Mechanisms of Water Block Damage of High Temperature Reservoirs Based on In-situ Characterization of Wettability and Subcritical Water Characteristics

Study on the in-situ wettability of deep reservoir rocks is of great importance to the in-depth understanding of the mechanisms with which a high temperature reservoir is damaged by water block and to the establishment of efficient measures for water block prevention. In this study, cores from a deep reservoir in a block in Bohai (China) were used to in-situ characterize the changes with temperature of the contact angles of different rock surfaces in a nitrogen environment of 20℃ – 200℃ and 8 MPa, and an empirical equation was established for predicting the changes with temperature of the contact angles of reservoir rock surfaces after the oils were washed off the rocks. Using atomic force microscopy, scanning electron microscopy and energy spectrum analysis, the mechanisms with which the changes with temperature of contact angles of rock surfaces were analyzed. The Experimental results showed that the contact angles of the reservoir rock surfaces after washing off the oils were reduced with temperature in different ranges; in 20℃ – 100℃, the rate of change of contact angle is −0.04 °/℃, and −0.24 °/℃ in 100℃-200℃. The adhesion work of water on the rock surfaces generally increases with temperature, the change of which is small though. The adhesion work of water on the rock surfaces with oils increases remarkably with temperature, with a rate of change at 160 ℃ of 155.27%. By measuring the micromorphology and element content of the rock surfaces, it was concluded, considering the physical-chemical characteristics of subcritical water, that the desorption and even the pyrolysis of the hydrocarbons adsorbed on the surfaces of the rocks under the action of subcritical water result in the significant change of the wettability of the oil-adsorbed surfaces of the rocks, and when in contact with the rock surfaces with adsorbed oils, fluids flowing into a well will present more serious water block damage to the reservoir formations. Based on the in-situ characterization of wettability and the physical-chemical properties of subcritical water, some new understandings about the mechanisms with which the wettability of the reservoir rock surfaces changes at elevated temperatures are obtained, and these new understandings are helpful in designing proper water block prevention program in high temperature oil and gas resource development.