Fracture Growth Experiment And Physical Properties For Gas Reservoir Caprock In Triassic Formation,Northeast Sichuan

Fracture development in reservoir rocks enhances the transport efficiency of the petroleum system,but fracture in caprocks can significantly risk reservoir seal integrity.In geologic time scales,the slow,subcritical fracture propagation can contribute to the fracture system development whose mechanism is still unclear and the influence under reactive fluid environments during petroleum migration and accumulation is lack of study.Using the double torsion(DT)technique in 59 sets of experiments,we measure the subcritical fracture growth properties of anhydrite and carbonate caprock core samples from study area,under different environments of ambient air(a),de-ionized water(w),and formation water(fw).Experimental results show that the aqueous fluids significantly promote subcritical fracture growth:(1)Compared to ambient air,the presence of aqueous fluids dramatically reduces both the fracture toughness(K_IC)and subcritical fracture growth index(n)of the caprocks;(2)The K_IC reduction is more prominent for the anhydrite(50%)caprock than the carbonate(30%)caprock,which indicates that mineral solubility is responsible for the fluid-weakening effect;(3)A systematic change of n with varying fluid conditions is observed:n_a>n_w>n_fw.Microstructural analysis suggests a predominantly opening mode for both the natural fractures and induced fractures during DT tests.Through analysis and comparative study,our results demonstrate that in normal pressure and temperature,anhydrite has a larger fracture toughness than limestone,the finer grain size will build up the capability to resist fracture growth,subcritical fracture growth process is also strongly related to fluid-rock chemistry such as dissolution.Then the relation between long-term strength and time to failure is deduced.With considering the in-situ condition of study area,anhydrite has a better capacity of sealing than carbonate rocks,but greater risks of subcritical fracture growth with the presence of fluid.These observations provide insights into the mechanisms controlling fracture growth in anhydrite and limestone,also introduce a new method and reliable data of caprock sealing evaluation.