Glorious mud

Techniques borrowed from the latest wireless networking standards are helping oil and gas engineers reach new deposits.

"Mud, mud, glorious mud, nothing quite like it for real-time bore-hole communications."

Okay, so perhaps mud-based telemetry doesn't have the same ring to it as the wallowy hippopotamus in Flanders and Swann's famous song, but for engineers involved in oil and gas exploration, there has been nothing quite like mud for delivering vital information. Perhaps more surprising is that mud-based communication has recently benefited from the introduction of techniques that are also used to boost data-rates in wireless networking.

Drilling for oil and gas these days is complicated: the easily accessible wells were drilled many years ago. A modern well will often have a vertical section, a curved part and then a horizontal path into the reservoir of oil or gas. Many reservoirs are made up of multiple small pockets, so part of the drilling process is to connect these into one well.

These subterranean labyrinths will run for thousands of metres, with typical ambient temperatures of 80°C to 110°C (or above 200°C in some cases). Pressure at the drill head will be between 15,000psi and 20,000psi, and vibrations during drilling could range from 3g to above 10g.

"You want to drill accurately, so you don't drill into formations like a previous well, and you want to drill quickly, bearing in mind that the day rates are at least $1m to run an offshore drilling installation," explains Ingolf Wassermann, a signal-processing expert at drilling technology and services company Baker Hughes Inteq.

Drillhead feedback

To do this, information has to be gathered while drilling to give feedback from the mechanically controlled drill head to the engineers on the surface. The drill head also needs to send information about the geological formations the drill is going through, using down-hole sensors that measure resistivity, porosity, pressure and temperature.

Getting sensor data from this hellish environment to the surface might seem like an engineering nightmare but the drilling industry has been doing it for the last 30 years using a technique much like banging on a pipe full of mud.

The mud, a specially engineered mixture of oil and water, is there for another reason. It's being pumped from the surface through the drill string and the drill bit and back to carry cut rock in suspension up to the surface, where it is filtered to remove the cuttings and then recycled back down the hole again.

The communications trick is to use the mud column to carry the sensor data up to the surface in the form of pressure waves. The mud flow can also be used to generate electricity to power the down-hole sensor electronics. On the surface, the pulses are received and measured by pressure sensors and converted back into digital signals.

Patent records suggest that drilling engineers were thinking about mud-based telemetry as early as 1929, but the first commercial mud-pulse system was introduced by Teleco in 1978. It supported the heady data rate of 0.4bit/s.

Until recently the maximum rate in such systems has been around 3bit/s. A couple of years ago, a group of German signal processing engineers lead by Wassermann came up with a system that achieves 20bit/s at depths less than 6,000m, and just over 3bit/s from depths of more than 10,000m. (Rates of 30bit/s have been achieved from 3,000m onshore in the USA and 40bit/s from 900m in a test area in Beggs, Oklahoma).

Higher data rates translate into more measurements per metre when drilling, making it possible to drill at a faster rate or drill at the same rate and gather more data. This is important because the economic success of many drilling operations depends on the availability and quality of the real-time information about the drilling process.

Signal to noise

The problem with boosting mud-pulse data rates has been a poor signal-to-noise ratio, which is also frequency-dependent with some frequencies highly distorted and others relatively clean. Most of the noise is due to the large surface pumps that cycle the mud around the system. The signals are also indistinct because the pressure waves travelling up the mud column get attenuated and partially reflected on their journey to the surface, with the attenuation increasing with both depth and the compressibility of the mud.

"Older systems delivered either strong, discrete pulses or low-amplitude continuous waves," Wassermann explained in a paper in the Oil and Gas Journal last year. "What engineers wanted instead was a new kind of pulser combining both options to ensure independence from the highly variable drilling environment: deep or shallow wells, oil- or water-based mud systems, complex or simple bottom-hole assemblies, high or low drilling rates or, indeed, any combination of these conditions."

To counter these problems, his team has taken inspiration from the multiple-input multiple-output techniques used in the latest wireless networking standards.

"If you look at wireless repeaters or routers, they have multiple antennas and transmit different data streams on each antenna. We've adapted this spatial diversity approach to our system," he says.

Using this approach, they've improved overall performance by transmitting on the least distorted bands and then by applying digital signal processing to get rid of the noise at the receiving end.

New pulsers

Their transmitter is an oscillating shear valve pulser driven by a very precise motor controller that can deliver a bandwidth of 40Hz or more. During each oscillation cycle, the valve runs through a zero-speed state, in which phase or frequency can be changed instantaneously according to changes in flow rate and mud weight. The valve can also transmit discrete pulses, as well as the passband-modulated signal, so in cases where high frequencies are attenuated, the oscillating valve can be set to discrete pulsing. With high-end motor controllers, even the pulse pattern can be changed at each oscillation cycle to create sinusoidal, trapezoidal, rectangular, or arbitrarily shaped signals. This enables maximum signal pressure at the source and optimum down-hole signal quality to give the best chance of decoding the signal.

Reconstruction

At the surface, signal reconstruction is used to remedy distortions caused by noise and frequency selectivity before handing the signal over to the demodulator and pulse detector.

"Noise is usually assumed to be additive and can be subtracted as soon as we have an idea of what it looks like, which is the difficult part," says Wasserman. "Noise is estimated from measurements and cancelled by applying linear prediction and space diversity."

Once the noise is cancelled, frequency-selective attenuation can then be estimated and equalised based on transmitted reference signals.

After noise cancellation and equalisation, the best set of parameters for signal transmission (discrete pulsing or passband modulation, data rate, and if applicable, signal frequency) can be determined.

Raw data rates are expected to increase in the near future with the help of better signal restoration schemes, says Wassermann. And in the most complex offshore drilling sites, such as those in the Sakhalin area of Russia, where wells are up to 10km long and approached horizontally by land, such highly detailed telemetry is becoming crucial.

"It basically means they can drill wells they couldn't drill before," he says.

So don't blame me if next time you fill up with petrol or turn up your central heating you find yourself quietly singing: "So follow me follow, down to the hollow, and there let me wallow in glorious modulated mud…"

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