Subterranean navigation


Subterranean navigation

The search for oil and gas reserves in new locations requires increasingly sophisticated surveying techniques. By adapting technologies first developed for aerospace, the company Gyrodata has pushed the boundaries of what is possible in accurately determining the position of underground locations. Gyroscopic techniques have not only improved the surveying and drilling of oil wells, they have also played a role in drilling relief wells and in rescuing the Chilean miners. Rob Shoup and Jean Paul Lips describe the technology’s role in oil and gas exploration and response to emergencies.

A rate gyroscope sits in its casing behind the drill. Directional surveys can help hit specified geological targets, avoid collisions with existing wells, and provide coordinates for relief wells. They are not affected by magnetic fields unlike systems that rely on magnetic north

A rate gyroscope sits in its casing behind the drill. Directional surveys can help hit specified geological targets, avoid collisions with existing wells, and provide coordinates for relief wells. They are not affected by magnetic fields unlike systems that rely on magnetic north

Oil and gas exploration has altered dramatically over the past few decades. For the first part of the 20th century, businesses could search for hydrocarbons and exploit reserves with relatively little effort. Reserves at existing locations were not always fully exploited as it could be cheaper and easier to set up and start new wells elsewhere.

By the mid-1970s, however, the search for fossil fuels was getting more complicated. In 1950, exploratory drilling achieved discovery rates of around 900 barrels of crude oil equivalent per foot of well drilled in an area with no existing oil or gas production. By 1977, this figure had dropped to 200 barrels of crude oil equivalent.

As a result, oil businesses were forced to exploit reserves located in much smaller, poorly defined rock formations with wells drilled to greater depths and in more complex geology. While traditional magnetic compasses and free-directional gyroscopes had been sufficient for mapping simpler wellbore trajectories – the holes to be drilled during oil and gas exploration and extraction – the data produced from these instruments was just not accurate enough for more complex operations. It was now crucial to develope more accurate instruments as uncertain surveys lead to miscalculations when positioning and drilling wellbores, as well as inefficient production operations.

A new approach

In 1980 Gyrodata was set up with funding from several major oil companies who needed more accurate instruments to carry out wellbore surveys in wells which were now directional rather than vertical. The solution was found in the defence sector where the science of navigation had reached a far higher level of accuracy than in the oil industry. Harnessing these designs to make them suitable for borehole applications could deliver a 10-fold improvement in directional accuracy. The combination of defence technology with Gyrodata’s oil industry experience culminated in the new generation of borehole survey equipment (see Gyroscopic survey instruments).

The company took inertial rate gyroscopic sensors, and built wellbore systems to make well surveys more accurate. Using the principle of a spinning mass gyro system to sense the Earth’s rotation, the system finds true north – the horizontal component of the Earth’s spin vector points to true north – and determines the azimuth or direction of the wellbore. The accelerometer provides local gravity measurements and measures the inclination, or deviation, from vertical. Combining these two measurements with the depth of the tool below the surface makes it possible to calculate a three-dimensional trajectory of
a wellbore.

The original survey system consisted of a probe – containing the sensors, telemetry and processors – connected to a power supply and computer. The company’s first probe was 6 m long and encased in a 6.5 cm diameter pressure barrel to protect the sensors and electronics from the harsh environment in the wellbore.

During drilling and production operations, the probe would be connected to the power supply by a single conductor wire, known as a wireline, and passed down the well. The surveyors could make measurements periodically at different depths, with the gyroscope and accelerometer providing data on azimuth and inclination angles (see Figure1).

This information would then be filtered, digitised, processed and transmitted ‘uphole’ to the surface, through the wireline to a computer. Combining these data with wireline depth measurements, made it possible to calculate, display and store the survey’s three dimensional coordinates.

Instrumentation such as Gyrodata’s original inertial rate-gyroscopic survey system heralded the beginning of more accurate surveying for oil and gas companies. Using these technologies improved the search for, and development of, new reserves as well as increasing the safety of drilling operations. More accurate placement of wellbores, and more efficient drilling operations and reservoir definition has improved oil and gas recovery worldwide.

Developing rate-gyroscopes

Since the original innovation, newer systems, based on rate-gyroscopes, (see Gyroscopic Survey Instruments overleaf), can define wellbore trajectories even more accurately. Accuracy from the original wellbore survey systems have improved from seeing lateral errors of 20m/k to 2m/k. Today’s rate gyro tools now collect directional data at pre-determined points within the borehole while the tool is stationary, and transmits this information to the surface in real time. The outside diameter of the probe has been reduced to 4.5 cm while its length is now less than 3 m.

There was significant progress in the technology in the mid-1990s, when a battery-powered version of the original system was developed. This made it possible to drop a probe through the hollow drilling pipe so that surveys could take place as the borehole was drilled. A fluid could be used to pump these battery-powered probes to the bottom of the drill-pipe.

While rate-gyroscopic navigation has steadily evolved, bringing improvements in survey measurement, data processing and electronics, the past decade has seen greatest progress in sensor design. In the 1990s, as global positioning systems (GPS) and satellites began to play a larger role in aerospace guidance, there was less technological incentive for suppliers in that industry to develop more advanced gyro-based inertial systems and sensors. As a result, Gyrodata decided to design and manufacture its own guidance sensors specifically for subsurface applications.

As a result, there have been several important developments in surveying technology over the past few years. For example, advances in downhole sensors, electronics, software modelling and mechanisms to increase resistance to shock and vibration have led to more robust rate-gyroscopic systems, such as the Surveyor X-4 (see Figure 2) which can carry out surveys with greater speed, accuracy and reliability.

