Crosswell seismic
Crosswell seismic tomography provides high frequency seismic and velocity profiles between two wells. Unlike surface seismic and VSP surveys both source and receivers are beneath the surface thus avoiding attenuation losses and preserving the high frequency component of the data. Cross well reflection tomograms (2D seismic sections between the wellbores) can contain frequencies of several hundred Hertz. Consequently the spatial resolution can be in the order of several feet.
The success of these surveys is dependent on many factors.
Clearly defined objectives
Thorough pre job modeling to determine optimal source and receiver intervals
Appropriate well spacing
Suitable source
Accurate survey time estimate to facilitate budgetary control
Crosswell survey schematics showing acquisition of common source ‘fans’ on the left and the ray paths utilized for velocity and seismic profiles.
Crosswell seismic applications
Resolution
Syarif et al, SPE 166490, 2013, Crosswell Seismic Guided 3D Seismic Interpretation Results in Successful Infill Well Location in Bunyu Field (Indonesia).
Comparison of Crosswell Seismic profile XR2-X7 (2011), re-processed surface seismic (2010). The new fault interpreted (green) separates the Bunyu Nibung & Bunyu Reuris blocks. The new well trajectory for X-30 is displayed in black.
Thermal
Bair et al, SEG January 1999, 1643-1646 Time‐lapse imaging of steam and heat movement in the Cymric 36W Cyclic Steam Pilot using crosswell seismology.
Velocity tomogram from steam injection shows up to 25% reduced velocity signature of injected steam. There is clear evidence of a vertical induced fracture near the injection wellbore.
CO2 Injection
Spyros et al, TLE 1997, 16(9), 1300-1308. Crosswell seismic imaging of reservoir changes caused by CO2 injection.
The image shows the difference in velocities from before and after CO2 injection. Lower velocity in warm colors. “There is a remarkable correlation between reflection event terminations and discontinuities in the tomogram difference”
Geotechnical
S. Boone et al, SEG January 2008, 1407-1410. Identification and interpretation of solution mining features on a crosswell reflection profile.
Direct evidence of solution salt mining cavities and cavity collapse features are detected on high resolution crosswell reflection seismic data in
surveys for site evaluation for bridge construction.
Crosswell seismic planning & acquisition
Crosswell seismic project require two wells; one to deploy the source and the other for the receiver array. Their distance apart is limited by the source generating sufficient energy to be transmitted to the receiver array. Typically the two wells are less than 1000’ apart. Additionally the wells should be in the same plane. The selection of which downhole source and receiver array to use is tied to the project objectives and geometry; what spatial resolution is required, where is the target within the 2D space and what is the overall depth of interest. Pre survey planning should provide recommendations on source and receiver spacing, overall survey time and likely resolution.
Downhole sources vary from small sparker sources most suitable for shallow engineering applications to high frequency piezoelectric sources to swept, high energy sources designed specifically for oilfield applications. Receiver arrays are those used in conventional VSP surveys however, because of the higher resolution required, these arrays typically need to be interleaved during acquisition to achieve the required level spacing. DAS using fiber optic cables can also be used subject to the directional sensitivity of the measurement.