Changes of Mechanics and Characteristics of Porosity and Permeability of Set Cement at High and Ultra-High Temperatures

In shale oil production using in-situ thermal recovery method, the temperature in the borehole is high or even ultra-high, and it is easy to result in failure of the sealing integrity of the cement sheath. It is thus important to study the changes of the mechanical characteristics and the porosity and permeability of a set cement under high or ultra-high temperatures. Using digital image technology, changes of the failure mode of a set cement after high temperature treatment is analyzed. Using low field NMR, the changes of the porosity and permeability of a set cement after high temperature treatment are studied, and the correlation of porosity and permeability is analyzed. The study shows that for a high temperature treated set cement sample, as the temperature increases, the monoaxial compressive strength of the set cement first increases and then decreases. The porosity and the permeability of the set cement increase continuously; at first, the increase goes slowly and then goes fast. This is because at lower temperatures, there are only micro and small pores produced inside the cement and these pores are not inter-connected with each other. As the temperature increases, some micro and small pores combine into medium and large pores, and pores of different sizes begin to connect with each other, thereby increasing the permeability of the set cement. The achievements of the study have important reference significance for protecting the sealing integrity of cement sheath and optimizing cement slurry formulation in in-situ thermal recovery of shale oil.