Electrical response law and water flooding mechanism analysis of water flooded layer in Gasikule Oilfield

To address the issue of high water cut in the Gasikule Oilfield, a series of laboratory experiments was conducted to examine the electrical response characteristics of water-flooded layers. Core samples representing high, medium, and low porosity and permeability clastic rocks were collected from the N1-N21 formations. The study focused on analyzing changes in lithology, petrophysical properties, pore structure, and cation content before and after water flooding, aiming to clarify the mechanisms responsible for the observed differences in electrical response. Experimental results indicate that in medium-to high-porosity and permeability reservoirs, when the salinity of the injected water exceeds 20, 000 mg/L, the resistivity-water saturation relationship follows an L-shaped curve. However, when the injected water salinity is below 20, 000 mg/L, the curve exhibits a U-shaped trend. In low-permeability reservoirs, the transition from an L-shaped to a U-shaped curve occurs at a much higher salinity of around 80, 000 mg/L. During freshwater flooding, as the degree of water invasion increases, the rock resistivity initially decreases and then increases, resulting in a clear U-shaped curve. The lower the water salinity, the more pronounced the U-shaped behavior becomes. Additionally, reservoirs with better petrophysical properties show lower water saturation values at the inflection point of the resistivity curve, indicating earlier changes in electrical response. These changes are primarily attributed to the hydration, swelling, dispersion, and migration of clay minerals during water flooding. These mineralogical processes lead to reductions in clay content and cation exchange capacity, while simultaneously improving pore connectivity and increasing both porosity and permeability. This research providesessential technical insights into the behavior of clastic reservoirs under water flooding and highlights the influence of salinity and reservoir quality on resistivity responses. The findings offer valuable guidance for the identification of water-flooded zones and the evaluation of remaining oil saturation. They also support the development of improved resistivity interpretation models and waterflood management strategies in similar sandstone reservoir settings.