The efficient exploitation of hydrocarbon resources is strongly dependent on the driller's ability to position wells accurately within the reservoir. This is particularly challenging for smaller accumulations and on long reach or horizontal wells.
Conventional wireline vertical seismic profiles (VSPs) can provide valuable assistance, but such operations require either the drill string to be removed from the hole or the pumping of the geophone tool through the drill string. Both of these methods add time to the well construction.
In long sections of open hole, wireline operations may present risks in the form of sticking and potential fishing operations, which prevent VSP surveys being acquired in look-ahead mode.
The costs in terms of rig time alone generally make this option prohibitive. Ongoing developments for cost-effective exploitation of mature and pressure depleted reservoirs have led to wider applications for coiled tubing drilling. The combination of a seismic tool with the coiled tubing drill string therefore provides the optimum configuration to reduce drilling costs and optimize well placement.
The efficient exploitation of hydrocarbon reservoirs is based largely on seismic images of the subsurface. For long reach wells, due to the complexity of the Earth's magnetic field, uncertainties in the well coordinates themselves, when derived from conventional techniques at reservoir depths, may amount to several tens of meters.
VSPs in which a seismic geophone is placed at regular locations in the borehole are an established technique for improving both the detail and positional accuracy of a reservoir image. They can be used to determine the well coordinates accurate to an order of magnitude greater than that achievable by conventional gyro techniques, and with a possible tolerance of +/- 10 meters.
VSPs have traditionally been acquired by geophones conveyed on wireline, generally as part of formation evaluation logging suites. This can reduce the effectiveness of the VSP technique because they are most commonly acquired "after the event," when remedial rather than preventative action is required.
The advantages of using VSPs, while reducing the risk to the hole, may be realized by including a geophone tool as part of the bottomhole drilling assembly. In this way, VSP data may be acquired without removing the drill string from the hole, allowing circulation at any time during the seismic acquisition and thereby substantially reducing the likelihood of sticking and subsequent fishing operations. Additionally, VSP data can be acquired at regular intervals or at critical stages of the drilling process, allowing corrective action to be taken if necessary.
With many of the world's reservoirs reaching maturity, it has been estimated by US industry and government agencies that the number of underbalanced drilling operations may double in the next five years, reaching 50% of all wells drilled. Coiled tubing drilling has certain advantages for this application, in that connections of jointed pipe are eliminated, preventing pressure surges caused by stopping and starting the mud pumps. This results in the maintenance of better circulation fluid stability and bottomhole pressures.
Additionally, the telemetry from coiled tubing geosteering systems may be made continuously along an integral wireline rather than attempting mud pulse transmission through the compressible fluids associated with underbalanced drilling. Coiled tubing operations have also been found to be a cost-effective alternative for side-track drilling through tubing and drilling long reach wells. Although the majority of drilling in the immediate future will be conducted by conventional jointed pipe, it is considered that an integral seismic-while-drilling tool has a significant part to play in the exploitation of mature fields.
Operationally, the tool is flexible in that it can be used to record seismic signals from a conventional surface source, or it can be used for inverse VSP acquisition. In the latter application, noise generated by the drill bit acts as a seismic source, and signals are recorded by sensors deployed at the surface or in neighboring wells.
The function of the downhole sensors becomes that of recording the drill bit source signature (pilot signal), a short distance from the bit. This reduces the pilot signal attenuation observed in conventional inverse VSPs in which the pilot signal is transmitted along the drill pipe to surface and extends the applicability of inverse VSPs to high deviation wells and to bits other than the roller cone variety.
Specific applications of the tool include steering the bit along highly deviated wells through faulted reservoirs, monitoring the bit when approaching drilling hazards, and determining absolute bit coordinates to assist in well navigation to specific targets on long reach wells.
The body of the tool is of solid construction to avoid parasitic bending modes and houses three accelerometers and one hydrophone to record seismic data, with an additional triaxial acceler-ometer system to record tool vibration.
The data from these sensors is transmitted at one-millisecond sampling along a single conductor for processing on a laptop computer at surface. A reference signal is transmitted along the same conductor and can be monitored continuously to ensure that the telemetry system is operating correctly.
The tool will now undergo a series of laboratory system evaluations and performance tests under the supervision of Shell (SEPTAR) prior to operational evaluation. Results of these and analysis of data recorded in test wells will be reported at the 2001 SPE/IADC Drilling Conference in Amsterdam.
Shell Exploration and Production provided financial support to build the production versions of the coiled-tubing-seismic-while-drilling tool and for helpful discussions during the design and construction.