The temperature and pressure limit of brine in oil & gas wells is typically below 205oC and 24,000 psi. Generally, the impact of temperature change on properties of brine is more significant than the effect of pressure. High temperature (above 100oC) related research is sparse because of experimental challenges, most research setup at low temperature should be fundamentally changed to be applied to high-temperature environments. Research at high-temperature high-pressure (HTHP) is the most challenging because of the need for special materials and cautious experimental procedures. In this research, by measuring the solubility of anhydrite at concentrated high [Ca] solution below 220oC and the following Pitzer modeling with updated database, as well as measuring induction time of barite nucleation using a novel laser-hydrothermal apparatus, we aim to quantitatively understand the effect of temperature on thermodynamics and kinetics of some sulfate scale minerals. Solubility measurement of scale minerals in electrolyte solution is a mature field, the research boundary has shifted to combined conditions of high temperature, high pressure and high salinity. In Chapter 2, the solubility of calcium sulfate anhydrite was measured in the presence of high NaCl and CaCl2 concentrations at temperatures from 120oC to 220oC. A static hydrothermal reactor method was used, solubility equilibrium was verified by continuously monitoring both [Ca2+] and [SO42-], as well as comparison of results with the literature. Solubility of anhydrite in CaCl2 solution ranged from 0-1.33 m shows a continuous decrease with temperature, when CaCl2 concentration increases at a constant temperature, the solubility of anhydrite increases at T>175oC due to higher ionic strength but shows a complex behavior below 175oC. Another setup is the solubility of anhydrite in the presence of mixed NaCl-CaCl2 solution with a constant ionic strength of 4 m, results show that solubility at low calcium concentration (0.25 m CaCl2, 3.25 m NaCl) is significantly higher than that with higher Ca2+ concentration, which can be explained by the Ca2+ common ion effect. When CaCl2 concentrations increase to 0.5 m, 0.75 m, 1 m, and 1.33 m, anhydrite solubility does not change significantly at a constant ionic strength of 4 m within the temperature range, suggesting that common ion effect is only significant at low Ca concentrations. In Chapter 3, solubility data from Chapter 2 are incorporated into a database of Pitzer-Debye-Hückel model, wherein thousands of thermodynamic data of sulfate minerals (barite, celestite, gypsum, and anhydrite) were integrated to give the best regression of ion-ion interaction parameters, especially the bivalent Ca-SO4 interaction parameters. The revision of Ksp values of sulfate minerals at HPHT and the multivariate linear regression procedure are also discussed in detail. In Chapter 4, we designed a laser hydrothermal reactor to examine barite nucleation induction time up to 250oC. A stainless 316 aging cell was equipped with two sight windows on its wall, through which a laser beam penetrates to monitor the turbidity change of solution inside. Barite supersaturation was established by injecting concentrated 0.5 ml of BaCl2 and Na2SO4 solution into about 200 g background solution (1 m NaCl, 0.025 m CaCl2), with saturation index (SI) range from 0.4 to 1.2 and temperature range 90oC to 250oC. Results show expected behavior that induction time decreases with both SI and temperature. The effect of two thermally stable chemical additives, branded polymer inhibitors of sulfonated carbonate copolymer (SCC) and polyvinyl sulfonate (PVS), was also examined at 200oC. Again, an expected inhibition pattern was observed, induction time decrease with inhibitor concentration (0-2 ppm) at constant SI value. These results imply that the change of induction time with SI and inhibitor concentration follows a similar trend from temperature range 4oC and 250oC.