With the further exploration of oil and gas, we have to search for new resources which buried in deep strata, and most of the deep and ultra-deep wells are categorized in high-pressure/high-temperature (HPHT) wells. The problem of high temperature and the challenge to the existing downhole equipments are becoming increasingly prominent, where the drilling depth is severely restricted. Most of the HPHT or ultra-HPHT wells with reservoir temperature about 175~220°C would be drilled with near-bit measurement and constructed. In such a temperature environment, the conventional measurement while drilling tools with common electronics will experience very high failure rates at these conditions.

There are two ways to deal with the downhole electronics system in HPHT wells. One technique is called the passive cooling, which aims to improve the anti-temperature performance of the components and add thermal insulation materials to the circuit module. In this way, the cost of the instruments would be greatly increased, and the capacity for anti temperature could not be improved indefinitely, especially in HPHT or ultra-HPHT environment for lone period. The other solution is called the active cooling, which commit to the construction of a downhole refrigeration system. The cooling system power by bettery or downhole generator ensures clectronics cabin always under suitable temperature.

According to a large number of research work by scholars worldwide, there are 2 main kinds of downhole active cooling techniques suitable for while-drilling environment. One is thermoelectric based regeneratiove cooling system, and the other technique is based on regenerative cryogenic refrigeration cycle. The thermodynamic cycle and working fluid are key concerns for the refrigeration. While reverse-Brayton cycle contains adiabatic compression, isobaric heat transfer, adiabatic expansion and isobaric heat transfer, so thermoacoustic coolers, stirling cryocoolers, and pulse tube refrigerators can all be suitable solution.

The study from this work demonstrates the active cooling method for downhole systems using in high-temperature environment, and provides a baseline framework for design methods. The preliminary laboratory test and application showed the feasibility of the method.

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