In order to investigate the effect of water-cement ratios on the microstructure and micromechanical properties of oil well cement slurry, mercury intrusion, thermogravimetric and X-ray diffraction techniques were used to characterize the internal pore structure, hydration process and phase content of oil well cement slurry. The statistical nanoindentation technique was utilized to probe the micromechanical properties of oil well cement slurry, in the meanwhile, both least squares estimation and maximum likelihood estimation methods were adopted for the deconvolution calculations, through which, elastic modulus, hardness, and volume fraction of solid phase in oil well cement slurry were gained. The BSI model is chosen to evaluate and select the most appropriate bin size because the bin size plays a significant role on the accuracy of the least squares estimation algorithm. The results of this study indicate that the water-cement ratio affects the porosity, pore size distribution, and pore properties of oil well cement slurry. A higher water-cement ratio favors the formation of hydration products. In addition, the elastic modulus of the hydration product decreases significantly with the increasement of water-cement ratio. The hydrated calcium silicate gel has a higher density under the low water-cement ratio condition and vice versa. Although the two deconvolution algorithms are based on distinct criteria, both of them are suitable for deconvolution calculations of cementitious materials.

The long-term strength retrogression of silica-enriched oil well cement poses a significant threat to wellbore integrity in deep and ultra-deep wells, which is a major obstacle for deep petroleum and geothermal energy development. Previous attempts to address this problem has been unsatisfactory because they can only reduce the strength decline rate. This study presents a new solution to this problem by incorporating fly ash to the traditional silica-cement systems. The influences of fly ash and silica on the strength retrogression behavior of oil well cement systems directly set and cured under the condition of 200 ℃ and 50 MPa are investigated. Test results indicate that the slurries containing only silica or fly ash experience severe strength retrogression from 2 to 30 d curing, while the slurries containing both fly ash and silica experience strength enhancement from 2 to 90 d. The strength test results are corroborated by further evidences from permeability tests as well as microstructure analysis of set cement. Composition of set cement evaluated by quantitative X-ray diffraction analyses with partial or no known crystal structure (PONKCS) method and thermogravimetry analyses revealed that the conversion of amorphous C-(A)-S-H to crystalline phases is the primary cause of long-term strength retrogression. The addition of fly ash can reduce the initial amount of C-(A)-S-H in the set cement, and its combined use with silica can prevent the crystallization of C-(A)-S-H, which is believed to be the working mechanism of this new admixture in improving long-term strength stability of oil well cement systems.

A comprehensive experimental program has been performed to characterize the hydration and engineering property evolution of a class G oil well cement under various curing temperatures from 30 to 90 ℃. The progress of hydration was monitored by isothermal calorimetry (atmospheric pressure); the viscosity evolution was measured using a high temperature and high pressure consistometer (up to 200 MPa); the ultrasonic property development was evaluated by an ultrasonic cement analyzer (up to 100 MPa). Test results indicate that the influences of curing temperature and pressure on the hydration, viscosity and ultrasonic property development can be modeled by a scale factor method that is similar to the maturity method used in the concrete industry. However, the key parameters of the scale factor model, namely the apparent activation energy and the apparent activation volume of cement showed obvious variations with test method and curing condition. The test results indicate that the curing temperature has a stronger effect on cement hydration rate than viscosity and ultrasonic property development rate, while the curing pressure has a much stronger influence on cement slurry properties before setting (viscosity) than after setting (ultrasonic property).