Simulation Study On Sand Plugging And Removal Mechanism For Gravel-Pack Sand Control Wells

Gravel pack completion is a commonly used sand controlling method in oil-fields,however,certain oil wells will enter a“low rate low productivity”status due to sand plugging phenomenon appears in gravel layer and near well formation,leading to the raise of workover frequency and the drop of production efficiency.Screen removing sand control and formation unblocking technology(SRSC)is a new low-cost low-damage sand control method which has the ability to effectively restore well productivity while decreasing formation sand plugging.At present nevertheless,due to the vacancy of quantitative analysis and operational experience,the field applicability of this technology can not be quantified,and the operation parameters design is with lack of of scientific evidence,leading to great blindness in its field utility.Regarding the issues above,this research carried out a study on the calculation model of sand production and formation plugging,SRSC operation applicability and well productivity prediction based on experiments,numerical simulation and data mining theories.Grey relational analysis for operation effectiveness is carried out based on production and operation data of sand producing wells in tested field area,and an evaluation method of SRSC adaptability level is established standing on the theory of fuzzy comprehensive evaluation.Experimental models for dynamic permeability prediction in plugging and unblocking precesses are established based on formation sand production,plugging,and removing simulation experiments.Based on COMSOL fluid-solid coupling(FSC)simulation,the spatial and temporal distribution of formation porosity and permeability is studied and the experimental model of permeability prediction is modified on the scale of time.Standing on the above research,optimum design suggestions of SRSC operation are given,and a dynamic productivity prediction model of sand producing and plugging wells is established.Calculation analysis and accuracy test of the above models are then carried out.According to the results,main factors affecting the adaptability of SRSC operation are formation sand sorting coefficient,porosity,formation pressure,layers of exploitation,etc.Using the method of fuzzy evaluation,the subordinate degree between six main controlling factors and productivity improving capacity is analyzed and evaluation grades are designed accordingly.The analysis of 30 tested records show that the evaluation results of 26 wells are complied with actual situation and the accuracy is 86.7%.The closer a position is to the wellbore,the more obvious the formation sand plugging is.The degree of sand plugging and sand removing effects both firstly increase rapidly with time and then tends to be stable.The reasonable rate interval for SRSC operation is 20-40 m L/min based on the observation of permeability change under different rate levels,the corresponding real production rate can be calculated according to the rule of experimental flow-rate conversion.Taking the tested parameter-set as example,COMSOL FSC simulation results show that the production rate of sand producing wells decreases from 22.1 m~3/d to 9.12 m~3/d and the sand-bridging-factor rises to 0.29 after 231 days of production.After approximately 62 days of SRSC operation under a production rate of 15 m~3/d,permeability is considered to have restored to its initial level.Based on bridged sand removal mechanism analysis,the upper limit of a proper operation rate interval is 16-18 m~3/d,which approves the conclusion from both the experiments and field practice.By carring out calculation anslysis on 6 tested SRSC operation wells using the productivity prediction model,the average prediction error of declining production rate for sand plugged wells is 11.15%,and that of estimated productivity after operation is 18.07%,both meet the requirements of accuracy.This research provides scientific basis for well selection,parameter optimization and matching technical design for screen removing sand control and formation unblocking technology,showing great significance to prolong sand production well operation cycle and enhance oil-field economic benefits.