Cracking the Code: 3D Seismology and Core Analysis
Most successful endeavors begin with a target. Without knowing where the bull’s eye is, it’s impossible to know where to steer, and easy to get off track. Having your sights set on a specific physical object is crucial in photography, archery and natural gas drilling.
This is especially true in the Marcellus Shale. As natural gas formations go, defining the Marcellus Shale as “big” is like defining Chuck Yeager as “fast.” The Marcellus is routinely called “monstrous,” “vast” and “enormous.” It covers 15 million acres, stretching from northern West Virginia through Southern New York. Penn State Professor of Geosciences Terry Engelder estimates it contains 500 trillion cubic feet of natural gas, enough to supply the entire United States for two decades.
Amid this immense land mass, natural gas producers have to decide where to drill two-foot-wide holes in the ground. And the shale is as obstinate and unyielding as it is vast. The rock is thicker in some places than in others, and drillers want to find a nice thick swath of shale to drill. Some of the shale holds more natural gas in its pores. Some of it allows the gas to flow more freely — it is more permeable — than other sections of the shale. And to add more mystery to the conundrum, most of the Marcellus in Appalachia is more than a mile underground.
Finding the sweet spots — those thick, porous, permeable spots that hold natural gas reserves — amid 15 million acres is like finding the proverbial needle in the haystack — a buried, invisible haystack. The successful completion of this “Mission Impossible” is vital to our nation’s clean energy future, as natural gas, and lots of it, will be required as our country looks for ways to wean itself from dependence on foreign countries and ways to lower carbon emissions. Methane is the lightest hydrocarbon, and the cleanest burning fossil fuel on the planet.
As our nation proves again and again — as innovators in energy, engineering and environmentalism keep showing – technology advances to meet the challenges. In this case, 3-D Reflection Seismology and Core Analysis are the answers.
A sonogram of the earth
3-D seismic utilizes the same technology that is used in an ultrasound in the medical community, but instead of a picture of the inside of the human body, it reveals a picture of the inside of the earth. The data is obtained through the process of transmitting sound waves into the earth via large trucks called “vibrators,” or by very small charges buried in the ground. These sound waves then bounce off rocks of differing physical properties and are reflected back to the surface where they are captured and recorded by small magnetic devices called “geophones.”
These seismic vibrations have shaken up the natural gas industry, changed the exploration paradigm, and opened new doors of opportunity to aggressive, shale operators like Chesapeake Energy. 3-D seismic helps geoscientists explore layers of rock formations beneath the earth’s surface, seeking out structural traps that could contain hydrocarbons.
The concept of seismic is not new, having earned a patent in 1916. But decades of refinement have made it commonplace as a prerequisite for the development of unconventional natural gas plays. The advances in high-resolution imaging and interpretation have been compared to the difference between conventional and high definition television. At Chesapeake, the high definition images can be viewed most effectively in the company’s 3-D Seismic Visualization Room. In this unique facility, 3-D stereo eyeglasses turn natural science into what resembles a video gamer’s paradise on 18-foot-wide stereo screens: fault lines come to life, underground formations float like space stations, intersected by yellow poles representing possible drilling sites. It looks like a virtual game board for natural gas exploration.
For anyone who might consider wildcatting a game, the ante is high. 3-D seismic helps even the odds. It tells drillers where to put the $7-million hole.
The technology will continue to evolve. In some of Chesapeake’s earlier surveys, recordings were made using 400 channels of six geophones each. Now companies have 4,000 to 5,000 channels. Instead of a narrow swath of phones, they have an active patch twelve miles long and four to five miles wide.
Today’s seismic survey is not only more detailed than its predecessors, it is far more expensive. Terrain is a factor in the cost. In the wide-open spaces of west Texas and Oklahoma, where vibrator trucks can be hauled in to produce sound waves, costs are less than in locations with rough or mountainous terrain such as Appalachia. Steep mountain ridges and deep hollows provide uneven purchase for vibrator trucks. In many forest areas, there are no roads to bring heavy equipment through the dense woods.
Rising Above
Chesapeake has literally risen above these problems by using heliportable equipment when it conducts seismic surveys in the challenging West Virginia environments. It leaves a smaller footprint because the company doesn’t have to cut roads or haul in heavy trucks or equipment. Instead, equipment is surgically dropped between trees.
Here’s how heliporting works: A small shot hole drilling rig is delivered by helicopter, attached to the end of a 160-foot cable. The 12-by-4-foot hole drill is carefully lowered into position and untethered from the hovering helicopter. Then, a 20-to-40-foot bore hole is drilled and a small charge or explosives is set in the bore hole. The helicopter then picks up the drill and moves it to the next drilling position. This is repeated until the area being surveyed is filled with a grid-like pattern of charge-filled bores, often 50 per square mile.
Next, a grid of geophone receivers is set over the entire survey areas to record the reflection of sound waves. Radio signals trigger the small explosive charge deep in the earth, to create the sound waves required for measurement.
Cracking the Code
Properly interpreted, such information is a roadmap to success in the natural gas exploration industry. There is no doubt that 3-D seismic technology has enabled the company to create value in some of the nation’s hottest unconventional natural gas plays like the Marcellus, but also the Barnett Shale of North Texas and the Fayetteville Shale in Arkansas. Every new gas discovery identified using this technology helps lead to the responsible and economic recovery of the resource.
Shale is often considered a mystery. Using 3-D to find the sweet spots helps unlock it, and another part of the solution is finding the best way to get the shale to release the gas. The rock’s ultra-tight fabric means that it is reluctant to release the natural gas that resides within. Chesapeake’s Reservoir Technology Center (RTC) analyzes shale cores that are pulled from various depths during the drilling process. Using highly sophisticated equipment, they determine how porous a rock is, how permeable, its gas and fluid saturations, grain density and other key characteristics. Their evaluations provide valuable information on whether the rock will yield gas. A scanning electron microscope and X-ray diffractometer are used to determine the sample’s composition, texture and pore-size distribution. The joint efforts of the RTC team and the seismic team keep Chesapeake focused on the sweet spots of a play.
Reaching a target is easier if you can see it. Natural gas drillers use a variety tools to help their vision – sound waves, x-rays and perhaps most importantly, the kind of vision that has nothing to do with eyesight. The natural gas exploration and production industry is driven by pioneers who are finding vast new reserves of clean-burning natural gas, and utilizing cutting-edge technology to provide the answers to the mysteries they encounter more than a mile underground, across 15 million acres. Successfully cracking the code, unlocking the Marcellus Shale, has become a game changer for America’s clean energy mission.
Larry Lunardi is Vice President of Geophysics for Chesapeake Energy
