Real-time checkshot and interval velocity reduces uncertainty ahead of the bit - prevents drilling surprises.

In multiple tests conducted offshore, a new seismic while drilling (SWD) system has reduced seismic uncertainty for a Caspian Sea operator from 2,500 ft to 30 ft (762 m to 9 m) to confirm and drill a target location. In the Gulf of Mexico, waveforms from the SWD tool were used to look ahead of the bit and identify features below the salt. The operator then drilled to within 60 ft (18 m) of the base of the salt.
The service is already being used to drill wells that would have been extremely difficult, if not impossible, to drill without it. The value if the new service, called SeismicVISION from Schlumberger, which involves delivering checkshot and interval velocity data in real time, lies in continuous correction of the seismic map using depth information for the driller and time data for the geoscientists. By acquiring and transmitting data in real-time, continuous correction reduces the uncertainty of drill bit location on the surface seismic map, without interfering with normal drilling operations.

Drilling into the unknown

Geologists and geophysicists cannot give the driller the exact depth of a particular event, but rather a "best guess' at the depth. As a result, due to this uncertainty, logistical problems are inevitable. Because of uncertainty in velocity, there is a corresponding uncertainty in pore pressure that compounds drilling issues. One such problem is critical mud weight. Guess one way and the operation becomes exceedingly risky - guess the other way, and penetration rate is reduced while mud costs skyrocket.

Methods of combating uncertainty

The current method of correcting this uncertainty is to stop drilling, pull the drillstring, run in a wireline survey tool, take a checkshot, pull the wireline tool, run back in and continue drilling. This procedure is repeated as often as necessary, as many as five times in a deepwater well. Each time, the procedure can take as many as 5 days and cost as much as US $2 million.

Until recently, the only available SWD technique used noise generated by the bit during drilling as a seismic source. Receivers placed on the surface, or seabed, recorded borehole seismic information in a time vs depth measurement. It has been found that the use of the bit as a source is generally ineffective under conditions of soft sediments, highly deviated or horizontal wells, drilling with polycrystalline diamond compact (PCD) bits or when well depth exceeds 15,000 ft (4,573 m).

The alternative to SWD is intermediate wireline checkshot or vertical seismic profiling (VSP) surveys. However, depth and velocity must be modeled, and accuracy can only be verified after the well is drilled. There is then the chance for unexpected hazards to be hit or desired targets to be missed.

Imaging ahead of the bit

The SeismicVISION service delivers traditional borehole seismic measurements, including checkshot and interval velocity data in real time, to reduce the uncertainty of events ahead of the bit. Full seismic waveforms are available in memory. On-time access to such calibration data is critical where large uncertainties in the time-depth relationship exist, or in wells where it is essential to set casing in a particular interval identified by surface seismic data.

Available in 63/4-in., 83/4-in. and 9-in. collar sizes, the system is comprised of a logging while drilling ((LWD) tool with seismic sensors positioned near the drill bit, a seismic source at the surface either on the rig or on a nearby support vessel and a measurement while drilling (MWD) system for real-time telemetry. The tool is fully combinable with any other Schlumberger LWD tool.

A patented technique enables source activation and data acquisition during drilling pauses, when the downhole environment is quiet. Suitable times to acquire data are during pipe connections while drilling or tripping.

The look-ahead tool, which contains a processor and memory, receives seismic energy from a conventional air gun or offshore vibrator for offset surveys. After acquisition, the seismic signals are stored and processed, and key information, such as checkshot data is transmitted uphole in real time using the MWD telemetry system. The time-depth data are used to position the well on the seismic map.
Waveforms, obtained from memory once the tool is removed from the hole, are downloaded later for VSP processing.

Real-time, at-the-bit, checkshot and interval velocity provide a viable antidote to drilling uncertainty. With look-ahead capability as great as 8,000 ft (2,439 m), the driller is armed with vital advance information of specific zones - a capability of significant value. The tool's unique abaility to deliver checkshot and interval velocity data in real time not only removes guesswork, but makes it possible to drill wells formerly believed to be undrillable.

Comments