Efficient Drilling of Ultra-HP/HT Wells in the Gulf of Thailand
A new measurement-while-drilling (MWD) tool has been designed that can operate reliably at 200°C and 207 MPa, providing real-time direction and inclination surveys, azimuthal gamma ray, annular and internal pressure while drilling, and shock and vibration measurements.
A new measurement-while-drilling (MWD) tool has been designed that can operate reliably at 200°C and 207 MPa, providing real-time direction and inclination surveys, azimuthal gamma ray, annular and internal pressure while drilling, and shock and vibration measurements. An operator in the Gulf of Thailand used this new MWD technology to drill wells in ultrahigh-pressure/high-temperature (HP/HT) reservoirs without the need to stop operation because of temperature limitations. Savings of ½ day per well have been achieved.
Offshore drilling in the Gulf of Thailand increasingly challenges logging operations, with temperatures greater than 200°C. Hundreds of wells need to be drilled per year in this environment to fulfill long-term gas contracts. High efficiency is required in such a high-volume operation.
Existing commercial MWD and logging-while-drilling (LWD) technologies are only capable of operating up to 175°C, with an inherent decrease in reliability at higher temperatures. In the planned ultra-HP/HT projects, the bottomhole assembly (BHA), which includes an MWD tool for directional and inclination measurements in real time, will be pulled out of the hole once the circulating temperature reaches 175°C. Then, a new BHA, without measurement tools, will be used to continue drilling to total depth. This “blind” drilling section could be several hundred meters long, introducing risks associated with well control and well collision.
The main limiting factors in having MWD/LWD tools operating in these ultra-HP/HT environments are the downhole electronic components. Industry research studies show that plastic-encapsulated components have a life expectancy of approximately 1,000 hours at 150°C; this drops to less than 100 hours at 175°C. Ceramic-encapsulated components last longer at 175°C, but they are bigger and heavier than their plastic counterparts. Because space is restricted in these tools, the best compromise yields a mix of both ceramic- and plastic-encapsulated components.
To find a solution to this industrywide problem, significant effort was made over the last decade to develop custom electronics that can withstand high downhole temperatures. The newly developed MWD tool equipped with these novel electronics components was deployed in ultra-HP/HT wells. In these wells, the real-time measurements were necessary to drill the sections with operating temperatures greater than 175°C. It allowed minimization of drilling risks, enabled proper well placement, and improved drilling efficiency by eliminating one run.
New HP/HT MWD Tool
The HP/HT MWD tool (Fig. 1) was designed and built with the objective not just to survive at high temperature but to perform reliably for a specified amount of time. The tool electronics can be mounted on a 6¾- or 4¾-in. collar and be fitted with flow kits to address different operational conditions. For this project, proprietary technology was developed to ensure reliable operation at 200°C and 207 MPa and lay the foundation for future HT developments.
The electronics are made of fully proprietary multichip modules (MCMs) and are powered by a downhole turbine. A downhole mud-pulse modulator enables real-time transmission above 6 bits/sec of data acquired by ultra-HP/HT sensors. These sensors provide directional and inclination surveys and gamma ray for well placement, formation evaluation, and geological correlation, as well as annular and internal pressure while drilling and shock and vibration measurements for risk mitigation.
The components are qualified by proprietary mission profiles that validate reliable operation. Such profiles subject electronics and sensors to the following:
- 2,000 hours at 200°C, –40°C/205°C cycles, 10 hours on plateau
- More than 1,000 shocks (500 g) at 200°C and aging in oven farm
- Electrical-connector qualification
- Highly accelerated life tests (HALTs), 60°C/min (up and down), –40°C/200°C cycles, cycling and with vibration 2 days per MCM
The final assembly undergoes pressure and temperature well tests at 200°C and 221 MPa, roll tests, and a functional flow-loop test to verify functioning under flow conditions.
Maturity of the product was evaluated by use of the four-axis methodology commonly used in reliability assessment:
- Performance: validation of the system level functions up 200°C in the laboratory
- Durability: more than 35,000 hours at temperatures greater than 200°C and 2,000,000 shocks at high temperature accumulated on the electronics
- Robustness: verified by HALT and by pushing duration of qualifications up to 2,000 hours at 210°C
- Variability: assessed on multiple batches of MCMs on four complete tools
Drilling of Ultra-HP/HT Wells in the Gulf of Thailand
The Arthit and Bongkot projects (Fig. 2 above) are operated in the concession area located in the southeastern Gulf of Thailand, with water depths ranging from 60 to 80 m. The Nang Nuan project is operated in the concession area of Block B6/27. This block is located in the west of the Gulf of Thailand, with water depth ranging from 25 to 35 m. The field development plan entails exploration and development wells with potential for gas production. The wells are deviated and encounter HP/HT conditions. The average total depth ranges from 3300- to 4300-m true vertical depth.
