Study On Interaction Between Water And Rock In Formation In CO 2 Injection And Storage In Depleted Gas Reservoirs

Carbon capture and sequestration is an important solution to address the environmental problems effectively with increasing attention. And depleted gas reservoir is an attractive geological carbon storage media because of the good sealing and large storage capacity. The interaction between CO2, brine and rock during CO2 injection and sequestration process in depleted gas reservoir, including water vaporization and the triggered mineralization reaction, affects the physicochemical properties of reservoir and injectivity. It is of great significance for storage design and mechanisam analysis to understand the interaction above.Based on the properties and phase characteristics of CO2, this paper identifies and quantifies post-flood dry zones, CO2-induced reactions and permeability changes in depleted gas reservoir after CO2 exposure. CO2 flooding experiment for water evaporation and salt precipitation, and CO2-brine-rock reaction experiments are carried out. The saturation equation during the evaporation process and the salting-out model is established in terms of the CO2-brine two phases flow theory. Besides, water vaporization effect and its influence on reservoir properties is studied in the gas reservoir from stage production to injection by applying numerical simulation. The sensitivity analysis of water vaporization and CO2 storage mechanisms are simulated. Through the above researches, the following conclusions can be reached:(1) Based on the experimental results of water vaporization, dry supercritical CO2 injection into core causes water evaporation and salt precipitation and the induced permeability reduction. It turns out that porosity is reduced by 29.76% to the most; permeability is reduced by 80.99% at most. It shows that the higher the formation brine salinity, the greater the degree of permeability damage; and the lower the initial formation permeability, the greater the permeability damage.(2) Weak dissolution and strong precipitation effect is observed in the CO2-brine-rock interaction experiments. After experiments, brine salinity is increased, mineral compositions are changed.(3) In terms of formation water evaporation, the CO2/brine two-phase radial model is established. The gas saturation at different positions can be expressed by using the equation and established model. Moreover, salt precipitation effect can be analyzed. The Kozeny-Carman equation is referred to develop the porosity-permeability relationship. And the damage can be quantified by applying the skin factor and injectivity index.(4) Numerical simulation shows that pressure drop during gas production results in rapid formation water evaporation in the near wellbore area, leading to severe salt precipiation in the range of 5m around the wellbore at the middle and later periods of gas reservoir development. The simulation shows that the total deposits are 18628mol,0.00036% of the total salt in brine. It results in a maximum porosity reduction of 2.07% and a maximum permeability reduction of 6.75%. The production suffers from salting out extraordinarily.(5) Vaporization of formation water and salt precipitation initiated during the production period is intensified by CO2 injection. The simulation indicates a drying area with a radius of 78m around wellbore and a total precipitated amount of 1.82×108mol,3.48% of the total salt in brine. Moreover, the ultimate porosity is reduced by 58% at most after injection and the corresponding permeability is impaired by 93.9% at most. On the other hand, skin factor increases extraordinarily after CO2 injection, indicating severe formation damage. Overall, injectivity is badly damaged at the end. The sensitivity analysis shows that higher injection rate, higher salinity brines, higher irreducible water saturation, lower initial permeability, higher temperature are conditions enhancing the salt precipitation due to dry CO2 injection. Moreover, capillary force promotes the evaporation and salt precipitation process.(6) 1000 years of carbon sequestration simulation shows that the solubility trapping dominates carbon sequestration. As expected, mineral trapping does little to the storage contribution. But the mineralization is quite effective for the sequestration security. During sequestration, mineral precipitation has a noticeable effect through time can be seen, the porosity decreases overall.