A microchip system capable of measuring temperature and pressure over the entire wellbore was developed and tested in the field. When used in the field, tracers will be injected together with the drilling fluid. As the tracer travels through the wellbore, it will measure the temperature and pressure throughout the wellbore and store the data in the on-chip memory.
Prototype
The prototype of the instrument system is shown in Fig. 1. The instrument system developed in this work includes two major components: the surface devices (Fig. 1) and the tracers (Fig. 2). A tracer (approximately 7.5 mm in diameter) consists of a small system-on-chip integrated circuit (SOC IC), which includes sensors, microcontroller, memory, transmitter and receiver circuits, and density-control material (hollow spheres), all encapsulated in a protective-chemical-coating shell. The surface devices include an initiator to reset the circuit on the tracer before the tracer is injected into the wellbore and a data collector to retrieve data from the tracer’s on-chip memory when the tracer is carried back to the surface by the drilling fluid. The initiator and data collector will use wireless communication to reset the circuit and download data from the tracer, respectively. A magnetic tracer separator (see Fig. 1) will be installed after the data collector to recycle the tracers. A lithium cell (battery) used in the tracer contains stainless steel, which can enhance the separation of tracers from the drilling fluid in the magnetic tracer separator.
When used in experiments or in the field, tracers will be injected together with the drilling fluid. Before a tracer is injected into the flowline, it passes through an initiator, which will reset the circuit for recording data. As the tracer travels through the wellbore, it will measure the temperature and pressure throughout the wellbore and store the data in the on-chip memory at a sampling rate set by the initiator. When the tracers are carried out of the borehole by the drilling fluid, they will pass through a data collector (controlled by a computer) through which the tracer will communicate with the surface devices to send the data stored in its on-chip memory.
Fiber-optic temperature and pressure sensors for harsh environments (up to 15,000 psi and 250°C) have been used in the field successfully during the last 10 years and will be integrated into the instrument system in this project. Other sensors can be integrated into the system when they are readily available.
When used in the field, magnetic rings can be placed on the tool joints (glued to the surface of the pin). An on-chip magnetic sensor can be fabricated on the SOC IC. As the tracer travels through the borehole, it could sense the weak magnetic field around each tool joint along the drillstring. This information can be recorded in the on-chip memory and used to determine the real-time location of the tracer in the wellbore.
SOC IC and Surface Devices. The device used in the initiator and the data collector is the same, a surface-device reader/writer module. The surface-device reader/writer and the tracer communicate through the wireless magnetic channel. The system can work in fully duplex mode or in half-duplex mode. In duplex mode, both the reader/writer and the tracer can transmit and receive simultaneously at different frequencies. In half-duplex mode, the reader/writer and the tracer transmit and receive at different time slots. Half-duplex mode can lead to simpler implementation at the expense of slightly slower communication rate in this particular application.
Each tracer has its own identification, and thus can be logged separately. The built-in lithium battery is rechargeable, allowing the tracer to be used many times.
Fiber-Optic Sensor. Currently, fiber-optic sensors are commercially available that are capable of measuring pressure and temperature up to 15,000 psi and 1,000°C, respectively, using a variety of methods. However, current realization of these sensors, including packaging, is not suitable for the current application. The focus, therefore, is the adaptation of proven sensor concepts to the unique environment and operating limits of the tracer.
One potential sensor technology is based on a Fabry-Perot-interferometer (FPI) structure. FPI-based sensors can be made small, having a cross-sectional area not much larger than the fiber diameter (125 μm) and lengths on the order of 1 mm. Both the extrinsic FPI and the fiber FPI configurations will be investigated. Both configurations can measure temperature or pressure with minimal crosstalk from the other measurement, with proper design.
Protective Chemical Coating. Tracers need to be protected against harsh downhole conditions. This can be achieved by encapsulating the stress-sensitive electronic parts of the tracer into a protective shell. The specific objectives of this protective shell are to shield the electronic parts of the tracer from high-pressure (up to 15,000 psi) downhole conditions, chemical attack, impact, and abrasion of the drillstring. The shell also plays a crucial role in reducing the effect of temperature variation on the integrity of the electronic parts. In addition, the density of the protective coating should be low enough to ensure the mobility of the tracer. A literature review of chemical-coating materials reveals that special types of porcelain, ceramic, thermoset, and composite materials have the potential to meet the required properties.
