Richard C. Liu

It is hard to believe that, in an era of rapidly developing information and communications technology, the communication method from downhole drilling systems to the surface in oil and gas exploration is still mechanical, with a data rate of 10 bits per second, versus a data rate of widely used wireless local area network at 52 megabits per second at 2.4 GHz IEEE802.11g standard.

Unlike the environment in air, electromagnetic waves decay very fast in earth formations, making it very hard to send data using electromagnetic waves.

However, combined with single-chip radio or MEMS radio and wireless communication technology, the downhole telemetry problem is being solved with a data rate in kilobits per second, which is hundreds of times faster than mud pulse technology.

Directional drilling and geo-steering are common practice nowadays. Helped by various sensors and actuators, the drill bit is controlled according to the predetermined drilling route and adjusted in real time by data from sensors. These sensors measure many physical parameters of earth formations such as electrical, sound and nuclear properties. There are also sensors used for drilling control such as gyroscope, magnetometer, gamma ray sensor and inclinometer. While most data is processed by downhole computers, communication to the surface data center is critically important for analysis and decision-making.

In oil and gas exploration, one of the most important factors to be considered is reliability. If the surface-to-downhole communication link is broken, drilling has to be stopped. Rig time and other costs will be significantly increased.

Transmitting data from downhole to surface at a high data rate is a constant pursuit of the drilling and logging industry. Many methods were attempted in the past decade. Besides increasing mud pulse data rate, the electromagnet method is an alternate. The electromagnetic telemetry system uses a transmitting coil downhole, encoded with data to be transmitted, sending electromagnetic signals to the surface. A large surface receiving coil is placed on the surface to collect the weak signal from downhole, which is decoded into original data. However, because of attenuation of the formation to the electromagnetic waves, the electromagnetic method has to use very low frequency (a few tens of Hertz). Therefore, the data rate is limited to a few tens of bits per second, with a limited transmission distance of a few thousand feet.

Wired solutions with coupling coils at each end of a drillpipe joint provided hopeful solutions. This method is a hybrid of wired and wireless solutions. A pair of wires or a coaxial cable is buried inside the drilling pipe and used as signal path inside the pipe. A receiver coil and repeater is installed at one end of the pipe and a transmitter coil is installed at the other end of the pipe. When pipes are physically connected, signals can pass through joints via the coupling transmitter and receiver coil pairs. The repeaters in drilling pipes are powered by batteries. However, drilling companies are hesitating in using such technology because of cost and reliability issues.

Another attempt at a solution was to use long, continuous-length, spoolable composite pipe or coiled tubing with embedded wires. One spool of composite pipe can be as long as a few thousand feet. When manufactured, wires and cables are buried into the coil tubing walls. Therefore, it solves the problem of coupling from section to section because composite pipe is one piece, with no connection necessary. This idea did not pass market tests because of its high cost and poor reliability.

Oil-base mud has a very low electromagnetic loss. Therefore, oil-base mud is ideal for use as a data transmission medium.

Recent developments in wireless communication and semiconductor technology made it possible to develop radio systems using a single integrated circuit (IC) chip. These radio frequency (RF) IC chips, or RF MEMS, work at low transmitting power, low supply power and high frequencies. The cost of each radio can be less than US $1. The new idea of solving this downhole telemetry problem is to use such radio chips in large quantities in oil-based mud. As shown in Figure 1, the radio chips can be deployed into the mud either from surface or released from downhole equipment. The distance between radio chips is a few meters. Radio chips either flow down from surface inside the drillpipe, or flow up the annulus when released from downhole devices.

The radio chip can also be pre-installed, at a fixed spacing, inside drillpipe. Once activated, these radios will act as repeaters, to relay downhole information to the surface. The radio chip is packaged with necessary battery, antennas and other components as shown in Figure 2. Each battery will last a few weeks to a few months.

Because of the high operating frequency, the data rate each individual radio can reach is in megabits-per-second level. Counting communication overhead and relay protocol overhead, the overall data rate this system can offer is in kilobits per second. Consider a well of 10,000 ft (3,048 m) - if maximum radio spacing is 10 ft (3 m) with redundancy included for reliability, this well will need 1,000 radios. If each radio system cost $10, the total communication cost is in the range of $10,000 per well. In this new data telemetry system, radio waves transmit only in the oil-based mud to increase transmitting distance. Each radio IC is a transceiver, so that acknowledgement signal will be received from the transmitter to guarantee reliable data transfer from radio to radio.

This innovative downhole data transmission method has been prototyped and tested in laboratory conditions at the University of Houston. Initial lab tests showed that the idea works as expected. More detailed work is being pursued such as more efficient antennas, more effective communication protocol and ways to further increase reliability. The researchers are now looking into an initial field test to verify this idea in a practical drilling system. A few companies are also interested in this project and expressed willingness to license the technology.