Shan Chen model Multiphase instability

Hi guys, when I’m using Shan-chen model to simulate water-oil system in pore space, The problem I encountered is that when I set
Water density=1
oil density=0.7
when the simulation starts, the fluids start behaving weird and have distribution like this intimate step 1000(shown below)
water oil system

however, when I set
water density=1
oil density=0.1. the simulation result makes sense(shown below)
water oil system 2

Does anyone have any idea about what happened here?
Thank you so much!