The near-bit measurement system in petroleum logging-while-drilling tools consists of two components: the near-bit measurement sub and the near-bit reception sub, which communicate via a wireless transmission system. The primary transmission methods include mud pulse transmission, electromagnetic wave transmission, and acoustic wave transmission.
The near-bit measurement sub is connected to the drill bit and mainly comprises sensors, a transmitting antenna, a control circuit, and a battery pack. The control circuit acquires downhole data such as geological and engineering parameters, processes the data, and sends signals to the transmitting antenna, which emits the signals in the form of electromagnetic waves to the receiving antenna.
Located above the screw drill, the near-bit reception sub mainly consists of a receiving antenna, a control circuit, a power supply circuit, and a memory module. It is responsible for receiving signals transmitted by the transmitting antenna, then processing and storing the signals. In the near-bit geosteering system, the reception sub can also communicate with the MWD (Measurement While Drilling) system to transmit data back to the surface via the MWD.
Mud pulse transmission technology is a widely used data transmission method in logging while drilling at present, with a maximum transmission rate of only 4–10 bit/s. It meets the demand for real-time data transmission to a certain extent.
Electromagnetic wireless transmission does not require drilling fluid as a signal carrier and has better adaptability to underbalanced drilling. However, due to signal absorption by formation media, its application depth in petroleum drilling is greatly limited, generally not exceeding 3000 meters.
Whether using mud pulse transmission, acoustic transmission, or electromagnetic transmission, the excessively low telemetry data rate has always been a difficult problem, which seriously reduces drilling progress and increases operating costs. Therefore, improvements in this area are necessary.
Acoustic information requiring real-time processing is telemetered to the surface via mud pulses, while a large number of processing results and raw waveform data are temporarily stored in high-efficiency memory. This reduces the transmission load and retains all raw data during drilling to the greatest extent.
Another approach is to adopt a downhole storage method: acoustic information for real-time processing is telemetered to the surface via mud pulses, while massive processed results and raw waveform data are temporarily stored in high-temperature memory, and the data is retrieved after tripping out. This reduces data transmission volume and maximizes the preservation of all original data during drilling.
The advantages are low cost and reliable data storage. The disadvantage is that real-time data cannot be obtained at the surface to guide drilling.
For logging-while-drilling applications with large data volumes, such as logging-while-drilling imaging, a combination of real-time transmission and downhole storage is usually adopted: real-time transmission for critical intervals and downhole storage for other intervals.
These memories for logging-while-drilling must have strong high-temperature resistance. They must enable data writing at high temperatures of 175°C or even above 200°C and maintain data integrity for long periods in high-temperature environments.