Abstract
How to control excessive water production in high-temperature and high-salinity reservoirs has always been a challenge, which has been facing many oil reservoirs in Tarim Basin (China), such as Y2 reservoir with an average temperature of 107 ℃, salinity of 213900 mg/L (Ca2++Mg2+>11300mg/L), and permeability from 2 to 2048 mD. In this work, we present experimental studies to determine the potential EOR process for Y2 reservoir from foam flooding, polymer gel/foam flooding, and microgel/surfactant flooding.
To simulate the permeability heterogeneity of Y2 reservoir, a 2-D sand-pack model was used for flooding experiments. Vertically, three layers (first 0.6cm, second 0.8cm and third 1.6cm from top to bottom, respectively) were packed with different size sand to simulate permeability heterogeneity (permeability increases from first to third layer). A 0.3 cm higher permeability zone was also filled inside third layer. Horizontally, permeability gradually decreases from middle to two sides. In this model, injection well was vertical, and production well was horizontal. The effect of impermeable interlayer was also studied by isolating the second and third layer.
The results show that conformance treatments using in-situ crosslinked gel or micro-gel are necessary before foam or surfactant injection under a high permeability heterogeneity. When an impermeable interlayer existed between the second and third layer, the additional oil recovery of N2 foam flooding, in-situ crosslinked gel/N2 foam flooding, and microgel/surfactant flooding was 16.34%, 20.37%, 17.50%, respectively, which was much higher than that without impermeable interlayer (9.84%, 13.62%, 12.07%). This implies that when multiple layers exist, crossflow between layers is unfavorable for improving oil recovery, which should be paid extra attention in EOR process. Foam flooding has not only a good mobility control capacity but also a good oil displacement ability (verified by visual observations of washed sand after experiments), which, together with the strong conformance control ability of crosslinked gel, makes in-situ crosslinked gel/N2 foam flooding yield the highest displacement efficiency. Generally, for high-temperature and ultra-high-salinity reservoirs with strong heterogeneity like Y2 reservoir, in-situ crosslinked gel/foam flooding can be a good candidate for EOR.
This work provides a potential EOR method with high efficiency, i.e. in-situ crosslinked gel assisted N2 foam flooding, for the development of similar reservoirs like Y2 with high temperature, ultra-high salinity, high heterogeneity and multiple layers. Moreover, this work also highlights that, despite that foam has the ability of mobility and profile control, a conformance treatment is necessary to block high permeability zone before foam injection when the reservoirs has a strong heterogeneity.