Since 1984 the Reservoir Characterization Project at Colorado School of Mines has been working on shale reservoir development. RCP began its first study at Silo Field, Wyo., recording the first land 3-D multicomponent (9C) seismic survey focused on characterizing the Niobrara fractured reservoir. The result of RCP’s work led to the first horizontal drilling in the Rockies in 1990 and led to the drilling of the most successful well in the field.
Today, the Niobrara is still undergoing development drilling in the deep Denver Basin in Wyoming and Colorado. How can multicomponent seismic monitoring enhance the potential of this development? Through RCP, the industry and academia work together to bring about innovation in shale reservoir characterization. For 40 years, we have conducted 3-D and 4-D (time-lapse) multicomponent seismic surveys as a means of improving recovery by better understanding the reservoir and processes operating within. Monitoring enables us to visualize changes in the reservoir and prompts us to quantify these changes in terms of dynamic reservoir properties.
In order to understand the value of multicomponent seismology in the application to shale reservoir exploration and development we need to step back and look at the physics of multicomponent seismology. Simply put, conventional P-wave seismology is a scalar process involving sound/acoustic propagation in the subsurface. Shear wave propagation is a vector process that involves ground shaking with particle motion largely in the horizontal plane. Shear waves are what take down buildings during large-scale earthquakes. Shear waves enable volumetric monitoring of the subsurface whereas P-waves are more suited to detecting acoustic change interfaces in the subsurface. As such, it is understandable that shear waves have added value in the characterization of shale reservoirs due to their inhomogeneous and anisotropic behavior. Simply put, shear wave azimuthal anisotropy links to permeability which is a critical reservoir property to characterize within shale reservoirs. The permeability field is dynamic, which is why we need to do time-lapse reservoir monitoring in shales.
Shale reservoirs are a historical part of Colorado’s hydrocarbon industry, as documented by Kira K. Timm and Matt Silverman in their Historical Highlights article in the May 2023 EXPLORER. Silo Field (figure 1) is part of that legacy, even though it occurs in the state of Wyoming. It was truly the start of the modern oil boom, because the field was the focus of the first horizontal drilling as documented by Harold Hamm in his book, “Game Changer.” What Hamm did not discuss is the role that RCP played in the lead up to the drilling. The first work that RCP conducted occurred from 1985 to 1987, which involved the acquisition of a five-mile, 9-C seismic line and a vertical seismic profile. The results of that work were presented at the Society of Exploration Geophysicists’ convention in 1987 and in a paper in The Leading Edge. This was early documentation of the value of shear-wave splitting in quantifying shear-wave azimuthal anisotropy and linking it to fracture density and fracture permeability. Key to this understanding is the phenomenon of shear-wave splitting and birefringence. When a horizontally polarized shear wave enters an anisotropic medium, it splits into two waves – a fast and a slow. Processing the shear wave data to optimize using this phenomenon was relatively new at the time. It required a rotation algorithm – known as the Alford rotation, developed by Rusty Alford, although Charles Neville also developed a rotation algorithm at that time. A rotation scan is necessary to separate the two shear waves into fast and slow. Once processed into fast and slow, either a time measurement or an amplitude ratio or both quantify the azimuthal anisotropy associated with shear wave splitting (figure 2).
How does this technology affect our ability to detect and monitor fractured shale reservoirs?
Natural fractures introduce azimuthal anisotropy (change in velocity with direction) in the reservoir and they tend to have a dominant fracture set. That set controls the fast shear wave direction as it passes through the reservoir interval. Knowing the direction of S1 or the fast shear wave enables one to drill horizontally to maximize access to the associated permeability structure of the reservoir.