The reservoir section is drilled in 155.6-mm hole size. Exploration wells can be normal pressure or high pressure and encounter temperatures from 165 to 230°C. Drilling time ranges from 15 to 35 days, depending on well design and depth. Development wells can be normal pressure or high pressure and encounter temperatures from 165 to 230°C. Drilling time ranges from 5 to 15 days, depending on well design and depth.
The drilling volume is estimated to be 500 wells offshore Gulf of Thailand in the next 3 years. Out of this volume, 225 wells are planned at temperatures greater than 175°C.
Case Study: Arthit Field Well AT-19-E
The well objective in an Arthit field well was to reach the target reservoir with a bottomhole static temperature greater than 200°C. The main challenge was to drill this well in one run using direction and inclination services from MWD. These measurements aid in risk minimization, confirm proper well placement, and reduce the cost of additional runs attributable to exceeding equipment specifications and running gyro surveys.
Existing Work Flow. At present, industry-standard HT drilling services in Thailand demand tripping out of the hole once bottomhole circulating temperature reaches 175°C. This is necessary as a result of the limits of the currently available technology. MWD and directional tools must be removed from the BHA, and the remainder of the well is drilled blind to total depth. Once the final depth is reached, an additional gyro-survey trip is necessary to confirm well positioning.
New Technology-Enabled Work Flow. Deployment of the new HP/HT MWD tool enabled drilling the well section in one run. While drilling, the highest temperature recorded in this well was 186°C. A subsequent run with wireline equipment measured a maximum temperature of 201°C. The well plan required drilling a tangent section to the total depth of the well, where the temperature was projected to be 190°C. The measurements provided at a 3-bits/sec real-time data rate were direction and inclination surveys, temperature, gamma ray, annular pressure while drilling, and drilling mechanics.
Planning and Execution. Extensive preparation was necessary to deliver these results. Two tools that went through extensive manufacturing acceptance tests in two stages were manufactured to meet the well timeline:
- Engineering validation
- Gamma ray validation (functional, azimuthal system, noise robustness, and stabilization at high temperature)
- Pressure-measurement validation
- Direction and inclination validation
- Firmware baseline checks
- System-level checks
- Functional system validation in a drilling test rig
- Pressure and temperature well test at 200°C and 221 MPa
- Hot shocks
- Roll test
- Flow-loop test in which water was pumped through the tool to simulate downhole power and demodulate the measurements
Job execution and tool maintenance were performed at the field location without the need to go through engineering centers, thereby minimizing turnaround time and increasing tool availability. The typical checklist of tool maintenance includes
- Disassembling of the tool and all electronics
- Inspection of sensors and mechanical components
- Replacing O-rings and face seals
- Inspection of the oil-pressure-compensation system
- Filling the tool with nitrogen to remove humidity (Fig. 3)
- Completing outgoing system checks to prepare the tool for the next well
The tool was deployed in Thailand in 10 HT wells during 2013 with nearly 1,000 hours below rotary table. The maximum temperature recorded during the operation was 193°C. Following the drilling of the first well, with a maximum recorded temperature of 186°C, the tool was serviced locally in Thailand to verify its condition. The tool was then sent back offshore to execute the next four wells without maintenance between operations. The maximum temperatures in these wells were 191, 189, 183, and 184°C, with a total of 256 operating hours. In the third cycle with four downhole runs, the tool encountered a maximum recorded temperature of 193°C.
The introduction of a new MWD technology designed for ultra-HP/HT operation enables an operator in the Gulf of Thailand to drill HT well sections in one run at up to 200°C. It improves safety and efficiency of operations in drilling ultra-HP/HT wells. Savings of ½ day per well were demonstrated. This highlights potential savings of 25 to 37 days per year by applying this new technology in the 50 to 75 wells to be drilled at greater than 175°C every year. Drilling risks were reduced by the availability of real-time measurements to place the well. Because of the success of this initial deployment, more tools are currently being manufactured to be used in these projects to maximize the efficiency gains.
These outstanding results would not have been possible with existing downhole MWD-tool technology. The application of this technology enables safer and more-effective drilling of ultra-HP/HT wells.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25392, “Efficient Drilling of Ultra-HP/HT Wells in the Gulf of Thailand,” by Joy Araujo, Schlumberger; Keittipong Kaotun, PTTEP; and Panupun Dumrongthai and A. Adrian, mSchlumberger, prepared for the 2014 Offshore Technology Conference, Houston, 5–8 May. The paper has not been peer reviewed. Copyright 2014 Offshore Technology Conference. Reproduced by permission.