Mineral dissolution and mobilization during CO2 injection into the water-flooded layer of the Pucheng Oilfield, China

Abstract

The Pucheng Oilfield, located in the northeast of Dongpu Depression, is one of the oldest and largest oil fields in the Zhongyuan Oilfield Company of SINOPEC. The CO2 captured from the Chinese coal industry tail gas has been injected into the Es1 reservoir of Pucheng Oilfield for geological CO2 storage and enhancing oil recovery. The Es1 formation is a high water-flooded reservoir and consists of sandstones with heterogeneous permeabilities. The Es1 reservoir has a formation temperature of 85 °C and a formation pressures of 15–24 MPa. After injecting CO2 into reservoir formation, complicated interactions of CO2-brine-rock induces the changes of reservoir properties, which have an impact on the performance of geological storage and enhancing oil recovery.The reactions of brine, CO2 and natural debris samples were studied by conducting laboratory batch experiments, which were conducted under reservoir conditions. During the batch experiments, Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) were used to evaluate the formation damage caused by mineral dissolution. Natural rock batch experiments demonstrated that calcite crystals were obviously dissolved and formed secondary minerals. Calcite content and feldspar content reduced, clay minerals content increased, and quartz content changed only slightly. Based on chemical equilibrium theory, mineral dissolution and deposition were discussed.Flooding experiments under various reservoir conditions were conducted to dynamically evaluate the influence of mineral dissolution and mobilization on formation permeability. The result demonstrated that minerals can migrate in sandstone with any permeability. For low permeability cores, the mineral secondary migration will cause the blocking of a porous medium, even though the flow rate is notably low. For medium permeability cores, the permeability firstly changes little due to the blocking, which is caused by mineral migration, can be canceled via minerals dissolution, carrying the plugging minerals out of the sandstone. Sufficient dissolution effect will weaken the blocking to enhance the permeability. For high permeability cores, it is easier to carry the mobilized minerals out of sandstone, and the mineral mobilization can first decrease the permeability and then increase it. Considering all of the permeabilities, mineral dissolution and mobilization can enhance reservoir heterogeneity.