Logging while drilling measurements are now widely available throughout the world for a variety of applications. Many of the first generation LWD measurements are similar to what was traditionally obtained using wireline methods. However, there are now measurements available that are either complementary or are entirely unique as compared to traditional wireline measurements.
The LWD measurements that are similar to wireline include natural gamma ray, induction-propagation resistivity, laterolog resistivity, bulk density, neutron porosity, magnetic resonance, and sonic velocity. These measurements have been proven to be equivalent to their wireline counterparts.
The drilling environment has allowed for LWD measurements that are unique to borehole geophysics. The ability to rotate the LWD sensors on the drill collars while drilling allows for the measurement of azimuthally varying formation properties and the presentation of this data in the form of an image. This is particularly important for well placement issues in high-angle wells. The various formation layers can easily be distinguished using azimuthal measurements allowing for the well to be positioned in the target layer. The use of these azimuthal measurements mitigates both the survey and geological depth uncertainties since the stratigraphy that the wellbore has been placed into is clearly evident.
LWD borehole seismic measurements are similar to their wireline counterparts but differ in the important element of timeliness of information. The ability to place the bit on the seismic map or to produce a seismic impedance map ahead of the bit, as the well is being drilled, allows for the accurate depth determination of drilling targets and the identification of pore pressure transgressions. Examples using this seismic image to guide the wellbore deviation as well as optimizing placement of casing strings are presented.
The new LWD Ultra Deep Resistivity measurement technology (UDR) meets the requirements for geosteering at the reservoir level with a sub-seismic resolution. Its major applications include delineation of oil water contact (OWC), distance to reservoir cap rock, and bottom shale.
Since the dawn of LWD in the 1980s, the industry wanted a reliable while-drilling borehole seismic measurement. However, a number of technical roadblocks prohibited its realization until recently. These difficulties included the nature and required accuracy of the measurement, timing issues, sensors, automation, and the lack of an electrical connection between the downhole tools and the surface in a while-drilling configuration.
Technological solutions to these roadblocks have since been found, and today seismicVISION? tool acquire and transmit seismic data during normal rig operations and without loss of rig time1.
In high-cost / high-risk environments, particularly in those with significant seismic uncertainties, this technology can positively impact the cost of exploration and development drilling. Seismic while drilling data is delivered in real time, so it can be used to make critical decisions while a well is being drilled. It also saves rig time by replacing intermediate wireline check-shot surveys eliminating the need to stop the drilling process and pull drilling equipment out of the hole for the duration of a survey.