplitude display (figure 5a). The gas probability display (figure 5c) indicates the gas anomaly as well defined, but is shrunk in size at the lower end as indicated by the light blue arrow. Again, the faults marked with purple block arrows on the multispectral coherence appear to be acting as fluid barriers, with the gas anomaly following them closely on all three displays. The gas anomaly also shows much better correlation of the dark blue areas on the odd phase component with the horizontal wells shown overlaid, which are operational for this zone.
Discussion on Reservoir Management at Stenlille
The volume of gas injected/withdrawn in zone 1-3 per year for the different wells is indicated by the curves shown in figure 6a and depict a gradually increasing trend during the establishment of zone 1-3 through the ‘90s. The downward swings in the curves are seen whenever wells were used for withdrawal. For zone 5 the injector/withdrawal curves as shown in figure 6b from 1995, where wells ST-2, 8, 14, 16 and 18 are seen as the main injectors, though the contribution of ST-2 has been minimal. However, if we look at a longer time span, say to 2017 as shown in figure 6c, we see that after 2010 it was used to inject a smaller amount of gas. Notice, most of the wells are annually made to shift between their roles from injection to withdrawal and vice-versa. The vertical black line on both graphs is a reference point for the Stenlille 3-D seismic acquisition between August and October 1997. The total gas (cumulative sum of injection minus withdrawal) volumes for zones 1-3 and 5 from the wells indicated in the inset are shown in orange and blue. The fall and rise in the gas volume as seen on the two curves is related to the injection and withdrawal cycles.
As stated earlier, the analysis attempted was carried out on poststack seismic data acquired and processed in 1997. Further research could include reprocessing of the data to benefit from modern processing techniques. Not only would prestack seismic data become available for running simultaneous impedance inversion to constrain the non-unique solutions, but an accurate velocity model could be generated and used for performing a robust depth conversion. Using the detailed historical injection/withdrawal data available, different volumetric calculations could be carried out and compared with the total gas registered at the time of acquisition of the seismic survey. Not only this, if the Stenlille area is declared to be the site for the first onshore CO₂ storage demonstration in Denmark, the maps that have been shown above would become very valuable in a reservoir management context, as small deviations between the maps could help to improve understanding of fluid movement within the reservoir. Also, the fluid migration simulations will be crucial for the reservoir management team, which could also benefit from additional time-lapse surveys.
Conclusion
In this article the application of phase decomposition has been introduced as a reservoir management tool, with the odd phase component (sum of plus-90-degrees and negative-90-degrees phase components) showing better correlation with the wells that control the injection and withdrawal of natural gas in the Stenlille reservoir zones. The present application of an odd phase component has been attempted on legacy data, but in the likely scenario in which the legacy data were reprocessed to take advantage of the modern reprocessing techniques and their value-addition, it is anticipated that more accurate results would be forthcoming.