SEM analysis on rock failure mechanism by supercritical CO2 jet impingement

•Rock erosion by supercritical CO2 jet mainly results from brittle tensile failure.•The mechanism is attributed to diffusivity and phase change of supercritical CO2.•Supercritical CO2 jet forms more fractures and complex structure than water jet.•Supercritical CO2 is more suitable in developing the unconventional resources.

Supercritical carbon dioxide (CO2) jet-assisted radial drilling is regarded as a potential alternative drilling method because supercritical CO2 jet has higher efficiency in rock-erosion efficiency than water jet. Although researchers have investigated how the rock-erosion performance by supercritical CO2 jet impingement correlates with the influencing parameters, there is little on the rock failure mechanism and change of pore structure. Scanning electron microscopy (SEM) analysis has been an important means in mineral identification, material damage detection, and aiding in water jet research. Therefore, rock-erosion experiments and SEM observations and analyses were carried out on different rocks including sandstone, marble and so forth. Secondary electron (SE) images show obvious grain shapes on the complex impinged surface of sandstone and marble by supercritical CO2 jet, and cleavages on the mineral grains, and cracks and holes between the grains in different sizes. It suggests that cementations in weaker rocks are firstly removed and then mineral grains broken, generating the intergranular and intragranular cracks and micro-fractures and fissures. Supercritical CO2 jet cannot bring obvious changes to hard limestone and shale at the same condition because the rock substances are tightly packed and filled with large cementing forces and ultra-low permeability and porosity, but can produce a few micro-cracks vertical to bedding planes. The features show that supercritical CO2 jet broke rock substances mainly in the brittle tensile failure mechanism and made the rock much easier to further break, accompanied with shear failure mechanism in particular locations of the perforation hole. In contrast, water jet impingement forms flat cutting sections in marble with continuous parallel breaking traces, indicating the shear failure mechanism. Low viscosity and high diffusivity of supercritical CO2 are primarily attributed to the rock tensile failure mechanism, they facilitate supercritical CO2 penetrates into deep into rock pores and produces static pressure on the inner wall and at the tips of the fractures, besides exerting jet impinging pressure. Moreover, the energizing effect caused by the phase change from supercritical to gas leads to the energizing effect and intensify rock breaking into more shattered pieces and, concurrently, improve general rock connectivity and porosity in the surface layers. As a consequence, supercritical CO2 jet appears to be more efficient and suitable than water jet in slim-hole radial drilling and hydraulic fracturing, particularly in unconventional reservoirs with low permeability.

Graphical abstractSEM images of marble samples impinged by a water jet (left, 2000x) and supercritical CO2 jet (right, 2000x). Supercritical carbon dioxide (CO2) is now regarded as an alternative drilling and stimulating fluid due to unique fluid properties and jet perforation performances. In the image above, Scanning Electron Microscopy (SEM) comparative analysis showed that a supercritical CO2 jet formed a more complex structure, unlike flat parallel breakage trace by a water jet, and produced more cracks than a water jet. Moreover, fractures and cracks exist in different sizes after the sample was exposed to the supercritical CO2 jet stream. Cementations among mineral grains are destroyed and grains are forced to break or separate from each other, and secondary micro-fissures are generated and pre-existing cracks expanded. Besides the rock failure results from impact pressure in normal direction, a supercritical CO2 jet is more likely to cause intergranular falling and intragranular breaking in tensile failure mechanism.Figure optionsDownload full-size imageDownload as PowerPoint slide