Experimental evaluation of tight sandstones reservoir flow characteristics under CO2–Brine–Rock multiphase interactions: A case study in the Chang 6 layer, Ordos Basin, China

Abstract

By the end of 2020, tight oil production accounted for 65% of total U.S. crude oil production (EIA, 2020). Carbon dioxide flooding is widely recognized as an efficient way of developing tight oil reservoirs. The flow characteristics of host rocks may be changed by CO2-Brine-Rock interactions during CO2 injection. It is necessary to reveal the impact of CO2-Brine-Rock interactions on tight oil recovery, which is critical in understanding the fundamental mechanisms of CO2 flooding for enhanced tight oil recovery. However, the flow characteristics alteration behavior and mechanism during CO2 flooding are still unclear. In this study, outcrop sandstone samples from the Chang 6 layer of Triassic Yanchang formation in Ordos Basin were used. First, CO2 solubility in formation water was measured. Then, the corrosion rate due to carbonic acid on rock minerals was evaluated under reservoir conditions. The porosity, pore throat diameter for the movable fluid, liquid permeability, mineral composition and cast thin sections in tight sandstones were compared before and after corrosion. The corrosion was found to have enhanced both porosity and liquid permeability, with liquid permeability exhibiting the highest increment. Pore and throat diameter for movable fluids in tight sandstones decreased after the reaction, implying reduced irreducible water saturation. XRD results showed that calcite, dolomite and feldspar were main corrosion minerals. The corrosion pores significantly increased after corrosion. These findings indicate improved formation petrophysical properties associated with CO2 flooding in tight sandstones. This study provides a theoretical basis for the efficient development of tight oil reservoirs during CO2 flooding.