Several technical advances have resulted in a wireline system that can tackle high-pressure, high-temperature well conditions.

New formation evaluation technologies have been put to the test logging some of the world's deepest wells. As wells get deeper, their pressures and temperatures rise, which increases the risks associated with their evaluation. However, despite the increased risks and costs, the number of high-pressure, high-temperature (HP/HT) wells being drilled is increasing worldwide. The depths, pressures and temperatures of these wells have challenged formation evaluation technology, resulting in significant technological developments in the areas of conveyance and logging tool design.
Strengthening cables
When logging ultradeep wells, extreme depths limit wireline conveyance technology because of the high surface tensions encountered, which result from increased drag and the weight of the logging cable. Deploying logging tools on drillpipe using a specialized conveyance system can overcome these limitations, but it is slow and much more costly due to the reduced efficiency associated with tripping drillpipe.
To solve the ultradeep well dilemma, a stronger yet lighter cable had to be developed. This would allow it to support its own weight and that of the logging tools it would convey as well as overcome the high drag associated with deep wells. Traditional materials and manufacturing methods could not produce sufficiently strong cables since they would be too heavy to be practical for deep logging jobs. Therefore, a new design and high tensile-strength materials were combined to improve the cable's strength member, or armor package.
The cable's core is responsible for carrying power down to the tool and data back up to the acquisition system. When the cable is under high tension, its core is subject to tremendous pressures from the armor package. Therefore, the new cable design also needed a newly designed core to prevent it from being crushed by the high tensions.
After extensive testing and design qualification, the new cables developed enabled Schlumberger to safely and efficiently log ultradeep wells on wireline. The efficiency gains resulting from wireline logging of these wells are passed to the operator in the form of cost savings.
Improving surface equipment
Improving surface equipment ratings also was necessary for safe, ultradeep wireline logging. During any logging operation, the cable is under its highest tension at surface. Handling this tension in a safe and efficient manner is critical to the success of an ultradeep logging job. Surface equipment must be designed to withstand high tensions, and it must pass certifications such as Det Norske Veritas.
Storing cables under high tensions can result in electrical core failure as well as catastrophic storage drum failure. Tension-relief systems must be employed to reduce these extreme tensions at surface before the cable can be safely spooled onto a storage drum. While several types of systems are available, the dual-drum capstan is preferred because of its operational simplicity and reliability.
The dual-drum capstan is a pulley-type conveyance system that decreases the cable tension before it is spooled onto the storage drum. Capstan control is fully integrated with the storage drum for ease of operation, allowing the winch operator to focus on actual well conditions. Positioned between the storage drum and the lower sheave, the two-drum system can relieve 20,000 lbf or more of tension at logging speeds in excess of 20,000 ft/hr (Figure 1).
Enhancing telemetry
The extra-long cables required for ultradeep well logging must be capable of transmitting sufficient power downhole to run high-power, complex tool strings while allowing large amounts of data to be transmitted back to surface. Unfortunately, standard logging telemetry systems have proved unreliable at extreme depths. When running long cables in deep wells, nonlinear effects such as temperature, cable tension and geometry work to reduce the amount of power that can be transmitted downhole. These effects also change the cable transmission characteristics, causing unreliable telemetry performance.
Analysis of early telemetry problems along with modern digital signal processing has resulted in telemetry systems that compensate for the signal distortion occurring on long cables as they are lowered into a well. Also, downhole power supply improvements enable numerous, complex tool combinations to be run. As a result, many recent deep-well logging jobs have been successfully run using 36,000-ft (10,980-m) cables with excellent results and data quality.
Broadening tool capabilities
Extreme HP/HT conditions can have different effects on each of the many sensors in a tool string. Therefore, each sensor must be addressed separately when it is modified for increased pressure and temperature limits and improved capabilities.
In many cases, a logging tool's sonde is compensated for pressure, but its electronics are not. The sonde contains the sensors as well as minimal electronics and other hardware components that provide the downhole formation measurements. The electronics contained in cartridges provide sonde control and power for the sensor packages. These electronics also digitize the data received from the sensors so that they can be transmitted back to the surface. Elastomer seals protect the sensor packages and electronics in the cartridges from crushing pressures. Even a small leak in one of the many seals of a modern logging string will flood the entire string and destroy it.
