Innovations in the field of fiber optics have resulted in the application of distributed acoustic sensing (DAS) for borehole seismic applications.
A hybrid optical-electrical cable has been developed to enable conveying all logging suites required during the entire well construction. From intermediate drilling section evaluations through well integrity and production, the optical fiber enables seismic acquisition during any wireline intervention.
Because the optical fiber inside the logging cable is the seismic sensing element, velocity information can be measured along the entire wellbore very efficiently. The acquisition of a borehole seismic program, such as a zero-offset vertical seismic profile (VSP) is reduced to minutes, instead of hours or days. Also, no additional rig time is needed if the seismic information is acquired during logging runs that involve remaining stationary in the borehole, such as fluid sampling and calibration of nuclear magnetic tools.
Improvements in drilling efficiency from using this technology are achieved in several ways: 1) elimination of a dedicated seismic run, which renders substantial rig time savings; 2) lowering risk of operations (i.e., avoiding fishing); 3) virtual elimination of nonproductive time due to downhole tool failure and the provision of many opportunities to gather data (logging runs in open or cased hole); 4) obtaining seismic information, however basic, in every well in a cost-effective manner, which is important for future well placement and derisking drilling operations; 5) optimization of seismic acquisition programs in combination with conventional downhole three-component (3C) seismic tools; and 6) seismic data quality assurance resulting from measurement during the drilling process with minimum disruption (intermediate sections and TD), avoiding sections with multiple casing strings or noncemented tubulars.
An example of a zero-offset VSP acquired on hybrid optical heptacable is discussed in the context of increasing drilling efficiency. The checkshot and VSP survey required only minimum rig time to acquire the data, with the hybrid fiber optic-conventional seismic program acquired at intermediate and final
sections that obtained VSP information for the entire well with significant rig-time savings and improved data quality.