Computation of Productivity Index with Capillary Pressure Included and Its Application in Interpreting Production Data from Low-permeability Oil Reservoirs

摘要

Capillary pressure might be ignored in high permeability rocks but could not be neglected in low permeability rocks. In order to study the effect of capillary pressure on production performance in low permeability oil wells or reservoirs, the formula of calculating water cut, dimensionless total and oil productivity indices were derived by considering capillary pressure. Productivity index and water cut data were computed using the new models with capillary pressure included. The results proved that the productivity index increases with water cut in high permeability rocks but decreases with the increase in water cut within a specific range in low-permeability rocks. Water flooding experiments have then been conducted in core samples with low and high permeabilities. The experimental data also demonstrated the same relationship between productivity index and water cut as the new productivity index model foreseen. Finally the productivity index data were calculated using production data from oil wells and the results were compared with the experimental data of the productivity index determined from core flooding tests. The curves of productivity index vs. water cut obtained from the production data of oil producers were consistent with those inferred from water flooding data in core samples. Note that the core plugs were sampled from the same oil wells. The new productivity index model could be used to explain the difference in production performance between high- and low-permeability oil wells.