Study on effective influence radius of liquid CO2 phase transition fracturing in high drainage roadway of deep coal seam

Deep underground, where the earth's weight presses down with crushing force, coal seams often hold vast quantities of natural gas. In these deep, high-pressure environments, the coal itself is so tightly packed that the gas cannot flow freely to the surface. This creates a dangerous situation for miners, as trapped gas can suddenly erupt in violent explosions known as outbursts. To prevent these disasters and to safely extract the gas as a fuel source, engineers must first break the coal apart to create pathways for the gas to escape. However, traditional methods of breaking rock are often too blunt or dangerous for these delicate, gas-rich layers. Scientists have been searching for a way to fracture the coal effectively without the risks associated with conventional explosives, seeking a method that is powerful enough to crack the rock but controlled enough to be safe for the mine environment.

In a recent study, researchers from a coal mining company and a university in China investigated a technique called liquid carbon dioxide phase transition fracturing. This method uses a specialized tube filled with liquid carbon dioxide. When heated, the liquid turns rapidly into a gas, expanding with tremendous force. This expansion builds up pressure inside the tube until a small, pre-set metal disc at the end of the tube shatters. Once the disc breaks, the high-pressure gas is released instantly, creating a shockwave that cracks the surrounding coal. The researchers wanted to know exactly how far this cracking effect reaches. If they knew the precise distance the force travels, they could place their drilling holes at the perfect intervals to ensure the entire area of coal is fractured and the gas can be drained efficiently, without wasting energy or leaving dangerous pockets of gas untouched.

To answer this question, the team combined three different approaches: mathematical calculations, computer simulations, and real-world testing in a deep mine. First, they calculated the energy released by the device. They found that the strength of the explosion depends on the thickness of the metal disc that holds the pressure back. A thicker disc requires more pressure to break, which results in a more powerful release of gas. By measuring the thickness of these discs, the researchers could determine the exact amount of energy being released, expressing it in terms of the equivalent weight of a standard explosive called TNT. They found that changing the disc thickness from 3.5 millimeters to 5.5 millimeters increased the energy from roughly the equivalent of 117 grams of TNT to over 220 grams.

With these energy values in hand, the team turned to a computer model to visualize what happens inside the coal. They created a digital representation of a coal seam and simulated the moment the gas is released. The simulation showed that the shockwave travels outward from the hole, creating a zone of intense damage close to the source and a wider area of smaller cracks further away. The computer results indicated that the most significant damage occurs within a radius of about 5 meters, where the coal is thoroughly shattered. Beyond this immediate zone, the cracks continue to spread, but they become less dense. The simulation suggested that with a smaller energy charge, the effective area where the coal is sufficiently broken extends about 15 meters from the hole. When they increased the energy charge, this effective area grew to nearly 18 meters.

To verify these computer predictions, the researchers took their method to the actual mine, specifically to a deep roadway in the Pingdingshan TianAn Coal Mining area. The mine is located nearly 1,000 meters underground, where the coal is under immense pressure and contains high levels of gas. They drilled a series of holes and installed the liquid carbon dioxide devices. In one set of tests, they used a charge equivalent to 300 grams of TNT, and in another set, they used a charge equivalent to 450 grams. After setting off the charges, they monitored the gas flow in nearby drainage holes at various distances to see how much the gas extraction improved.

The field tests confirmed that the technique works, but the real-world results were even more impressive than the computer models had predicted. When using the 300-gram equivalent charge, the researchers observed that the gas drainage improved significantly in holes located between 16.59 and 23 meters away from the fracturing hole. When they increased the charge to 450 grams, the effective range expanded further, reaching distances between 21.08 and 30.16 meters. This means that by simply adding one more tube of liquid carbon dioxide to the hole, they increased the effective influence radius by roughly 30 percent. The data showed that the gas flow increased dramatically in these zones, proving that the coal had been successfully fractured to allow the gas to escape.

The study concludes that for deep coal seams with low permeability, using a higher energy charge is the most effective strategy. The researchers recommend placing the drilling holes about 5 meters apart and using a charge equivalent to 450 grams of TNT. This combination ensures that the fractured zones from adjacent holes overlap, creating a continuous network of cracks that allows for maximum gas drainage. While the computer simulations provided a useful lower-bound estimate, the actual field tests revealed that the technique is capable of influencing a much larger area than initially calculated. This finding offers a clear, practical guide for mining engineers: by adjusting the energy of the fracturing device and the spacing of the holes, they can safely and efficiently unlock the gas trapped in the deepest, most difficult coal seams.