Polymer “viscoelastic effect”; does it reduce residual oil saturation*

Abstract

There exist several mentions (Qi et al., 2016; Vermolen et al., 2014; Wang et al., 2013) of hydrolyzed acrylamide (HPAM) polymers leading to reduced end point residual oil saturation, SOR, relative to water floods. Observed lowering of SOR is commonly referred to as “viscoelastic effect”, but literature results are scattered when it comes to identifying a significant and reproducible impact. This claim has typically been investigated in tertiary type core floods i.e. polymer has been injected after a long preceding water flood. Huh and Pope (2008) have, however, proposed that polymers act as to maintain oil phase continuity, meaning that an impact can only be induced before oil breaks up into discontinuous ganglia. Hence, if their proposed mechanism is correct, tertiary mode investigations would be misleading and cannot capture the full potential effect.The present work was therefore initiated with the aim of testing the hypothesis of Huh and Pope (2008). A reservoir rate core flood study was performed in a “micro-CT” imaging system enabling pore scale (≈7µm) resolution of fluids. Water wetting rock samples, 5mm diameter, were drained with both light and viscous model oils and placed in a core holder situated in the X-ray beam. Water salinity, polymer concentration and molecular weight were chosen to be in a reasonable range for an offshore field case. Polymer injection following water flooding, at SOR, did not change oil saturation, which was in agreement with the hypothesis. It was in addition observed that the spatial location of the discontinuous oil phase remained virtually un-altered. Thereafter oil was injected into the porous medium, which already contained polymer, to increase oil saturation. Subsequent polymer injection led to higher SOR, which was in complete contradiction to the hypothesized lowering of SOR. This observation of higher residual oil saturation occurred irrespective of the rate of preceding oil injection i.e. whether oil saturation was only slightly increased or driven back to around initial water saturation, Swi. This hysteresis did, however, not seem to occur when polymer was present only as an adsorbed layer during water injection.The results reported herein, from purely water wetting porous media, did consequently not fit with the expectations from the hypothesis at question. On a general basis the results do therefore not support claims that polymer flooding, due to the viscoelastic nature of the fluid, should lead to a lower SOR.