Predictions that US shale plays could hold enough natural gas to last as long as a century fail to consider an important caveat. Although these reservoirs often are quite large, they lie trapped in rock formations so impermeable that horizontal drilling and multistage hydraulic fracturing techniques are required to extract them.

These technologies can cause well costs to soar, and because production tends to decline rapidly in unconventional reservoirs, full-scale commercialization of a field typically requires hundreds of wells to be drilled annually over many years. To make their exploitation economic, operators must be able to fully understand the overlap of regions of favorable reservoir architecture with regions where most of the gas is stored and then use the stress states of these formations to pinpoint precisely the areas where production can be commercially viable. So far, geophysical technologies are simply not up to the task of scanning these regions effectively. In particular, seismic data are insufficiently accurate to compute reliable models.

One way to improve the situation is to acquire more high-quality seismic data – perhaps a hundred times more than typically are gathered today – to generate detailed, accurate images that reveal variations in the subsurface. This can enable more effective production from unconventional gas reservoirs and generally improve subsurface understanding in other reservoir settings – all at a reasonable cost.

Although Shell has been very active in deploying conventional seismic data acquisition technologies, existing systems typically are limited to 20,000 channels and do not have such advanced capabilities. Deploying significantly more channels can lead quickly to prohibitively high costs, limiting the application of very large systems to fewer assets.

The million-channel vision

Shell's vision is for seismic surveys to be run using as many as 1 million channels. While this is a tall order, it could be achievable within the next five years thanks to Shell and HP's joint development of a super-sensor network that could revolutionize the acquisition of onshore seismic data.

Combining HP's technology development capabilities with Shell's advanced geophysical expertise in seismic imaging, the partnership began in February 2010, not long after Shell convened an internal conference to review new breakthroughs in nanotechnology.

Among the conference presenters was Stan Williams, a senior technology fellow for HP, who discussed his company's revolutionary digital micro-electromechanical systems (MEMS) inertial sensing technology. MEMS accelerometers – sensors that measure vibration, shock, or change in velocity – are up to 1,000 times more sensitive than high-volume products currently on the market.

A key area of synergy for the project is HP's position in supplying MEMS devices, the underlying technology and commercial scale of which are applicable for HP's novel inertial sensing technology. It became clear to Shell scientists that with further R&D, a truly revolutionary seismic sensor could be designed from HP's core technology and that HP had the wireless networking, software and systems integration capability to design the entire seismic system. Shell approached HP about devel- oping a wireless, ultra-sensitive, million-channel sensor system that could be deployed easily and at low cost.

The HP seismic MEMS sensor is compact, ultra-sensitive, and requires very little power. (Images courtesy of Shell)

System advantages

The result of this collaborative effort is a seismic receiver significantly more advanced than conventional sensors. Unlike geophones used in existing systems, this sensor is compact and lightweight. It also has a very low power requirement and operates on small, built-in, long-life batteries. In addition, the system is completely wireless.

These characteristics alone could overcome two of the biggest challenges associated with conventional seismic acquisition systems: size and weight. Typically, geophones must be connected by heavy, cumbersome cables to transmit power and signals, which pushes their total weight to more than 1,000 tons for a million-channel system. Even with 20,000 channels, the systems require hundreds of workers and numerous trucks to deploy. Increasing that channel count significantly could make them prohibitively expensive and, due to the sheer size and complexity of their deployment, a logistical nightmare.

By comparison, a system that incorporates 1 million Shell/HP sensors could be set up and operated without increasing cost or deployment time, thus requiring no additional workers or equipment and trucks to carry them. As a result, it could be deployed more easily and quickly in rugged or remote regions or in densely populated areas.

The Shell/HP sensor offers other advantages as well. Not only will it allow for single-sensor data processing, but the sensor itself also has better noise performance. It can measure at ultra-low frequencies that are well beyond the range of conventional seismic technologies, which can use data only from 8 Hertz (Hz) to 80-100 Hz to create images of the subsurface; outside this limited span, the spectrum can deteriorate quickly. Low-frequency signals travel much farther through the subsurface than do higher frequency waves. They also carry critical information not contained in higher frequencies, opening up sometimes nonlinear ways of subsurface imaging. In addition, low frequencies are required for accurate full waveform velocity modeling as well as the inversion of seismic data into reservoir models. For these reasons, Shell and HP want to be able to gather data down to 1 Hz or below and as high as 200 Hz.

An HP sensor network prototype features the ultra-sensitive MEMS accelerometer.

Testing landmark

Development of this next-generation seismic acquisition system passed a major hurdle when field tests at the US Geological Survey's Albuquerque Seismological Laboratory in New Mexico showed the sensor not only met but surpassed its developers' expectations, even accurately measuring sound waves generated by an earthquake 800 miles off the California coast. Now Shell and HP expect to connect up to a million of these super-sensors into a reliable, low-power field network that could transmit data about seismic quality control and system status to a command center. Operators in the center would track and manage the progress of the survey and monitor the sensors and network to ensure system wellness. They also would identify situations in which environmental conditions could interfere with the survey and monitor logistics and the effectiveness of the data collection process.

Flexible survey design is extremely important to providing a reliable solution across multiple environments and multiple target scenarios. To address this need, the system also could be operated in different modes, providing the flexibility to expand or reduce the network and therefore reduce costs for each unique deployment. The goal is to develop a seismic acquisition system so robust it can operate in a "blind" mode, minimizing deployment costs and providing the required flexibility for networked communication and quality control.

Ultimately, the super sensor could make it easier to find reserves that lie buried deep below ground; are hidden under complex geological strata such as salt, basalt layers, or ice shields; or are situated in small accumulations that are difficult to detect with conventional seismic systems. Its application also could lead to other innovations such as large-scale computing and storage for seismic data processing and visualization services as well as continuous sensing and reservoir monitoring.