The number of HP/HT wells is increasing in many areas of the world, including the North Sea, the Gulf of Mexico, and Southeast Asia. Several factors are responsible for driving the search for hydrocarbons in these hostile environments. Existing fields are becoming depleted, and easier targets are scarce. The high pressure means that relatively more hydrocarbon is contained in these fields compared with normally pressured fields. Moreover, as long as the fields are large enough, they make commercially attractive targets even with the technical difficulties.
Application of hybrid circuitry (hc) in tools has expanded temperature limits. The circuits use ceramic substrate instead of typical polyimide materials. Each company using this technology has created proprietary methods of attachment for individual components. Hybrid circuits are used in the quartz-based pressure transducer for very high accuracy and reliability.
Typically, companies put conventional logging tool electronics in Dewar flasks. This helped meet the early challenges of HP/HT environments. As logging expands into more extreme environments that push beyond the safe operating envelope achieved with flasks, new technology is needed to meet the ever increasing demands.
New developments in elastomer technology have allowed operations up to 321°C (610°F). Parfluor Ultra material (PTFE) is used exclusively for these temperatures. The use of metal seal technology has expanded this upper limit to more than 427°C (800°F) for limited times downhole.
Special qualification is needed on all components and modules used in the production logging string. Heat qualification is needed for all tools prior to running in the well. Clear operations plans are needed to limit the time in the well at elevated temperatures. It is very important to work together and make sure all limitations are understood. The detrimental effects of temperature can be seen when the electronics of a new tool are compared with a previously used circuit board. Rigorous job preparation is critical to success in HP/HT logging.
Intensive post-job maintenance is needed for all HP/HT equipment. Many sensitive components, though in working order, are routinely replaced to ensure optimum performance on subsequent logging jobs since temperature cycling often results in failure. High temperatures are particularly wearing on O-rings and seals, so all are replaced before the tools are reassembled and tested.
Logging HP/HT wells
Electronic components such as integrated circuits are frequently the weakest link in a logging tool. Commercial-grade components require careful testing and qualification if they are expected to go to 175°C (347°F). Temperature increase inside the tool depends on two factors: the heat flow from the outside environment and the heat dissipated by the power consumption of the internal components. The use of low-power components and efficient circuit design helps reduce the internal temperature increase, and the use of a heat-sink to increase the thermal mass inside the Dewar flask slows the rate of temperature increase.
Dewar flasks are double-walled, vacuum-jacketed tubular vessels. They are installed into separate pressure housings, protecting the instruments from temperature and pressure. Materials with poor thermal conductivity in the opening of the flask help to slow the rate of external heat
entering the flask and increasing internal temperature.
The use of memory-based tools instead of the standard surface read-out helps in simplifying wireline operations. No high-temperature conductor line is needed, only slick-line. Slickline is a nonconductive solid wire that has a much smaller diameter than even the smallest conductor lines. The smaller diameter allows the use of less weight in the toolstring to overcome the effect of high surface pressures on the line at the surface seal. Less weight equals shorter lengths of pressure control equipment.
No complex pressure control gear is needed to maintain well control when using slickline. Internal tool temperature limit is controlled by the maximum temperature rating of the battery that is used to power the downhole tools. Most battery manufacturers use lithium thionyl chloride chemistry to achieve high power density as well as very high temperature ratings.
Operations in the geothermal environment are considered to be in the ultra-HP/HT or even HPHT-hc classification. Pressures typically are not very much over normal pressure gradient, but temperatures can typically be above 315°C (600°F). An example logged in northern Japan was a hot dry granite well being prepared for steam production. The bottomhole pressure was only 2,700 psi, but the temperature was 345°C (653°F). A special flask using a metal O-ring as the primary seal was run in this well. Time downhole was just over 5.5 hours.
Steam injection is another HT application. Steam injection is used as a method of lowering viscosity of oil in the formation. It allows oil movement in the formation and finally the production to the surface.
Production logs are used for determining zonal contribution, measure steam quality, and compute enthalpy. This is true for both injection and production wells. Density can be computed using nuclear density tools. A recent steam injection logging campaign in Russia involved the use of nuclear fluid density tools to measure the density of saturated steam. In conjunction with pressure and temperature data, density information was used to infer steam quality and enthalpy of the injected steam. Maximum temperature (at surface, in the case of steam injection) was around 282°C (540°F), which classified these wells as HPHT-hc.
HP/HT operations can be completed successfully if proper job planning, tool preparation, and maintenance are completed. When operating in these very hostile environments it is essential that everything is considered.
HP/HT tools can be organized into three categories, selected according to commonly encountered technology thresholds. Classification system boundaries represent stability limits of common well-service tool components, elastomer seals, and electronic devices.
- Standard tools are rated up to 150°C (300°F) bottomhole temperature and 10,000 psi bottomhole pressure.
- HP/HT tools are rated for 150°C to 204°C (400°F) bottomhole temperature and 10,000 to 20,000 psi bottomhole pressure.
- Ultra-HP/HT tools are rated for 204°C to 260°C (500°F) bottomhole temperature and 20,000 to 35,000 psi bottomhole pressure.
- HP/HT-hc tools are rated for 260°C to 315°C (600°F) bottomhole temperature and 35,000 to 40,000 psi bottomhole pressure.
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