Study On Stress Sensitivity Of Long-term Acid Fracture Conductivity In Carbonate Reservoirs

Acid fracturing is the main mean to develop carbonate reservoirs,and the acid etched fracture conductivity is a crucial index to evaluate the acid fracturing results.Therefore,it is very important to study the conductivity of acid-etching fracture.Acid fracture conductivity is seriously affected by the gradually attenuated reservoir pressure,and affected by the production time in the oil and gas production as well.However,there is little research on how long-term acid fracture conductivity varies with production time.Therefore,this paper focused on this problem from the following aspects.(1)Simulation study on the conductivity of acid fracture.Taken the influence of closure stress variation,rock creep and hydraulic erosion on the fracture width into consideration,the model of long-term conductivity of acid-etching fracture under variable closure stress is established to study stress sensitivity.Through the sensitivity analysis of the model parameters,the following is found: First,the closure stress is the main factor determining the conductivity of the acid etched fracture;Second,the UCS of the acid etched surface,the mean value and standard deviation of the acid etched surface asperities and the correction coefficient of the real contact area have a less importance on the conductivity of the acid-etched fracture;Third,the creep of asperities and hydraulic erosion affect the change of the conductivity at the different stage of production.(2)Experimental study on stress sensitivity of long-term acid fracture conductivity.The results of stress sensitivity experiments show that if the acid contact time is the optimal time,the stability of acid-etching fracture conductivity during oil and gas production is best.Through comparison of simplified experiment results and the model calculation results,it is found that the model results conform to the experimental data,thus the model can reflect the variation of the acid fracture conductivity in the oil and gas production process.Through the further study of the acid etched surface asperities and UCS measured by scratch test,the conclusion that the optimal acid contact time leads to the highest UCS of acid-etched fracture and the asperity height distribution owns the most resistance to reservoir pressure attenuation has been drawn.The following has also been discovered: the acid-etched fracture is propped by the asperity composed by the acid insoluble mineral,and the UCS of the acid fracture surface increases linearly with the height of the asperity.(3)Field practice based on stress sensitivity of long-term acid fracture conductivity.An example was given to calculate the optimal acid contact time of different types of acid.According to the conclusion of the optimal acid contact time from th long-term conductivity stress sensitivity experiment,the long-term conductivity model of variable stress is used to analyze the change of long-term conductivity of different acid contact time under the condition of reservoir stress attenuation.And then,the best acid contact time is obtained based on the simulation results.At last,the acid-rock reaction rate is used to convert the optimal etching time at different acid etching conditions.