crosswell reflection tomography

Crosswell reflection tomography is a borehole seismic imaging technique in which a seismic source deployed in one well and a hydrophone or geophone receiver array deployed in an adjacent well are used to record both direct transmitted P-wave arrivals (traveling straight across from source well to receiver well) and reflected P-wave and converted S-wave arrivals (traveling from the source, reflecting off impedance contrasts between the two wells, and arriving at the receiver array), with the combined traveltime dataset from both direct and reflected ray paths incorporated into a tomographic inversion algorithm that simultaneously reconstructs the two-dimensional seismic velocity field and the positions of reflecting interfaces in the interwell volume at resolutions of 1 to 10 m, far exceeding the 10 to 50 m spatial resolution achievable with conventional surface seismic at the frequencies used for deep exploration; crosswell reflection tomography extends standard direct-ray tomography by adding reflected arrivals whose ray paths sample geological boundaries between the wells, improving velocity boundary sharpness and imaging contacts (reservoir-seal, gas-water, steam chamber) that direct-ray tomography smooths over. In WCSB reservoir monitoring and enhanced recovery programs, crosswell reflection tomography finds primary application in SAGD projects in the Athabasca oil sands and Cold Lake deposits, mapping the steam chamber between the horizontal injector and producer well pair; steam displaces cold bitumen, reducing acoustic velocity from approximately 2,800 m/s (cold bitumen-saturated sand) to 600 to 900 m/s (steam-filled pore space at 200 to 260 degrees Celsius), creating an impedance contrast mapped with meter-scale resolution using high-frequency piezoelectric sources (200 to 2,000 Hz) in observation wells 50 to 200 m from the SAGD well pair. The method is also applied in WCSB CO2 storage monitoring at the Weyburn-Midale CO2 EOR and Sequestration project in Saskatchewan, where CO2 injected into the Midale carbonate at 1,400 m depth reduces acoustic velocity 5 to 15 percent, producing tomographic images of the CO2 plume that validate containment and support regulatory compliance reporting.

• Crosswell seismic acquisition geometry and source-receiver configuration in WCSB monitoring programs: Crosswell seismic surveys in WCSB reservoir monitoring programs use one well as the source well and one or more adjacent wells as receiver wells, with the source tool (sparker, air gun, or piezoelectric transducer) lowered to successive depth stations in the source well at 1 to 5 m intervals and the receiver array (string of hydrophones or geophones at 1 to 3 m spacing) clamped to the casing in the receiver well to record arrivals at all receiver depths for each source position. In WCSB SAGD monitoring, the crosswell source-receiver separation is typically 50 to 150 m, and the survey covers from 10 m above the Clearwater cap rock to 20 m below the McMurray-Devonian contact, spanning the productive McMurray interval. High-frequency borehole seismic sources (200 to 2,000 Hz) used in WCSB crosswell programs produce seismic wavelengths of 1 to 10 m at the McMurray acoustic velocity of 1,500 to 2,800 m/s (cold to hot sand), enabling resolution of steam chamber boundaries at meter scale that is impossible with surface time-lapse seismic (dominant frequency 60 to 80 Hz, wavelength 20 to 40 m in the McMurray). The crosswell survey is repeated at 3 to 12 month intervals (time-lapse crosswell tomography) to track steam chamber growth rate, lateral conformance (whether the chamber is growing uniformly along the SAGD well pair length), and heat loss into overlying non-reservoir shale, providing WCSB SAGD operators with real-time information for steam injection rate optimization and well spacing decisions for the next development well pair.
• Direct-ray versus reflection tomography inversion and image resolution in crosswell programs: Standard crosswell direct-ray tomography inverts only the first-arrival traveltimes of direct P-waves traveling across the interwell volume, producing a smooth velocity model in which velocity variations are resolved with a horizontal resolution approximately equal to the first Fresnel zone width (typically 5 to 20 m for source-receiver separations of 50 to 200 m at crosswell frequencies); this resolution is adequate for tracking bulk velocity changes from steam saturation or CO2 injection in WCSB monitoring, but insufficient for imaging sharp geological boundaries such as the top of the McMurray Formation or the oil-water contact within the reservoir. Crosswell reflection tomography adds the reflected arrival traveltimes to the inversion, treating the reflections as ray paths that constrain both the velocity field and the geometry of the reflecting boundaries simultaneously in a joint inversion; the reflecting surfaces are parameterized as depth-migrated horizons in the inversion grid, and their positions are iteratively updated alongside the velocity model until both direct and reflected traveltimes are simultaneously minimized. The joint inversion improves boundary sharpness by 30 to 50 percent over direct-ray-only tomography in WCSB SAGD programs, resolving the steam-cap rock boundary to 2 to 3 m precision and detecting asymmetric steam chamber growth invisible in a smoothed velocity model.
• Crosswell reflection tomography for CO2 plume monitoring at WCSB EOR and sequestration sites: At the Weyburn-Midale CO2 EOR project in the Williston Basin of southeastern Saskatchewan, crosswell seismic surveys between producing and injection well pairs (spaced 200 to 400 m apart in the Midale Marly and Vuggy carbonate reservoirs at 1,400 m depth) complement time-lapse surface 3D seismic by providing higher-resolution images of the CO2 saturation front at 2 to 5 m resolution within the 25 m thick productive Midale unit. CO2 injection into the Midale carbonates reduces P-wave velocity from approximately 5,500 m/s (brine-saturated dolomite) to 4,800 m/s (CO2-saturated dolomite) near the injection well, a 12 percent velocity decrease that appears as a low-velocity anomaly in the crosswell tomographic image growing outward from injection wells over successive quarterly monitoring surveys. The Weyburn crosswell monitoring program contributed to the international CO2 storage database by demonstrating that crosswell reflection tomography can quantitatively track CO2 plume geometry and verify that injected CO2 is not migrating vertically out of the target Midale reservoir into the overlying Frobisher-Alida unit, supporting regulatory assurance of geological containment for both EOR royalty accounting and carbon credit verification.
• Hydraulic fracture geometry monitoring with crosswell seismic in WCSB tight gas and shale programs: Crosswell seismic methods have been piloted in WCSB tight gas and shale monitoring programs to image the geometry of hydraulic fractures created during stimulation treatments, because the fracture plane and the proppant-filled fracture aperture create a seismic impedance contrast that can be detected as a reflection or a diffracted arrival in crosswell data collected from an observation well adjacent to the stimulated well. In WCSB Cardium tight gas pilot programs where a horizontal observation well was drilled 100 m from the stimulated well to act as the receiver well during hydraulic fracturing, crosswell microseismic and active-source crosswell data recorded simultaneously during fracture treatment provided complementary information: microseismic passive events located the fracture cloud in 3D space, while active-source crosswell data after the treatment confirmed proppant placement by detecting the high-amplitude reflection from the propped fracture plane (proppant acoustic impedance approximately 2.5 times the formation sandstone impedance). The combined crosswell fracture imaging technique supports WCSB Montney and Duvernay shale completion optimization by validating whether hydraulic fractures are propagating in the designed direction, height, and half-length, providing an independent check on the fracture geometry model used to design stimulation treatments and production forecast models.
• Crosswell seismic equipment, deployment challenges, and cost in WCSB borehole programs: Crosswell seismic surveys in WCSB cased production and observation wells require the source tool and receiver array to be deployed through production tubing or through 4-1/2 to 7 inch casing, imposing maximum tool OD constraints (typically 1-11/16 to 2-1/8 inch OD for through-tubing deployment); piezoelectric transducer sources for WCSB crosswell frequencies (200 to 1,000 Hz) fit within these OD constraints and can be powered by wireline cable from the surface. The primary operational challenge in WCSB crosswell surveys is clamping the receiver array to the casing in the receiver well to suppress tube wave noise (acoustic energy traveling along the wellbore fluid column that arrives simultaneously with or before the formation arrivals and masks the crosswell signal at frequencies below 300 Hz); pneumatic or mechanical decoupling clamps at 3 to 5 m intervals along the receiver array suppress tube wave noise by 20 to 30 dB relative to unclamped hydrophone strings. WCSB SAGD crosswell reflection tomography costs $180,000 to $350,000 per survey repeat versus $800,000 to $2,000,000 for equivalent time-lapse 3D surface seismic; the resolution advantage justifies the cost for SAGD monitoring where steam chamber conformance governs recovery of $30 to $50 million capital per well pair.