Extreme bottomhole temperatures also challenge the electronic components. As temperatures increase, the life expectancy of electronic components is reduced exponentially. Temperature increases that occur inside a logging tool originate from two sources: the heat flow from the outside environment and the heat generated internally from power consumption. During the past decade, reduced demand for high-temperature electronics has eliminated 350°F (176°C) integrated circuits as a viable option for logging tool design. As a result, most new logging tools are rated to 300°F (149°C).
The Dewar flask provides one solution for HP/HT logging tools that contain modern electronics. The flask insulates the electronics from the borehole's high temperatures and increases the pressure ratings for the logging tools. Although it is a relatively easy matter to flask cartridge electronics, the sensors and electronics in the sonde must be able to withstand the well's full temperature and pressure ranges.
A new logging suite meets the challenges of HP/HT logging. The sensors and sondes of this string are designed to provide high-quality, accurate formation evaluation data at pressures as high as 25,000 psi and temperatures to 500°F (260°C). Table 1 lists the measurements that can be obtained in deep HP/HT environments.
The logging suite can be combined with many other tools, depending on the specific logging requirements and bottomhole conditions.
The array induction tool provides five depths of investigation and three high-quality, vertical-resolution measurements. Tool output is corrected for borehole and environmental effects. Its invasion profile, water resistivity and saturation, and true and invaded-zone resistivities allow a better understanding of reservoir characteristics.
The sonic logging tool uses a digital algorithm for detecting first arrivals and provides robust borehole-compensated formation slowness for evaluating unconsolidated and altered formations. The tool also can record cement quality logs for evaluating cemented casing.
The density pad tool measures formation bulk density and photoelectric factor using full spectral data from a two-detector array, and includes a powered caliper that enhances the measurement in washouts and rugose holes.
The array porosity sonde uses a neutron generator rather than the traditional chemical source to measure epithermal neutron porosity and formation capture cross-section for clay indication, salinity computations and grain size estimations.
At top is the HP/HT logging head, which generates real-time, downhole head tension and mud temperature measurements. Below the head is the integrated telemetry, gamma ray and accelerometer cartridge, which can be used to provide speed corrections to correct for irregular tool movement downhole.
In addition, the logging suite has been designed to meet shock and vibration specifications for enhanced reliability. It may be drillpipe-conveyed or combined with a variety of knuckle joints and centralizers to meet various borehole challenges.
Tools modified for HP/HT also include sidewall coring, fluid sampling and pressure testing tools. High pressures and temperatures greatly increase the challenges associated with formation coring and fluid sampling since the specialized tools require that more electronics be exposed to full wellbore temperatures and pressures. They also have many moving parts that must operate over wide pressure and temperature ranges. Additionally, there are safety concerns when handling highly pressured fluid sample chambers at surface.
Specially designed seals are used to allow HP/HT fluid sampling. Advanced seal technology also allows the samples to be safely returned to surface without losing seal integrity, allowing quality reservoir fluid samples at near reservoir conditions to be analyzed later in the laboratory. On one descent into a well, a specially designed tool can acquire multiple formation fluid samples and pressures at hydrostatic pressures and bottomhole temperatures of up to 25,000 psi and 400°F (204°C).
In addition, filtrate contamination levels can be monitored while formation fluids are pumped out of the formation and into the borehole. Software is used to continually monitor contamination levels and predict how long formation fluids must be pumped into the borehole before the desired contamination level is reached, allowing the operator to optimize the sampling program in real time and save rig time as a result.
Looking toward the future
Wireline conveyance and formation measurement technologies are continually being developed to meet the requirements of HP/HT environments. Other deep well issues are the smaller borehole sizes, which often are less than the published minimums for some downhole tools. A slimhole version of Schlumberger's well logging platform helps overcome these limitations while exceeding the HP/HT ratings of the larger tool. Technological innovations have enabled numerous formation measurements to be combined into an advanced 3-in. design. The integrated slimhole logging tool string can be run in ultradeep wells having diameters as small as 37/8 in.
References
Henkes, I.J. and Prater, T.E.: "Formation Evaluation in Ultra-Deep Wells," SPE/IADC Paper 52805, IADC/SPE Drilling Conference, Amsterdam, Holland, March 9-11, 1999.
Baird, T., Fields, T., Drummond, R., Mathison, D., Langseth, B., Martin, A. and Silipigno, L.: "High-Pressure, High-Temperature Well Logging, Perforating and Testing," Oilfield Review, Summer 1998.

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