A continuous all-attitude surveying system has been developed, in which the system provides trajectory profiles while descending a wellbore. Previous continuous surveyors had to wait until the probe had reached a certain angle of inclination, but this can now take place while the probe is moving at high speeds from a horizontal to vertical position.

The latest innovation is the new gyroscopic sensor. The company is developing this to provide improved accuracy and greater ruggedness. The new sensor has allowed Gyrodata to deploy ‘Gyro While Drilling’. This development takes a spinning mass gyroscope that it sensitive enough to detect the Earth’s rotation but rugged enough to sit inside a drill string for weeks where it is subjected to shock and vibration and temperatures from freezing to 150 °C while still delivering accurate reliable data from many kilometres underground.

Relief work

A dual axis gyroscopic sensor that spins at 24,000 rpm, the first dynamically tuned sensor to be specifically designed for oilfield use

A dual axis gyroscopic sensor that spins at 24,000 rpm, the first dynamically tuned sensor to be specifically designed for oilfield use

While rate-gyroscope surveys are crucial to determining wellbore trajectories in oil and gas production, the instruments have also been widely used to map emergency relief wells. Relief wells are drilled to cut into an existing well that is leaking crude oil or natural gas. They are intended to provide a conduit for injecting dense mud and cement into the ruptured well, plugging the out-of-control flow of oil or gas at its source.

While it is a standard tactic in the oil and gas industry to drill relief wells, they are not straightforward to engineer. For example, a relief well may have to intersect a well that is just centimetres in diameter and several kilometres beneath the Earth’s surface.

Despite the challenges, Gyrodata’s rate-gyroscope systems have been used at most major relief well emergencies since the mid-1980s. For example, the technology has assisted relief operations in Mobile Bay in Alabama, the UK’s North Sea, El Tejero in Venezuela, Nigeria and Bahrain, and more in recently Alaska, Libya and Syria. Indeed, at the El Tejero operation in Eastern Venezuela, in 1991, a total of 26rate-gyroscope surveys led to an exact intersection with the blowing well at a record depth of 4,907 metres.

The oil and gas industry is not the only beneficiary from these techniques. This year, the company was called out to a relief well of a very different kind. On 22 August, 33 miners became trapped in the San Jose mine, which runs like a corkscrew for more than 7 km, beneath a mountain in the Atacama Desert in Chile. The Chilean government sought the best minds, organisations and technologies throughout the world, including NASA, in a concerted effort to rescue the trapped men. The South American oil company, ENAP/Sipetrol, contacted Gyrodata for help with accurately measuring a wellbore trajectory to reach the target well and to rescue the miners. The company responded by offering to provide services free of charge until the wellbore was complete.

Edison Pena, centre with glasses, emerges from the capsule that brought him, and 32 other miners, to the surface from the collapsed San Jose gold and copper mine near Copiano, Chile © Eva Vergara, Associated Press

Edison Pena, centre with glasses, emerges from the capsule that brought him, and 32 other miners, to the surface from the collapsed San Jose gold and copper mine near Copiano, Chile © Eva Vergara, Associated Press

The story of the rescue attempt is now well known. Rescue teams initially drilled a number of exploratory boreholes in an attempt to find the survivors; 17 days after the mine’s collapse, the rescuers found a note attached to a borehole probe referring to the miners’ location some 700 m beneath the surface, in a refuge area.

The first task for Gyrodata’s continuous all-attitude (CAAT) surveying system was to accurately measure the trajectory of the Paloma1wellbore. The CAAT tool can run in continuous mode from vertical to any inclination and can take readings every 30 mm if required. This was to be used to send food, water and supplies to the trapped miners. The system also determined the coordinates of several small shafts that connected the miners with the surface so that the drilling rigs would know where to drill relief shafts. Three drilling operations then took place simultaneously. The first two, known as plans A and B, involved a ‘raise-bore’ system of drilling a narrow pilot shaft or wellbore and then widening this to accommodate a rescue capsule. The miners worked in shifts to remove falling debris from the wellbore.

Plan C used a powerful Canadian oil drilling rig that brought rubble up to the surface while drilling a wellbore wide enough for the rescue capsule. Gyrodata’s Surveyor X-4 provided gyroscopic guidance of this drilling assembly, measuring the trajectory of the shaft as it drilled towards the trapped miners. Plan C made good progress, was on target, but started two weeks after the other plans and it was Plan B that was the first to break through to the miners.

Plan B used a percussion hammer drill that by using four hammers could progress more than 40 metres a day. The miners removed 700 tonnes of drilled rock that came down the 70 cm diameter shaft in the five and a half weeks of drilling. After being trapped underground for 69 days, all of the Chilean miners were rescued on 11 October in a worldwide televised operation that saw the 33 miners carried up to the surface in a capsule that was lowered up and down the drilled shaft.

Next steps

The use of rate-gyroscopes has increased greatly over the past few decades. They are now used for precision wellbore surveys in a variety of industries – energy, mining, environmental and construction.

A key focus for Gyrodata now, is to fabricate smaller rate-gyroscope systems that will deliver similar high levels of accuracy and survivability in harsh drilling environments.

A gyro-while-drilling-system is also being developed that is capable of being run in wells from vertical to horizontal as part of the actual drilling assembly. Such a system would give drillers real time wellbore positional data and allow drilling parameters to be changed ‘on the fly’.

These and other new technologies will improve the reliability and speed both for industrial business uses and assist with any other Chilean-type emergency relief wells of the future.

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