Cryogenic fracturing using liquid nitrogen on granite at elevated temperatures: a case study for enhanced geothermal systems in Kazakhstan

Abstract

Cryogenic fracturing using liquid nitrogen (LN2) is a novel stimulation technology that enhances porosity, permeability, and rock-fluid contact area in subsurface formations targetted for geothermal energy extraction. In our experimental study, granite cores collected from the Zhylgyz region in South Kazakhstan were equilibrated at various elevated temperatures before treatments involving LN2 exposure time. Compression, Brazilian, and fracture toughness tests were performed on granite with starting temperatures ranging from 100 to 500 °C to quantify the impact of initial temperature on cryogenic fracturing and to compare with baseline geomechanical tests at 50 °C without LN2 exposure. The results show that LN2 cooling of hot granite induces mechanical rock failure and permeability enhancement. Moreover, the degree of thermo-fracturing augments with initial granite temperature, total freezing time, and number of freezing–thawing cycles. The peak load before failure of granite specimens, both in compression and Brazilian tests, reduces with the increased sample temperature difference and length of LN2 treatment. The fracture toughness of our semi-circular bend (SCB) LN2-treated specimens diminished with increasing temperature difference between granite and boiling point. In both experimental LN2 treatment processes, the specimens with an initial temperature of 500 °C before LN2 treatment formed many new fissures and extensions of pre-existing ones, showing that the plastic behavior is augmented. While cryo-fracturing experimental confirmation is recommended with site-specific samples in planning geothermal operations, these results in our work indicate a threshold downhole temperature, e.g., > 300 °C, for enhanced stimulation outcomes.

Research on Mechanical Properties of Rock Mass with Tiny Cracks under FTCs Conditions

“…To obtain the freeze-thaw coefficient of a rock sample, the measured compressive strength data of the rock were inserted into Equation (2). The variation in the freeze-thaw coefficient with an increasing number of FTCs is shown in Figure 10.…”

“…In low-temperature environments, the freeze-thaw cycle of a rock mass can exacerbate the damage, deterioration, and fracture process of the rock mass, further affecting stability and engineering safety [1,2]. Therefore, for research and application in related fields, it is necessary to study the freeze-thaw cycle damage [3], deterioration, and fracture process of joints and rock masses.…”

Laboratory Study of Liquid Nitrogen Cryo-Fracturing as an Environmentally Friendly Approach for Coalbed Methane (CBM) Reservoirs

“…In their mechanical experiment, Sandstrom et al [59] found that the quantity of freezing-thawing cycles on rocks influenced the mechanical characteristics and fissures in the material. It should also be mentioned that the LN 2 process, as the waterless hydraulic fracturing method, has been used by different research groups in shale [41,60,61], carbonate [17,62], sandstone [63,64], and granite [65][66][67] rocks with successful results as it initiates the mechanical and physical deterioration of the samples of varying mineralogy and composition. This work explores the changes in pore structure characteristics that are seen in coal samples that are obtained from the Karaganda area after they are exposed to liquid nitrogen (LN 2 ), which is used as a waterless fracturing method for coalbed methane (CBM) reservoirs.…”