The greatest advantage of the azimuthal focused gamma detector is that it upgrades traditional 1D GR depth logs into 2D circumferential borehole radioactive imaging. This dimensional leap changes geosteering from blind drilling to visual trajectory steering. Mastering the full workflow from raw sector counting to geosteering decision-making allows operators and integrators to accurately assess the field performance of ZTFT-A type LWD tools.
Sector Division & Real-Time Azimuthal Imaging
In conventional downhole operation, the detection window of a focused gamma sensor adopts an azimuth opening of 90° or narrower. Equipped with a tool face sensor, the tool divides the 360° borehole circumference into 4, 8 or 16 sectors during rotation, and accumulates gamma ray counts independently for each sector.
As the drill string rotates, the acquisition system allocates every gamma pulse to the corresponding sector storage by tool face angle. After statistical smoothing within seconds to tens of seconds, a complete borehole radioactive unfolding map is formed, known as azimuthal gamma imaging. It provides intuitive circumferential formation radiation distribution for real-time geosteering.
Physical Mechanism of Formation Boundary Identification
Boundary early warning relies on obvious radioactive difference between reservoir and surrounding rock formations. In typical sand-shale sequences, reservoir sandstone shows GR values of 30–60 API, while overlying shale reaches 120–180 API.
During horizontal drilling in sandstone, when the bit approaches the upper shale boundary, gamma intensity from upper sectors rises sharply due to shale penetration of radiation paths. On the imaging map, the corresponding sector displays higher gamma counts with brighter color. The asymmetric sector response directly indicates the relative distance and direction between the bit and formation boundary.
Look-Ahead Detection & Well Trajectory Optimization
Installed at near-bit position with forward-tilted detection windows, the azimuthal focused gamma detector realizes effective look-ahead detection. It identifies abnormal formation radiation ahead of the bit before crossing formation boundaries.
Combined with real-time LWD geological model updating, geosteering engineers can adjust build and drop angles in advance, keeping the well trajectory stably in the reservoir middle layer. This prediction-adjustment-verification closed-loop control boosts reservoir penetration rate to over 90%, greatly superior to the lagging response of conventional single GR logging.
Data Compression & Mud Pulse MWD Transmission
Azimuthal gamma imaging produces much larger data volume than traditional single-channel GR logs, while common mud pulse MWD systems only support ultra-low transmission bandwidth of several bits per second.
To adapt to limited downhole bandwidth, the downhole processor adopts optimized solutions: sector merging from 8 to 4 quadrants, transmitting only maximum-value sector direction and count rate trend, and uploading compressed imaging slices.
ZITN focused gamma detectors output standard negative pulse signals directly. All data processing and compression are completed by the built-in acquisition & processing board. The modular structure enables system integrators to flexibly match and configure different MWD/LWD transmission systems for various downhole working conditions.