Time-lapse seismic analysis of a heavy oil reservoir undergoing steam injection is a critical step in the evaluation of production efficiency and reserve potential after a period of steaming. Through the identification of changes in seismic characteristics, steam chamber development can be identified and investigated in an effort to delineate resources unaffected by steam. Unheated portions of the reservoir will play a role in future exploitation strategies, targeting resources currently being neglected by reservoir production.
However, for time-lapse analysis to be most effective, a number of considerations must be addressed in the development of an accurate 4D monitoring survey. Acquisition and repeatability of source and receiver locations, identical data processing flows and interpretation techniques all play a role in the successful and accurate delineation of missed reserves and other reservoir properties of interest.
Approximately 42 line-km of high-fold reflection seismic data were recorded in and around the city of Christchurch, New Zealand, following a devastating Mw 6.3 earthquake on February 22, 2011. The goal of the seismic program was to map previously unknown faults in and around the city for hazard assessment and to assist in the post-earthquake recovery effort.
Seismic data were collected along six 2D lines, two of which were within the Christchurch metropolitan area and four were in rural areas west of the city. Recording conditions were challenging within the city, but good quality images were obtained along all of the seismic lines, with events interpretable to a depth of approximately 1.5 km. Numerous faults were imaged along the lines and these were interpreted in two groups – older faults that showed clear offsets in deep (> 1 km) reflections and younger faults that showed displacement in shallow reflections. Some faults in the latter group were interpreted to be directly associated with hypocentres of the earthquake and aftershocks.
Geophone orientation azimuths were found from 3D VSP data, acquired near Lousana, Alberta, in order to examine any dependence of computed geophone orientation on source-well offset or azimuth. Additionally, a comparison was made between analytic and hodogram methods. The dataset was divided based on source-well azimuth into bins with centers trending 0°-180°, 45°-225°, 90°-270° and 135°-315°. There appeared to be little dependence on source-well sector azimuth, which is expected for flat, isotropic geology near the well. Offsets were binned into ranges of 0-600 m, 600-950 m, 950-1300 m, 1300-1650 m and greater than 1650 m. Scatter in rotation angles was shown to be strongly dependent on offset, with the most constrained results in the 1300-1650 m offset bin. The optimal offset range for geophone orientation calibration was found to be between 1 and 2 times the receiver depth. Standard deviation in orientation azimuths were found to be 1.66° using the analytic method, and 2.17° using the hodogram method from a total of 249 source locations. While the both methods performed well, the analytic method produced more consistent results.
A large 3D/3C seismic survey will be acquired in northeast British Columbia (NEBC) in 2012. In order to assist in the design of this survey, a high resolution multicomponent 2D line was acquired to give information about shear wave properties particularly for the near-surface. Shear (SH) and compressional (P) sources and 3-component geophones were used in this acquisition. Knowledge about the velocity-depth structure of the near-surface and feasibility of acquiring multicomponent data were the main drivers for this acquisition. Also, it is important to know the structure of the near-surface for the investigation and integration with deeper data. As a result, Vp/Vs analysis was carried out for both the shallow and deep formations. The analysis targeted both the near surface study and deeper horizon registration for PP and PS reflection data.
From the shallow P-wave data, one refractor was detected and the presence of a glacial channel was confirmed at the east end of the line. The depth of this refractor ranges from 140 m to ~230 m in the channel. The average velocity for the first layer is 1950 m/s and for the second layer is 2800 m/s. From the S-wave data a different model was determined, with two refractors detected to the west end of the line and one refractor to the east of the line. The depth of the first refractor is ~70 m and the second ~140 m; to the east the refractor detected is at ~180 m. The S-velocity for the first layer is 350 m/s to 420 m/s, and 650 m/s to 800 m/s for the second layer to the west and 1400 m/s for the third layer. Finally static corrections for the reflection analysis were computed. For SH-wave data the static corrections range from -150 ms to -250 ms and for the P-wave data, values range from 10 ms to -15 ms.
Vp/Vs analysis was performed for the near-surface structure with the values of velocities obtained from the shallow analysis and also through PP-PS registration for deeper structures. Good agreement was observed when comparing these results to Vp/Vs values from well log data. Values of Vp/Vs ranged from 5 in the near-surface to 2.2 in deeper formations.
Fractures in the subsurface are known to impact the quality of seismic imaging. This multi-component, time-lapse study focuses on the fracture-induced anisotropy within a potash mining region in the Williston Basin. The area of interest is the Dawson Bay Formation, a fractured carbonate overlying the Prairie Evaporite Formation which contains the potash ore deposits. PP and PS full azimuth volumes were divided into 4 sub-volumes consisting of a stack of source-receiver ray paths consisting of a 45 degree aperture for both a baseline and monitor surface. Through their interpretation and travel-time analysis, weak azimuthal velocity anisotropy was observed within the Dawson Bay Formation. Analysis of Vp/Vs indicates that there is a significant decrease in shear-wave velocity in the centre of the survey between the Birdbear and Winnipegosis formations, including the Dawson Bay Formation. These Vp/Vs results, along with travel time differences, confirm the presence of fracturing in the subsurface, and more specifically, within the Dawson Bay carbonates.