The study looked at how stress affects low permeability sandstone. By increasing outer stress in the lab, researchers found that low permeability sandstone is sensitive to stress, which reduces its permeability as pressure decreases. The double effective stress theory suggests that effective stress is not just the net confining pressure, but also affected by inner pore pressure and porosity. This means that changes in reservoir pressure or net confining pressure have minimal impact on the permeability and porosity of low permeability sandstone during production.