NETL researchers have developed a cutting-edge fiber-optic sensor to monitor conditions in extreme environments thousands of feet below the Earth’s surface and obtain essential data needed to safely sequester greenhouse gas in underground reservoirs.

“This breakthrough invention by NETL researchers can be used to obtain accurate pH measurements from formation fluids in harsh subsurface conditions where temperatures range up to 300 degrees Celsius (572 degrees Fahrenheit) and pressures range up to 30,000 pounds per square inch,” said Barbara Kutchko, a senior research scientist at NETL.

A member of the team that received a U.S. patent on the technology, Kutchko explained that obtaining pH levels from these extreme environments is critical to ensure the safe sequestration of carbon dioxide (CO₂) in the subsurface, a major component of the nation’s plan to achieve a net-zero carbon emissions economy by 2050.

Chemical reactions in water are affected by the acidity or alkalinity of the solution. “Accurate pH measurements at downhole wellbore conditions are needed to understand formation fluid water chemistry and predict corrosion and scale potential of the equipment that’s used to inject CO₂ into the subsurface,” Kutchko said.

The lack of robust pH measurements also requires that large safety margins be taken in the selection of corrosion-resistant materials, which has a significant impact on the cost of carbon storage and drilling operations. In addition, measured pH values can be used to determine the chemical composition of CO₂ in fluids contained within geological formations, which is helpful to evaluate and monitor subterranean sites for carbon sequestration.

Most electrical and electronic components used in sensor applications cannot withstand extreme subsurface temperatures and pressures. Also, because gases and solids can leach from solutions as downhole samples are transported to surface laboratories, it is important to develop technologies for accurate pH measurements downhole in the native condition at reservoir temperatures and pressures.

Kutchko and her colleagues have addressed these concerns by developing fiber optic-based sensing layers composed of metal nanoparticles incorporated into a silica matrix coated on an optical fiber. The new sensing materials have exhibited strong and reversible optical responses to pH variation at high temperatures and pressures under saline conditions. Deployment of these sensors will allow for embedded, real-time, remote pH sensing capabilities in extreme subsurface environments.

“The temperatures and pressures present in subsurface environments are beyond the limits of most electrical and electronic components used in sensor applications, which often fail due to the instabilities associated with packaging, wires and interconnects. For this reason, approaches that replace the need for electrical components and connections at the sensing location can also eliminate a common mode of failure for conventional sensor devices,” Kutchko said.

Information about this NETL invention, Method of Evaluating pH Using a Metallic Nanoparticle Incorporated Nanocomposite-based Optical pH Sensor, can be obtained on the U.S Patent and Trademark Office’s website.

Enter the patent number 11408827.

NETL is a U.S. Department of Energy national laboratory that drives innovation and delivers technological solutions for an environmentally sustainable and prosperous energy future. By leveraging its world-class talent and research facilities, NETL is ensuring affordable, abundant and reliable energy that drives a robust economy and national security, while developing technologies to manage carbon across the full life cycle, enabling environmental sustainability for all Americans.