An Experimental Method Study On Working Fluid-induced Damage Evaluation For Commingled Production Of Multi-layer Natural Gas Reservoirs

In recent years,natural gas in coal measures strata has developed into a hot field of global oil and gas exploration and development.Different from the single-layer development of conventional natural gas reservoirs,coal measures strata are characterized by the symbiosis and frequent alternation of multiple reservoirs in the vertical direction,so the commingled production technology of multi-layer natural gas reservoirs in the same wellbore is the primary choice.Under the condition of commingled production,the degree of damage induced by the working fluid is significantly different due to the differences in lithology and physical properties of different reservoirs,and the contribution of each layer to production is not consistent.In order to select a suitable working fluid for commingled production operation to ensure high and stable production of gas wells,it is important to comprehensively evaluate the working fluid-induced overall damage during commingled production.However,the current conventional industry methods based on permeability index are only designed for single-layer reservoir,and there are still doubts about the method of layer-by-layer evaluation to obtain the average instead of characterizing multi-layer reservoirs.The multi-layer reservoir cannot be integrated and the overall damage cannot be directly quantified in the multi-core parallel mode.Moreover,the experimental parameters setting based on conventional industry standards is far from the real reservoir conditions.In this study,on the basis of summarizing the defects of traditional industry methods,we firstly analyzed the production mechanism of natural gas in coal measures strata,compared the principles of experimental methods for rock seepage,and discussed the feasibility of those methods under commingled production conditions.The principle of constant pressure method based on steady state for evaluation of working fluid damage during commingled production was determined.And combined with the geological and engineering characteristics of coal measures strata,a production simulation device for multi-layer natural gas reservoirs was established.Then,by carrying out the comparison experiments for core samples cleaning,drying,as well as establishing the initial water saturation,and through constructing a mathematical model converting in-field production into in-lab flow rate for vertical wells,the samples pretreatment method and the test parameter formulation method were optimized respectively.Next,the flow rate index was theoretically selected according to the previous physical experiments of multi-layer commingled production,and the feasibility of quantifying the reservoir damage during commingled production by the flow rate index was discussed by establishing the mathematical equation between the flow rate and the permeability index.Finally,a comparative experiment was carried out between the proposed new method and the conventional industry standard method using the natural rock samples from four typical wells in the Linxing block,and the rationality of the new method was verified via the skin factor of the field well test.Results showed that: In the single-layer development,double-layer and triple-layer commingled production modes,the instantaneous flow rate index was almost equal to the permeability index in characterizing the degree of reservoir damage,the maximum errors of the two under the three corresponding modes were 0.33%,0.59% and 1.21%respectively.The cumulative flow rate index was significantly different from the permeability index in characterizing the degree of reservoir damage,the maximum errors of the two under the three corresponding modes were 39.12%,29.12% and34.25% respectively.The classification of damage degree obtained by the proposed method had a coincidence rate of 90% with that of the skin factor of the corresponding wells or production layers in the field,which was much higher than the coincidence rate25% of the conventional industry method.Under the optimized test parameters conditions,the damage rate of permeability considering the production contribution during commingled production was closer to the damage rate of the instantaneous total flow rate than the damage rate of average permeability for each layer of commingled production.In conclusion,the proposed evaluation method of working fluid-induced formation damage for multi-layer natural gas commingled production is in line with the actual development mode in coal measures strata,it can effectively present the stress state,interlayer differences and gas seepage characteristics under the in-situ reservoirs conditions,and can accurately quantify the overall damage caused by the working fluid,which overcomes many defects in the application of conventional industry methods,thus providing better guidance for the design of future field development plans.