Resistivity Measurements at the Bit Provide Real-Time Formation Evaluation Before Invasion
Authors: Bergt David 
Published: December 2007

Real-time formation evaluation with today's conventional horizontal drilling techniques is limited by the distance between the bit and resistivity measurements. Logging-while-drilling (LWD) sensors reach the formation long before wireline measurements, and so generally view it before wellbore degradation, but not before some invasion has occurred. Rapid invasion, called spurt, may mask true resistivity in some formations. The solution to this problem is to relocate logging measurements to the bit itself.
A new LWD resistivity tool improves and simplifies formation evaluation by allowing geologists to visualize and log the formation around the wellbore before mud invasion or wellbore damage has occurred. The tool is normally run as a near-bit stabilizer on a rotary bottomhole assembly or just above the motor in a steerable assembly. It makes five formation evaluation resistivity measurements and an azimuthal gamma ray measurement.
One resistivity measurement uses the bit as part of the measuring electrode to provide the earliest possible indication of a change in formation resistivity. This real-time correlation capability leads to rig time savings during searches for casing or coring points, in effect allowing immediate "geostopping." The top of a reservoir can be cored without danger of drilling up the interval first. At-the-bit resistivity also allows the fastest detection of pore pressure anomalies.
Four additional resistivity measurements are high-resolution focused electrode resistivities. One measurement uses a ring electrode to make an azimuthally averaged resistivity that is accurate up to 20,000 ohm-m. The other three use button electrodes to make azimuthal resistivity measurements that scan the borehole as the tool rotates. Together the ring and three button electrodes give four depths of investigation with identical vertical resolutions.
Conventional 2-MHz LWD resistivity measurements are limited to environments favoring induction-type settings; that is, resistive mud (i.e., fresh or oil-based mud) and conductive rock. The solution to this problem is a high-resolution, focused electrode resistivity that provides accurate Rt measurements for the first time in "laterolog" environments of high formation resistivities and salty mud. An example is carbonates, where 2-MHz tools are inherently less accurate. This high resistivity measurement has excellent vertical resolution, allowing evaluation of beds as thin as 3 in. and detection of beds as thin as 1 in. Multiple depths of investigation of the button measurements, in combination with the focused electrode resistivity measurement, allow evaluation of permeable beds from early-time invasion detection.
The array of three button electrodes provides azimuthal resistivities that scan the borehole as the tool rotates. These measurements form the basis for advanced processing and interpretation of full-bore resistivity images (see Fig. 1), introducing the potential for fracture detection, dip determination, and improved formation evaluation and characterization.
The technical leap that allows measurements to be made at the bit is a wireless telemetry system that sends data from sensors near the bit to the measurement-while-drilling (MWD) tool up to 200 ft. behind the bit, bypassing the intervening drilling tools. Data sent to the MWD system are then transmitted to the surface in real time using mud-pulse telemetry.
Resistivity at the bit is measured by attaching the tool directly to the bit and driving an alternating electric current down the collar, out through the bit, and into the formation. The current returns to the drillpipe and drill collars above the transmitter. In water-based mud, returning current is conducted from the bit, through the mud, into the formation, and back to the bottomhole assembly. In oil-based mud, which is an insulator, current returns through the inevitable but intermittent contact of the collars and stabilizers with the borehole wall, leading to a qualitative indication of resistivity. Formation resistivity is obtained by measuring the amount of current flowing into the formation from the bit and normalizing it to the transmitter voltage.
Although drilling with at-the-bit measurements is still in its infancy, petrophysical measurements near the bit provide numerous benefits for geologists, petrophysicists, and drillers and extend the range of conditions under which it is possible to measure formation resistivity accurately while drilling.
SI Metric Conversion Factors
ftx3.048* E-01=m
in.x2.54* E+00=cm
*Conversion factor is exact.