Crosswell Tomography

Key Takeaways

• Acoustic crosswell tomography is the most common variant — typical operations include deploying a seismic source (typically a controlled-frequency acoustic source) in one well and an array of seismic receivers (geophones, hydrophones, or accelerometer arrays) in the neighboring well; the source generates seismic energy at multiple depths, with the resulting signals being recorded at the receivers; the multi-depth source operation combined with the multi-receiver array geometry provides hundreds to thousands of source-receiver paths through the inter-well formation; tomographic inversion of the resulting data produces an image of the acoustic velocity distribution between the wells, with the resulting velocity image supporting structural and stratigraphic interpretation of the inter-well formation.
• Electromagnetic crosswell tomography uses EM signals to characterize subsurface conductivity — the EM transmitter generates electromagnetic signals at controlled frequencies (typically 1 kHz to 100 kHz) in one well, with the receivers in the neighboring well detecting the resulting EM field; the multi-depth source operation provides the diverse raypaths needed for tomographic inversion of the conductivity distribution; the technique is particularly useful in EOR applications where conductivity changes from saturation evolution can be tracked through time-lapse surveys, with the resulting EM tomography supporting reservoir surveillance that conventional methods cannot provide.
• Operational considerations for crosswell tomography include well availability (the technique requires two wells with appropriate spacing and configuration for the planned survey), source-receiver geometry (the inter-well distance must be appropriate for the specific technique, typically 100-500 m for acoustic tomography), formation conditions (the technique works best in formations where the planned signal type propagates effectively), and operational coordination (the multi-well operations require careful coordination across the operating teams); the operational complexity makes crosswell tomography a specialty technique used selectively for high-value applications.
• Time-lapse crosswell tomography provides reservoir surveillance capability that bridges between wellbore and surface scales — by repeating crosswell surveys at intervals during reservoir production, the technique characterizes the changes in formation properties (typically saturation evolution from waterflood or other production effects) between the wells; the resulting time-lapse data supports reservoir surveillance applications including waterflood monitoring, EOR project evaluation, and field-wide saturation mapping; the surveillance value of time-lapse crosswell tomography supports the operational investment in specific applications where the inter-well surveillance provides unique value.
• Modern crosswell tomography services from specialty providers (Schlumberger, BJ Services, various specialty contractors) include integrated technology spanning the source-receiver equipment, deployment operations, data processing, and interpretation methodology; the technical sophistication of modern services supports successful application across diverse operational contexts; the relatively limited adoption of crosswell tomography compared to other characterization techniques reflects the operational complexity and cost rather than technical limitations, with the technique providing unique characterization capability when the operational requirements support its use.