Time‐lapse crosswell seismic tomogram interpretation: Implications for heavy oil reservoir characterization, thermal recovery process monitoring, and tomographic imaging technology

Abstract

Time‐lapse crosswell seismic data acquired with a cemented receiver cable have been processed into P‐ and S‐wave tomograms which image heavy oil sand lithofacies and changes as a result of steam injection. Twenty‐seven crosswell surveys were acquired between two wells over a 3.5 month period before, during, and after a 34‐day, 30 MBBL [Formula: see text] steam injection cycle. Interpretation was based on correlations with reservoir data and models, observation well data, and engineering documentation of the production history and steam cycle. Baseline S‐ and P‐wave tomograms image reservoir sand flow units and areas affected by past cyclic steam injection. S‐wave tomograms define lithology and porosity contrasts between the excellent reservoir quality, “high flow” turbidite channel facies and the interbedded “low to moderate flow” bioturbated levee facies. The reservoir dip of approximately 20° is defined by the velocity contrast between lithofacies. P‐wave baseline tomograms image lithology, porosity, structure, and several low velocity zones caused by past steam injection. Previous steam‐heat injection caused the formation of gas which reduced velocities as much as several thousand ft/s (600 m/s), an amount which obscures the velocity contrast between lithofacies and smaller velocity reductions as a result of temperature alone. Time‐lapse and difference P‐wave tomograms document several areas with small decreases in velocity during steam injection and larger decreases after cyclic steam injection. Velocity reductions range from 300 to 900 ft/s (90 to 270 m/s) adjacent to and above injectors located 20 to 50 feet (6 to 15 m) from the tomogram cross‐section. Poisson’s ratio tomograms show a significant decrease (.10) in the same area, and include low values indicative of gas saturation. Continuous injectors located 50 to 350 feet (15 to 100 m) from the survey area also caused a progressive decrease in velocity of the “high flow” channel sands during the time‐lapse survey. Interdisciplinary interpretation indicates that tomograms not only complement other borehole‐derived reservoir characterization and temperature monitoring data but can be used to quantitatively characterize interwell reservoir properties and monitor changes as a result of the thermal recovery process. Monitoring results over 3.5 months confirms that stratification has controlled the flow of steam, in contrast to gravity override. This suggests that tomographic images of reservoir flow‐units and gas‐bearing high temperature zones should be useful for positioning wells and optimizing injection intervals, steam volumes, and producing well completions.

Ray-based time-lapse traveltime tomography

“…The work of Farmani et al (2008); Chang and Alumbaugh (2011) compared snap shots radar tomograms obtained independently to map velocity changes due to variations in water content. In seismics, work from Mathisen et al (1995); Bauer et al (2005); Saito et al (2006); Daley et al (2008) also relied on independent snap shot comparison, while Vesnaver et al (2003) used a "manual" iterative approach to minimize artifacts between the time-lapse models. Spetzler et al (2007) and Ajo-Franklin et al (2007) inverted traveltime differences, thus assuming stationary raypaths.…”

Joint inversion of geophysical data for site characterization and restoration monitoring. FY97 annual progress report for EMSP

“…Surface and borehole geophysical data have been used for a number of years for site characterization and clean-up monitoring (e.g., Ramirez et al, 1993Ramirez et al, , 1995Wilt et al, 1995a,b) and monitoring steam-flooding in hydrocarbon reservoirs (e.g., Harris, 1988;Mathisen et al, 1995), but these data may only indirectly measure the site structure and fluid flow parameters that control the storage and movement of subsurface fluids. The current practice in geophysics is to interpret a single geophysical data set to obtain an image of a single geophysical parameter, such as seismic velocity or electrical resistivity.…”

“…Current practices also do not exploit the complementary capabilities of seismic and electrical methods. Seismic methods are best for resolving subsurface structure and porosity (e.g., Lines et al, 1993;Mathisen et al, 1995), whereas electrical methods are preferred for identifying fluids, saturation, and permeability (e.g., Wilt et al, 1995a,b).…”

Environmental management science program FY1997 progress report

“…Harris, 1988;Mathisen et al , 1995), but these data may only indirectly measure the site structure and fluid flow parameters that control the storage and movement of subsurface fluids The current practice in geophysics is to interpret a single geophysical data set to obtain an image of a single geophysical parameter, such as seismic velocity or electrical resistivity The geologic parameters of interest (i e., the permeability, porosity, and fluid distribution) are usually estimated by overlaying a series of these geophysical images Current esttmation techniques are very subjective, and geologic parameters ate not obtained directly Current practices also do not exploit the complementary capabilities of seismic and electrical methods Seismic methods are best for resolving subsurface structure and porosity (e.g , Lines et al, 1993, Mathisen et al, 1995, whereas electrical methods ate preferred fat identifying fluids, saturation, and permeability (e g , Wilt et al , 1995a,b).…”