High bottomhole temperatures can significantly degrade the life of electronics used in LWD and MWD tools. In the last decade, the number and temperature of hot wells has steadily increased. While MWD/LWD tools rated to 175°C/347°F can drill most of them successfully, the conditions are difficult and it is not unusual for failures to result in loss of data about the hole location, drilling the final reservoir section without logging data, or having to make a costly trip to change out tools or time to run wireline.
This reality is compounded by even hotter drilling prospects where reservoirs may reach an estimated static bottomhole temperature (BHT) exceeding 220°C/428°F. The solution to these growing temperature extremes is a better-informed approach to how the tools are run, maintained and ultimately designed.
Consumer electronics are not required to work above 65°C. This is the result of one hour's exposure to 190°C.
A time-based conditional maintenance programme being applied by Weatherford and a major operator in the Gulf of Thailand is providing insights into failure modes in these conditions, and how individual MWD/LWD electronic components respond to temperature cycles and vibration. These insights are helping improve performance for less non-productive time (NPT), as well as providing the basis for a new generation of tools for 200°C-plus reservoirs. The approach depends on a detailed understanding of component performance under operational conditions and how the conditions relate to tool failure.
Heating up the GOT
Each year in the Gulf of Thailand (GOT), operators drill more than 350 wells where bottom hole temperatures exceed 150°C/302°F. The number of wells and the BHT have been steadily increasing.
Thailand high temperature market.
In 2008, when Weatherford and a major operator embarked on a conditional maintenance program for MWD/LWD tools, there were three high temperature GOT runs that accounted for 125 operating hours and a maximum temperature of 308°F. The next year there were 27 runs for 1665 total hours; ten of those runs exceeded 320°F and a maximum temperature of 327°F was recorded.
By 2010, a total of 84 runs accounted for 4551 operating hours and 13 of those runs were greater than 340°F with a maximum of 358°F. In 2011, there were 88 runs; 41 were more than 340°F and the maximum temperature was 358°F. The following year, a maximum of 374°F was reached.
In 2013 and 2014, the number of runs and temperatures continued to increase—last year there were 106 runs and four were more than 360°F. Half way through 2014, there have been 100 runs, including 11 that were more than 360°F. The maximum reached was 374°F.
As part of the condition based maintenance program, preventive maintenance temperature tracking (PMTT) was employed to reduce non-productive time and contribute to development of 200°C-plus tools. The program played a significant role in Weatherford’s development and deployment of a suite of HeatWave HEL (hostile environment logging) tools for MWD/pulser, vibration, pressure, azimuthal gamma ray, and thermal neutron porosity measurements. The tools have a maximum temperature of 374°F (190°C) and a pressure rating of 25 000 psi in 9 ½ in. and 8 ¼ in. diameters, and 30 000 psi in the 6 ¾ in. and 4 ¾ in. diameter versions.
In their initial GOT application, the tools performed for the full 83-hour run. For 29 of those operating hours, temperature was above 338°F (170°C). The tools were deployed because the anticipated BHT was approximately 374°F (190°C), which exceeded the previous generation of HEL tools rated to 356°F (180°C). The performance saved a 24-hour trip to save an estimated US$250 000.
Much of this advance has been based on understanding the performance of the electronic components used in these tools. The relatively small L/MWD market for these component means the industry must depend on electronics built for the general consumer market, and these systems are not required to work above 60°C.
Understanding how these electronics perform in downhole conditions is critical to building condition based maintenance programs and improving overall performance. A key aspect of this process is rigorous testing under wellbore conditions.
For example, Weatherford tests electronic components to simulate heating and cooling cycles that occur as tools are run in and out of the hole. This baking and function-testing process increases temperature from 77°F (25°C) to a 302°F (150°C) dry-out period to remove moisture from the components. This is followed by a ramp-up to 392°F (200°C) and an extended period at 365°F (185°C.) The temperature is then dropped to 14°F (-10°C) before concluding at 77°F. This cycle is repeated 10 times.
Histogram tracks battery temperature to inform conditional maintenance programme.
Monitoring temperatures in the hole also provides valuable data. By monitoring battery temperature during bit runs, histograms are compiled to show the time spent at various temperatures. Acquiring this data provides operational context for developing conditional maintenance schedules in which preventive actions for various sensors and components are based on indexed hours at specific conditions. Since a component might last 5000 hours or 30 000 hours depending on conditions, this context is critical to taking timely, informed and efficient preventative action.
In addition to temperature, vibration is an important conditional factor in tool life. The GOT programme also employed a preventive maintenance vibration tracker (PMVT) to develop a torsional dynamics indicator histogram and capture high frequency torsional oscillation.
The dynamic sensor employs a novel angular rate gyro to acquire a very high sample rate measurement of angular rotation, which is used to generate torsional, lateral and axial vibration metrics. A key measurement is high frequency torsional oscillation that is a major source of vibration damage to electronics, mud motors and PDC bits. It is prevalent in drilling carbonate formations, which are characteristic of GOT drilling.
The high-resolution data is used in preventative maintenance algorithms that constantly monitor the downhole environment and generate warnings at surface when conditions require mitigation or exceed operational guidelines. The data contributes to drilling optimisation and assessment of the damaging nature of particular formations.
Current and future advances
Testing and downhole monitoring of temperature and vibration is the basis for condition-based maintenance that is yielding significant advances in LWD/MWD tool performance in high-temperature, hostile environments. In addition to improving reliability and reducing NPT, the data gleaned from this process is critical to an understanding of component performance and downhole failure modes needed to design LWD/MWD systems for even greater extremes.
Thermal testing subjects LWD/MWD electronic components to downhole temperature cycles.
Adapted for web by David Bizley