CO2 Explosion Fracturing Stimulation of Hot Dry Rock Reservoirs

Literature Overview

This paper investigates the use of CO2 explosion-induced fracturing to stimulate hot dry rock (HDR) geothermal reservoirs. The technique involves the controlled detonation of CO2 charges within pre-drilled boreholes to generate shock waves and gas expansion forces that create complex fracture networks in hot, dry rock formations. The study evaluates the stimulation effectiveness through permeability measurements, microseismic monitoring, and temperature response analysis.

Core Technical Mechanisms

CO2 explosion fracturing operates through two primary mechanisms: the initial shock wave generated by detonation (peak pressures of 5–20 GPa at the charge surface) and the subsequent gas expansion phase where CO2 expands by a factor of 1000–3000 times its liquid volume. The combination of these mechanisms creates radial fractures extending from the borehole wall and branching fractures that propagate along natural weaknesses in the rock mass.

The choice of CO2 as the explosive medium offers several advantages over conventional explosives in geothermal applications:

• Reduced environmental contamination risk compared to nitrate-based explosives
• Lower blast vibration levels suitable for near-surface applications
• The CO2 byproduct can be re-injected or utilized for enhanced oil recovery
• Lower sensitivity and handling requirements compared to primary explosives

Pressure Equipment and Containment Considerations

The HDR stimulation process requires high-pressure equipment for CO2 storage, transport, and injection. The CO2 must be stored in liquid phase at temperatures below 31.1°C and pressures above 7.38 MPa, or in supercritical phase above the critical point. This creates specific requirements for the design and fabrication of containment vessels.

For the storage vessels used in HDR applications, the following design considerations are critical:

1. Material selection: The base material should be a low-alloy steel such as 18MnMoNbR or SA-516 Gr.70, with an internal overlay of 304L or 316L stainless steel to resist CO2 corrosion at operating temperatures above 40°C.
2. Weld overlay specifications: The overlay thickness should be no less than 3 mm to provide adequate corrosion allowance. The weld procedure must be qualified per NB/T 47014 with a bond strength requirement of at least 250 MPa.
3. Thermal cycling resistance: HDR applications may involve temperature swings from ambient storage conditions to formation temperatures of 150–250°C. The overlay welds must maintain bond integrity under these conditions, which requires careful control of the weld metal composition to minimize thermal expansion mismatch at the interface.

Fracture Network Characterization and Reservoir Engineering

The effectiveness of CO2 explosion stimulation is evaluated through multiple indicators:

• Permeability enhancement: Measured through flow tests showing increases from baseline 0.01–0.1 mD to 1–10 mD after stimulation
• Fracture network complexity: Assessed through microseismic event mapping showing distributed event clouds rather than single plane failures
• Temperature response: The heat extraction rate from stimulated reservoirs typically increases by 30–60% compared to unstimulated wells
• Long-term stability: Fracture conductivity must be maintained over the operational lifetime of 20–30 years

From a materials engineering perspective, the high-temperature geothermal fluids extracted from HDR reservoirs present additional challenges for downstream equipment. The produced fluids may contain dissolved CO2, silica, and various mineral scales that can cause erosion-corrosion in carbon steel piping. This necessitates the use of clad or lined piping systems, where the cladding material must be selected based on the specific fluid chemistry at operating temperatures of 150–250°C.

Key Questions and Engineering Reflections

This technology raises several important questions from a pressure equipment and materials engineering standpoint. First, the repeated use of CO2 explosion charges in the same borehole creates a cumulative damage scenario where each detonation slightly enlarges the fracture network, potentially leading to stress concentration at fracture tips that could initiate fatigue cracking in the surrounding casing and tubing. The casing materials should be selected with adequate fracture toughness (KIC > 100 MPa·m^0.5) to resist crack propagation under these conditions.

Second, the transition from liquid to gaseous CO2 during detonation creates rapid pressure transients that can exceed the design pressure of surface equipment if the detonation occurs unexpectedly. Pressure relief systems must be designed with adequate capacity to handle the worst-case scenario of a full charge detonation in the injection line.

Third, the long-term corrosion behavior of overlay cladding in geothermal environments remains an area requiring further research. The combination of high temperature, CO2, dissolved salts, and silica in geothermal fluids can attack even austenitic stainless steel overlays under certain conditions. Engineers should consider nickel-based overlay alloys such as Inconel 625 or Hastelloy C-276 for applications where the geothermal fluid contains high chloride concentrations or where the temperature exceeds 200°C.

This study demonstrates that the successful implementation of CO2 explosion fracturing for HDR stimulation requires not only geological and geophysical expertise but also rigorous pressure equipment engineering, with particular attention to material selection, weld quality, and corrosion management under extreme operating conditions.