Crosswell Reflection Tomography Detecting SAGD Steam Chamber Asymmetry

A WCSB Athabasca SAGD operator noticed unequal heat response between two temperature observation wells flanking a 900 m horizontal well pair in the Lower McMurray Formation at 390 m depth: the east observation well showed rapid temperature increase while the west side lagged by 60 days at the same distance from the injector. A crosswell reflection tomography survey was acquired between the east and west observation wells (140 m separation, 200 Hz source, 2 m receiver spacing over 80 m vertical coverage). The tomographic inversion revealed a 15 to 20 m-wide zone of high velocity (2,800 m/s, indicating cold unswept sand) centered on a shale barrier intersecting the injector at 420 m depth that was not visible in the original 3D seismic interpretation (resolution 25 m). The steam chamber had grown over and under the barrier on the east side (thicker, more permeable sand above the barrier) but had not penetrated to the west side (thinner sand with the barrier intercepting the chamber at the producer level). The operator re-drilled a lateral from the producer into the west side of the blocked zone, recovering 18,000 m3 of bitumen over the following 24 months that would otherwise have been bypassed.

Related Terms

Crosswell seismic is the broader category of borehole-to-borehole seismic methods; crosswell reflection tomography is the advanced variant that adds reflected arrivals to the direct-arrival tomographic inversion to sharpen velocity boundaries and image geological contacts. Seismic tomography reconstructs the subsurface velocity field from traveltime data; crosswell reflection tomography is the interwell application, producing 2D velocity images at 1-10 m resolution between WCSB SAGD and EOR monitoring wells. Steam-assisted gravity drainage (SAGD) in WCSB Athabasca and Cold Lake oil sands is the primary application for crosswell reflection tomography; the large acoustic velocity contrast between steam-swept and cold bitumen-saturated McMurray sand drives the detection sensitivity. Time-lapse seismic (4D seismic) is the surface-seismic alternative to crosswell monitoring in WCSB EOR programs; crosswell provides higher resolution but narrower areal coverage than 4D programs repeated over the full SAGD development area. CO2 sequestration monitoring at the WCSB Weyburn-Midale site uses crosswell reflection tomography to image CO2 plume geometry and verify geological containment within the target Midale carbonate reservoir.