Seismic shift for imaging

The application of newly developed low-frequency acoustic technology to borehole systems will help oil and gas companies exploit previously untapped reserves, according to a company commercialising the technique.

US-based Technology International has now completed testing of a borehole imaging system designed to aid exploration and extraction.

Imaging the rock formations through which a drill bit is boring has always been a critical requirement for the industry. Such seismic acquisition and processing techniques enable operators to optimise the path of the drill towards its target — in a process known as geosteering — and prepare them for any anomalies, such as changes in pressure in the well that might arise during the process.

A seismic technique perfected in the early 1980s acquires a profile of the underlying rock using the drill bit itself as an acoustic source. The source, projected in front of the drill bit, is reflected off the interfaces between different layers of rock, then returns to the surface where it is picked up by an array of sensors, later to be analysed to create a seismic image.

However, as older roller cutting bits on rigs were replaced by much quieter polycrystalline diamond compact (PDC) cutters, the acoustic technique lost favour with the petroleum industry and an alternative seismic source is something that oil and gas companies have been looking for ever since.

A number of alternative techniques have been proposed or even deployed since then, including hydraulic pulsation devices and explosives, but none has enjoyed any significant commercial success.

As part of a project run by the US Department of Energy, engineers at Technology International claim to have come up with a solution through the introduction of the Seismic Pulser, a seismic-while-drilling system that can provide the type of data that will allow operators to more accurately steer their drill bits in the process of oil and gas extraction.

It was the harsh, high-pressure, high-temperature environments found in both onshore and offshore oil rigs that drove the development of the new system, according to Robert Radtke, the company’s president.

More specifically, the extreme depths at which such rigs operate had made it impractical for operators to use more traditional high-frequency borehole pulsed acoustic sources to perform the tasks of determining pressure in the well, or carry out geosteering or reservoir model verification.

What was needed was a system that could generate the low-frequency sounds that could be transmitted over long distances through rock, a problem that Radtke claims his technique has overcome.

At the heart of the system lies a sparker — a device that comprises two electrodes across which a high voltage potential is applied. Because the sparker operates in a liquid environment, the high voltage potential causes a bubble to be created in the fluid. That bubble then collapses, creating an acoustic signal that can then be detected.

‘When an underwater high-energy spark occurs, a bubble is formed that expands outward until the pressure inside the bubble reaches ambient pressure and then the bubble collapses,’ said Radtke. ‘The process produces two high-energy pressure pulses, one at the initial impulse and one upon bubble collapse and together these produce a very broad acoustic spectrum,’ he added.

The sparker itself, however, is not the fundamental key to the claimed advance. Sparkers have been developed before but have found limited use due to the fact that the signal frequencies generated by the collapsing bubbles are high, so using such sources has meant that it was only possible for operators to deploy them when drilling down to 900m.

Radtke and his team of engineers have applied research into fundamental physics to generate low frequencies at less than 5Hz from the sparker’s high-frequency source, a method that is being patented.

He said that the system, with its power supply and control, would typically be delivered to the bottom of a borehole using a drill string, a column of drill pipe found on an oil rig that is used to convey drilling fluid via mud pumps, as well as rotational power to the drill bit itself.

The energy to power the sparker would be created from the movement of the drilling fluid or mud itself. When this flows though the system’s turbine alternator it generates enough power to charge large capacitors. When released across the electrodes of the sparker, the charge in the capacitors is sufficient to create the vapour bubble that then forms and then collapses several milliseconds later.

If the system was deployed with the drill operating, the energy generated by the low-frequency sparker would normally have to compete with the other noise generated at the rig site from the pumps and the rotating drill string. However, because the new system stores electrical energy in its capacitors, the sparker can be programmed, for example, to generate a signal every 20 seconds with the pumps turned off.

Once the system is built into, or attached onto, a drill string, it will emit low-frequency sounds at less than 5Hz, which, according to seismic calculations from Technology International, can be transmitted to surface receivers from depths beyond 9,000m. According to Radtke, this is the only system that meets the requirements of companies that plan to drill high-pressure, high-temperature wells at these depths. Once the data has been acquired by the sensors, computer software then performs seismic data processing on the signals; this enables a velocity profile to be created showing the speed at which the sound was reflected from the rocks beneath the surface. From those profiles a seismic image is then created, allowing operators to determine what types of rock are being drilled, whether they contain oil or gas, or the pressure ahead of the drill bit.

Radtke claimed that the system will give an accurate location of the drill bit relative to reservoir models and provide the operator with real-time images roughly 300m ahead of the bit without interfering with normal drilling operations. It also offers new operational capabilities by allowing operators to visualise and steer towards more optimal targets when drilling deep formations.

To trial the new system the company first went to the Devine seismic test site, south of San Antonio, Texas. Its engineers performed a run at 550m depth to test out the effectiveness of the tool mounted on a wireline — a cable used by operators of oil and gas wells to lower equipment or measurement devices into the well for reservoir evaluation — rather than use a drill string itself.

A second test gave the developers a true idea of the potential of the system at low frequencies. Conducted at the US Department of Energy’s Rocky Mountain Oil Field Test Centre they deployed the system on a drill string, and demonstrated its effectiveness in capturing the signals transmitted from it by using accelerometers distributed on the surface that received the signal successfully from the source.

After declaring the initial tests a success, the engineering team at Technology International said it is now talking to a number of oil, gas and associated service companies in the industry.