In recent years, the development of domestic petroleum logging tools has been accelerating. Today, instead of discussing their structure and functions, we will focus on the processing of data after acquisition, namely signal transmission and data storage. As we all know, logging-while-drilling (LWD) tools can collect formation information near the drill bit in real time. How to efficiently and completely obtain such deep subsurface information is closely related to signal transmission technology and high-temperature memory technology.
The key technology of logging-while-drilling is signal transmission, among which mud pulse telemetry is widely adopted as a common method for LWD tools. It converts measured parameters into mud pressure pulses, which are then transmitted to the surface through mud circulation.
The advantages of mud pulse telemetry are cost-effectiveness and convenience, while its drawback lies in the low data transfer rate (bits transmitted per second), with a maximum rate of only 4 to 10 bit/s. This can only meet real-time data transmission demands to a certain extent.
In recent years, electromagnetic wave transmission technology has been trialed to improve the transfer rate. With this technology, the LWD tool is placed inside a non-magnetic drill collar, and an insulating sub is installed between the non-magnetic drill collar and the upper drill pipe to facilitate the propagation of low-frequency electromagnetic waves carrying measured information to the surrounding formation. On the surface, the voltage difference between the drilling rig and surface electrodes is detected. Early electromagnetic wave transmission was not commercialized due to severe signal attenuation, short transmission distance and high cost. However, with technological improvements in recent years, it has entered the market. Its advantages include a high transfer rate and immunity to mud performance. Nevertheless, its application depth in petroleum drilling is greatly restricted by signal absorption from formation media, generally limited to no more than 3,000 meters.
Whether it is mud pulse transmission, acoustic transmission or electromagnetic wave transmission, the excessively low telemetry data rate has long been a thorny issue, which severely reduces drilling operation efficiency and increases operational costs. Therefore, improvements in this regard are necessary.
This calls for an alternative approach: a downhole storage method. Acoustic information requiring real-time processing is telemetered to the surface via mud pulses, while a large volume of processed results and raw waveform data are temporarily stored in high-temperature memories, with data retrieved after tripping out. This reduces the transmission load and retains all raw data during drilling to the greatest extent. It features low cost and reliable data preservation, yet has the disadvantage of failing to provide real-time data to the surface for drilling guidance. For LWD operations with massive data volumes, such as logging-while-drilling imaging logging, a combination of real-time transmission and downhole storage is usually adopted: real-time transmission for critical well sections and downhole storage for other sections.
Moreover, memories used for LWD must possess excellent 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.
For continuous writing operations at 210°C, the LHM series high-temperature memories are generally selected. They can retain data at high temperatures for up to 500 hours and have a service life of 2,000 hours, with no failures reported within their service life to date. However, the LHM series has limited capacity, with conventional models including LHM128M, LHM256M and LHM512M. Ultra-large capacity requires series/parallel connection of multiple units or customized development of higher-capacity models. For applications with a maximum operating temperature of only 175°C, the LDMF series memories are available, offering conventional large capacities of 1GB and 4GB, which are generally sufficient for logging-while-drilling data storage.
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