Nine-component seismic recording involves a shear wave source, normally a vibrator with a base plate that can move horizontally. If two orthogonal directions of base-plate motion are conducted at each source point, then a vector rotation can be applied to the data and two volumes of data can be obtained S1 and S2. Measuring the time interval or isochron over the reservoir interval enables the display of a volume of azimuthal shear wave anisotropy. In terms of percentage anisotropy, it is numerically calculated as Isochron S1-S2/S1 X 100. Quantifying the anisotropy is important as it links to crack density in a volume sense, which in turn links to fracture permeability. The advantage of 9-C recording of pure shear waves is that there are two passes through the reservoir (down and back) and with the slower velocity of shear waves compared to conventional P-waves the potential for improved imaging and detection of reservoir anisotropy and heterogeneity. In addition, the travel paths of the two split shear waves are identical. With converted waves or waves that are generated from offset P-wave sources that cause shear wave conversion at interfaces below the reservoir that travel upward through the reservoir, only one pass through the reservoir and thus lowers the potential for anisotropy measures in thin reservoirs with interval time measurements.
The first land 9-C seismic survey was conducted by RCP in conjunction with CGG in January 1987 at Silo Field. It was acquired with 6,000 single-component geophones, one set conventional P with vertical elements, and two other sets with individual horizontal element receivers. One set had elements oriented north-south the other east-west. Multiple geophones were used employing 12 receivers per group for each component. Source points involved a set of P-wave vibrators followed by two orthogonal directions of horizontal motion at each vibration point from sets of shear wave vibrators. The survey area encompassed 6 square miles centered on the heart of Silo Field as evidenced by vertical well production (figure 3).
The shear wave seismic data were processed into fast and slow volumes and shear wave azimuthal anisotropy was computed on the basis of interval time measurements over the reservoir interval. Up to 7-percent anisotropy occurred in the heart of Silo Field (figure 4).
Negative anisotropy, which mathematicians would declare as impossible, is due to our fixing a fast shear direction, and if that direction flips orthogonally as when the northeast fracture direction becomes dominant then the S2 wave travels faster than the S1 giving us a numerically negative number. Where zero or near zero occurrences could imply no fractures or it could imply equal fracture intensity in the two major directions. Thus, one needs to be careful how to interpret these anisotropy volumes based on time measurements alone. Careful preservation and utilization of amplitudes can help in the unravelling of these complexities and are key to enhancing the resolution of shear wave splitting data.
Confirmation of our interpretations occurred when the LeMaster well was drilled and a formation microscanner was run in the horizontal leg. Figures 5 and 6 show the outcome of the analysis of the data. The dominant fracture sets are northwest oriented but there is a smaller northeast subset in parts of the reservoir.
Other observations that were noteworthy included the amplitudes of split shear waves changed dramatically in the fractured areas of the reservoir. The shear impedance dropped on the polarized wave that crossed the open fracture sets so indeed we could confirm the direction of the open dominant fracture sets by amplitude analysis. It requires lining up the two volumes on events above the reservoir that are isotropic and then scanning the amplitudes of the split shear waves over the reservoir interval. Warping algorithms are useful for determination of these amplitude ratios between fast and slow.
Another noteworthy observation is that small faults appeared more evident on the split shear wave volumes than on the P-wave volume. This observation has occurred in several other studies we have undertaken. These faults at Silo Field caused many of the horizontal wells to go out of zone and ultimately affected the expected ultimate recovery of these early wells.
Conclusions, and To Be Continued …
What is the potential impact of multicomponent seismic surveying on shale reservoir development based on our investigations at Silo Field?
- The areas of highest fracture density in the reservoir can be mapped.
- Reservoir models can be created to more accurately locate horizontal well drilling locations and spacing.Optimizing the well locations and spacing on the apparent interconnectedness of the natural fractures could help the bottom line and ultimately improve the recovery efficiency of shale reservoir development.
- Finding faults that are subseismic resolution or just can’t be seen on P-wave data volumes with shear wave data gives us a better chance of geosteering the wells in the development phase. The faults when mapped accurately can assist in the prediction of enhanced fracture areas.
How can multicomponent seismic bring change to shale reservoir development? Time-lapse (4-D) seismic surveys can bring insight and will be the subject of the next Geophysical Corner.