Experimental Study on the Variation of Porosity and Permeability of Shale With Supercritical Carbon Dioxide Fracturing

Abstract

Porosity and permeability of supercritical carbon dioxide (scCO₂) treated shale are studied under the changed pore gas, temperature, pore pressure and moisture. Based on the modified triaxial mechanical apparatus and Chinese shale core, we conduct 40 experiments, which included 2 kinds of injection gas (scCO₂ and N₂), 5 groups of moisture (0, 0.5%, 1.0%, 1.5%, 2.0%), 4 groups of pore pressure (5, 10, 15, 20 MPa) and temperature range (25-70 °C). At last, SEM and XRD tests are carried out to compare the microstructure and mineral composition of shale before and after the scCO₂ treatment.

Waterless fracturing for shale gas exploitation using scCO2 is both effective and environment-friendly, so it is a popular research topic. In literatures, after scCO2 treatment (supercritical condition: >32 °C and >7.3 MPa), some minerals in shale are corroded and microfractures propagates. But there are few quantitative researches on the change of permeability and porosity of scCO₂-treated shale, especially for the Chinese shale.

Some experimental results have been obtained. (1) For the N₂ control group, the porosity and permeability do not changed obviously; however for the scCO₂ group, the porosity and permeability are larger than that in N₂ group. (2) With the increase of shale moisture, the porosity and permeability have a tendency of first rise and then drop. Because the scCO₂ reacts with water to generate acid, corroded-minerals and leave microfractures, and the mineral contents are changed. The high moisture content results in clay hydration expansion, which leads to the closure of micro-fractures. (3) When the temperature is below 32°C, porosity and permeability of shale do not change obviously; with the increase of temperature (>32 °C), the porosity and permeability increased significantly. (4) Under the scCO₂ conditions, the changing of shale porosity and permeability with pore pressure is similar with that of temperature. (5) A prediction model of porosity and permeability with pressure, temperature and moisture of scCO₂-treated shale is finally established.

It makes some contributions: (1) obtaining the variation regularity of porosity and permeability of shale after various conditions of scCO₂ treatment, (2) presenting a new experimental method and apparatus for testing porosity and permeability parameters under scCO₂ condition, (3) providing the experimental basis and a new model for the scCO₂ fracturing design.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018