Laboratory Testing
Validation of Tracer Passing Through Bit Nozzles. One of the major concerns of the tracer is whether it can pass through drill bit nozzles. In order to prove that the tracers can do this, they were tested on a flow loop with a tricone drill bit attached at the end. Tracers were injected into the flow loop by the tracer-injection system. Injected tracers traveled through the flow loop and then passed the drill-bit nozzles at the end of the loop. Results show that the tracer injection functions well and a controllable tracer-injection rate can be achieved. In addition, all tracers passed the drill-bit nozzles without any difficulties.
Temperature-Sensor Test. Changes in the temperature of mineral oil in which the tracer was immersed were recorded every 20 seconds. At the same time, measurements by the tracer were taken at the rate of one sample per second. The integrated circuit was programmed to take a total of 500 data points. Therefore, the measurement took 8 minutes and 20 seconds to complete. After the test was completed, data collected by the tracer were transmitted to the data-acquisition system wirelessly. The tracer output is inversely related to the actual temperature change. These two curves were in a good agreement in terms of slope, response time, and curve trend.
High-Pressure/High-Temperature Survival Test. Tracer samples were placed in a chamber, and then pressure and temperature were applied to simulate harsh downhole conditions. Tests were conducted up 12,000 psi and 100°C. Results show that tracer samples can withstand 12,000 psi and 100°C without undergoing any structural problems.
Tracer-Mobility Test. Another major concern about the tracer system is tracer mobility. Tracers might travel a long distance in a wellbore and then have to flow back to the surface. In order to prove that the tracer can be transported by the drilling fluid, a tracer-mobility test was conducted on a full-scale flow loop. The full-scale flow loop had a 100-ft-long 8×4.5-in. annular test section. Tracers were injected from one end of the flow loop by use of the tracer-injection system developed in this study. After injecting the tracers, one can see that the tracers travel at a very fast speed in the test section.
Field Test
The first field test was conducted in an onshore field in Saudi Arabia. Before injecting tracers into the well, a tracer-retrieval system consisting of magnetic strips on the shale shakers was installed on the rig. In addition to the magnetic strips, an aluminum mesh basket was installed at the end of the discharge line. The aluminum mesh basket serves as the last point to trap the tracers.
Testing Well Information. For the first field test, the main goal was to validate the concept and find out if the tracers can be carried out of the wellbore by the drilling fluid and then be retrieved on the shale shaker. To simplify the test procedure so that we can focus on the major functionality of the microchip system, the tracer-injection system developed in this study was not used for the first field test. The tracers were deployed by directly dropping them into the drillpipe during the pipe connection. Fig. 3 shows the first three tracers to be placed into the drillpipe.
A stopwatch was started when the tracers were dropped into the drillpipe to record the time needed for the tracer to return to the surface. Circulation of the drilling fluid was started right after the pipe connection was completed, and a flow rate of 500 gal/min was maintained. It was estimated that the tracer should return to the surface approximately 50 minutes after the circulation started.
After injecting 13 tracers, seven tracers were found that returned to the surface. Out of the seven returned tracers, two tracers were intact and the other five were damaged.
Post-Test Processing. The two intact tracers were moved to the control room, and data were downloaded from one of them. During the download procedure, the circuit of the second complete tracer was found to be damaged. Pressure and temperature readings were downloaded from the on-chip memory of the intact-tracer circuit. It was observed that all the broken tracers had fractures on the plane where either the battery or the circuit board is located, which means that, by manufacturing the tracer in three layers, the integrity of the tracer was reduced. It is more difficult to make the tracer in one step, but, for greater mechanical strength, one-step manufacturing of the tracers may be required in the future.
Conclusions
- The concept of the microchip was proved successfully. Operations such as initiation, deployment, fluid capability to carry the tracers back to surface, and retrieval methods were tested and proved to be successful.
- The prediction of time for the tracer to return to the surface calculated by the simulator was close to the recorded time.
- Two intact pieces were retrieved out of 13 pieces deployed. In addition, five broken pieces were retrieved at the surface. By improving the fabrication process, the survival rate should be improved.
- No plugging of drill-bit nozzles occurred during the tests. Therefore, more pieces can be deployed in the future to increase the retrieval rate.
- The overall return rate was greater than 50%, much better than expected.
This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 159583, “A Distributed Microchip System for Subsurface Measurement,” by Mengjiao Yu, Sufeng He, Yuanhang Chen, Nicholas Takach, SPE, and Peter LoPresti, The University of Tulsa; and Shaohua Zhou, SPE, and Nasser Al-Khanferi, SPE, Saudi Aramco, prepared for the 2012 SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, 8–10 October. The paper has not been peer reviewed.