Scientists from Chongqing University of Science and Technology with participation of major oil-and-gas companies of China (CNOOC and SINOPEC) and University of Barcelona made a step capable of changing the future of shale gas production. For the first time, they described in detail and provided a mathematical rationale for the innovative method hydraulic fracturing using carbon dioxide and the so-called “slickwater”.
For many years, a traditional frac job was used for shale gas production across the world – pumping in a mixture of water, sand and chemical additives. However, upon contact with the rock such fluid caused the upswelling of clay minerals, decreased the permeability and “greased” the fractures causing them to implode faster. It resulted in the situation, when a significant portion of gas remained locked in the subsoil: according to many estimates, the recovery factor made only 10–15%, and 85-90% of the reserves were not engaged in the development.
In 2017, Chinese Yanchang Petroleum was the first to apply the mixture of supercritical CO₂ and the so-called “slickwater” — the fluid decreasing the friction and facilitating the penetration into the rock. The experiment turned out a success: the volume of the reservoir engaged in draining increased 3-5 times, and the production grew pro rata. In addition, CO₂ was not emitted into the atmosphere – part of it remained in the subsoil bonded with minerals and turning into stable compounds.
But given the efficiency of the method, nobody could disclose its mechanism – how did CO₂ interact with the shale and why the combination with water gave such a strong effect.
The researchers from Chongqing gave an answer recently. They performed a series of lab tests using shale samples from Lungmasa formation in Sichuan Basin. At first, the rock was kept in the slickwater, then it was exposed to supercritical CO₂, i.e., the substance in a special state between gas and liquid. After that, the fractures were scanned with micron accuracy.
It turned out that CO₂ literally “engraved” the rock: increased the wall roughness and the contact area, created a micro network of channels for gas to freely come outside. On the contrary, water stabilized the structure of fractures preventing their collapse and maintain the conductivity.
For the first time, the scientists provided a qualitative description of this effect, linking the pressure, porosity, chemical corrosion and change of shale micropattern. Based on the tests, they built a physical-mathematical simulation accounting not only for gas movement, but also for rock elasticity and deformation. Digital platforms COMSOL and MRST integrated into a single system called C–R solver was used for calculations. With its help the researchers could simulate the reservoir behavior given different combinations of pressure, temperature and composition of the pumped-in fluid.
The comparison with the real gas field data showed that the calculations almost fully coincided with the field survey results. Now, engineers will be able to determine the optimal frac job parameters – pressure, volume and CO₂ / water ratio, to recover maximum gas at minimum costs.
