Influence of stress and formation water properties on velocity sensitivity of lignite reservoir using simulation experiment

To study the velocity sensitivity of a lignite reservoir under different stresses and formation water conditions, orthogonal experiments are performed with pH, salinity, and effective stress as the main factors, and evaluation indexes are extracted at different stages. The effects of the main factors on each index are analyzed by means of range analysis, variance analysis, and effect diagram to study the mechanism of velocity sensitivity in a lignite reservoir. Under a given effective stress condition, the change in lignite permeability with the variation of fluid pressure undergoes three evolutionary stages (stable-rise, decline, and fluctuation) and two effects (sensitization and velocity sensitivity effects) exist. On the basis of the seven evaluation indexes, an evaluation index (Dm) suitable for describing velocity sensitivity damage in the presence of sensitization stage is proposed. Extreme and variance analyses show that salinity is the main factor that controls the lignite sensitization (rise) stage. In the velocity sensitivity (decline) stage, the permeability attenuation rate is controlled by pH and salinity, and the sustained range of velocity sensitivity is affected by pH and effective stress. The fluctuation stage is mainly affected by effective stress. It is analyzed that pH mainly controls the migration and agglomeration of pulverized coal particles through hydrophilicity, and salinity controls the dispersion and agglomeration of pulverized coal particles through the zeta potential generated by an electric double layer. Effective stress influences the sensitivity of lignite reservoir by effecting pore structure.