Ultradeep Resistivity Tool Maps Waterfloods Effectively
Authors: Carpenter Chris 
Published: December 2022

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 207385, “Ultradeep LWD Electromagnetic Directional Resistivity for Waterflood Mapping: A Game Changer,” by Parmanand Thakur, SPE, Maniesh Singh, SPE, and Saif Al Arfi, ADNOC, et al. The paper has not been peer reviewed.

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Abu Dhabi’s thick Lower Cretaceous carbonate reservoirs experience injection water overriding oil. The water is held above the oil by negative capillary pressure until a horizontal borehole placed at the reservoir base creates a small pressure drawdown. This causes the water above to slump unpredictably toward the horizontal producer, increasing water cut and eventually killing the well under natural lift after a moderate amount of oil production. Water slumping is difficult to forecast using the reservoir model. This paper showcases the successful deployment of an ultradeep electromagnetic (EM) directional resistivity instrument to map injection-water movement.

Ultradeep Electromagnetic Technology
Two of the most-significant drivers of the depth to which an EM field will penetrate the formation are the spacing between transmitter and receiver antennas and the transmission frequencies. These can be modified at the tool-design stage and with ultradeep EM tools by placing the transmitter and receiver antenna on different drill collars. Modifying the bottomhole assembly (BHA) allows the spacing between the antennas to be optimized for reservoirs of differing thickness (Fig. 1). Increasing the transmitter-receiver spacing, coupled with decreasing the transmission frequency, generates an EM field that penetrates further into the formation and allows a greater depth of investigation.
The transmitter is placed closest to the bit, with two receivers placed farther back in the BHA. To map reservoir units close to the wellbore in greater detail, the first receiver can be connected directly to the transmitter. To increase depth of investigation in thicker reservoirs, the spacing can be increased. The farther the receivers are placed from the transmitter, the greater the depth of detection.
Lower frequencies are used by these tools in the range of 2–64 kHz. In low- to mid-range resistivities, the lower frequencies provide a greater depth of investigation but with lower spatial resolution. At higher resistivities, greater depths of detection can be achieved with the higher-frequency options. In one field example, using the lowest frequencies and a transmitter-to-receiver spacing of 133 ft, it was possible to identify resistivity boundaries up to 225 ft away from the well.
Measurements from ultradeep azimuthal EM tools are pulsed to the surface as components of the EM field from multiple spacings for multiple frequencies. Inversion algorithms are used to convert this to a model representing the subsurface, depicting resistivity boundaries. The position of these boundaries can be tracked many feet from the wellbore and used to place the well in the optimal position during drilling. This technology can be used to map units that the well may never penetrate, such as the water zone in this case, which was anticipated to be approximately 80 ft above the well.