Study On Evaluation Method Of Working Fluid Damage In Pressure Transfer Of Tight Reservoir

The potential reserves of oil and gas resources in tight reservoir are abundant and they have great potential for exploitation.Tight reservoirs have the characteristics of low porosity and low permeability,complex pore structure,and full of various clay mineral.Generally,the matrix permeability of gas is less than 0.1×10^-3μm².Therefore,it is necessary to use horizontal branch wells and staged layered fracturing techniques to increase production,which however may make working fluid enter into tight reservoirs.The entry of the working fluid leads to the damage of tight reservoirs and affects the efficiency of mining.The domestic evaluation of sensitivity and working fluid damage is completed mainly by adopting the professional standard SY/T5358-2010.However,it is not suitable for damage evaluation of tight reservoirs,mainly because the pore flow rate is difficult to be measured accurately or even cannot be measured,leading to problems such as measurement errors,high time costs,and high requirements on experimental instruments.Therefore,it is urgent to establish an effective evaluation method for fluid sensitivity and working fluid damage in tight reservoirs.According to the fluid permeability model based on the principle of pressure transfer,an experimental platform for evaluation of working fluid damage in tight reservoirs was constructed.Combined with the concept of capillary bundles,a three-dimensional model of dense cores was established.The relationship between the downstream volume and the pore volume of the core was determined.The downstream volume of the damage platform was determined as 1 mL.Compared with the permeability and stability of cores with different lengths,it is preferable to use a core with length of 1 cm for damage evaluation.The whole experimental platform is composed of pressure transmission systems,control systems,metering systems,simulation systems and data acquisition systems.The evaluation process of sensitivity and working fluid damage was determined in detail,the monitoring of flow rate in the professional standard was replaced by monitoring of the downstream pressure change,the evaluation of the velocity sensitivity was completed by gradually increasing the pressure gradient,and the pressure gradient of other damage evaluation was determined by using the critical pressure gradient obtained from velocity sensitivity.In the process of fluid damage and flowback,the time required for the core self-priming 0.8PV was used to simulate the time of damage fluid entering the core and the flowback time of the working fluid,thereby initially establishing evaluation method of sensitivity and working fluid damage in pressure transfer of tight reservoir.Based on evaluation of the basic properties and petrological characteristics of the tight sandstone of the Penglaizhen Formation in western Sichuan,the evaluation of sensitivity and working fluid damage were performed.The evaluation results showed that the velocity sensitivity of tight sandstone in the Penglaizhen Formation in western Sichuan was medium and moderately weak,the water sensitivity and alkali sensitivity were medium and moderately strong,salt sensitivity and acid sensitivity were strong;the drilling fluid and fracturing fluid had higher damage rates and the recovery of permeability was poor after the working fluid return.Through comparing the results obtained by the pressure transfer method with that by the professional standard method,the water sensitivity,acid sensitivity,alkali sensitivity,and the correlation and stability of drilling fluid damage rates were analyzed.The results showed that the range of the matrix permeability of gas applied to pressure transmission method is between 0.001⁵.0×10^-3μm².When the permeability was 0.001⁰.1×10^-3μm²,the correlation of the damage rate was weak,and the test results with the pressure transfer method was more stable;when it was0.1⁵×10^-3μm²,the correlation was strong,and the test results with the two methods were both stable;when it was 5¹0×10^-3μm²,the professional standard method was more stable.The average coincidence rate of different core damage levels was also calculated to be 95%,verifying its reliability.Finally,compared with the professional standard method,the advantages and disadvantages of the pressure transfer method were discussed.The pressure transfer method was considered to have certain application potential for the fluid damage of tight sandstone,and it can provide some suggestions for evaluation of working fluid damage in matrix of shale and coal rock.