Influence And Prediction Of Water-locking Effect Of Coalbed Methane Reservoir In South Of Qinshui Basin

The south of Qinshui Basin,as an important coalbed methane(CBM)industry base in China,has a large productivity difference among gas wells in its production process,with a high proportion of low-producing wells,and the overall production condition is not ideal.As the most reservoir modification method of CBM,hydraulic fracturing produces water lock after the external fluid enters the reservoir,which becomes a non-negligible reason for low gas well production.The production data of Qinshui Basin show that nearly 2/3 of CBM wells have the problem of low productivity due to water locking effect.Therefore,the research on water locking effect in the study area is helpful to prevent and mitigate the productivity reduction caused by water locking effect.This paper mainly studies the water locking effect in the study area from two aspects of experimental research and formula derivation,and the main achievements are as follows:(1)The gas-water relative permeability test was carried out on the coal samples,and the water-lock damage degree of the samples was analyzed and calculated after the normalized treatment of the gas-water two-phase data.The results showed that there was a high degree of water-lock damage in the coal samples in the study area.(2)To carry out the nuclear magnetic resonance(NMR)displacement experiment,observed displacement process of gas liquid two phase flow is as follows:free water in the process of CO₂gas displacement displaced out first,and then under the effect of CO₂ gas adsorption water into free water displaced,CO₂ can change the wettability of the gas,almost no change in the whole process of capillary water.According to the T₂ cutoff value of NMR T₂ spectrum,the potential water lock damage of coal samples was analyzed,which proved that the potential water lock damage of coal reservoir in the study area was more likely.(3)The capillary curve of coal samples was obtained by mercury injection experiment.The phase permeability curve of coal samples was calculated by capillary model,and the degree of water lock damage was calculated.The results showed that most coal samples had a certain degree of water lock damage.The influencing factors and mechanism of water locking effect are analyzed and summarized.(4)The productivity equation of gas well considering water locking effect was derived,and a new method for calculating water locking damage based on production data was derived by using the productivity equation.The influences of water locking intrusion radius,damage degree,bottom hole flow pressure and reservoir pressure on gas well productivity were analyzed and summarized.(5)The water locking effect was predicted by the water locking index method and the total water accumulation method,and the predicted results were basically consistent with the experiment,indicating that the prediction method had a certainly accuracy.