Objective
To demonstrate the technical/economic viability of an innovative new borehole seismic reflection/transmission tomography technique combined with advanced high temperature/pressure borehole instrumentation for the imaging of fluid bearing fracture zones in hdyrocarbon reservoirs.
The fieldwork involved two crosshole seismic surveys at the Chalmers Hot Dry Rock research site at Fjallbacka, Sweden. CSMA's downhole sparker seismic source and string of four hydrophones were used to acquire the crosshole data. The first survey was performed under ambient borehole conditions. The second survey was performed at an elevated downhole pressure of 3.35 MPa overpressure. The elevated pressure was restricted to a zone below 350 m measured depth to inflate known horizontal fractures predominantly around 460 m. The borehole spacing was approximately 90 m.
In excess of 3000 rays were recorded for each survey and the seismic traces were of very good quality. The dominant frequency of the direct P wave arrival was around 1500 Hz and the signal to noise ratio was typically more than 10. During the pressurized survey 3332 traces were recorded in a 12 hour period. The data quality and acquisition rates demonstrated that the acquisition system is reliable and well suited to potential applications in hydrocarbon wells.
Velocity tomograms and diffraction stack migration wavefield images were formed from the unpressurized and pressurized data. Changes due to the pressurization were enhanced by subtracting the un-pressurized from the pressurized images to form difference tomograms and wavefield images. Zones of fracture opening were readily interpretable from the difference images and the images correlated well with permeable fractures identified in the boreholes from hydraulic tests.
The results of the imaging demonstrated the capability of the processing techniques to image fracture opening during pressurization and together with the success of the data acquisition it has been clearly demonstrated that the aim of the project was achieved.
Borehole seismic tomography is a powerful geophysical exploration technique using variations in seismic wave velocity to image geological features such as naturally fractured reservoirs. A seismic source is placed in one borehole and receivers in one or more adjacent boreholes. The conventional "transmission" tomogram is a velocity contour map constructed from a large number of intersecting seismic rays paths transmitted across the inter-borehole region. However, the innovative combined reflection/transmision enables images to be constructed both within and outside the inter-borehole region.
Demonstration of the technique is made possible by the contractors technical advances in the fields of high temperature/pressure borehole instrumentation, operational techniques and data processing systems.
The viability of the technique will be demonstrated by imaging a pressurized sub-horizontal fluid bearing fracture zone in a well characterised geological environment.
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Coordinator
TR10 9DU Penryn
United Kingdom
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