An experimental study on the pore characteristics alteration of carbonate during waterflooding

This study presents experimental results on the effect of dissolution and precipitation to porosity, permeability, and oil recovery in four types of carbonate rocks, which were collected from different outcrops. The mineral composition of the four specimens was analyzed using X-ray diffraction and X-ray fluorescence. Also, the dissolved ion concentration of effluent water was measured by inductively coupled plasma. Before and after waterflooding experiment, pore size distribution was observed to the change of pore structures. The core flooding experiments were conducted under the reservoir condition for n-decane and water systems. From the experiments, when the carbonate has a pore size distribution dominated by pores in the range of 0.1-5 μm range without flow channels, precipitation becomes higher due to the low permeability. However, dissolution becomes higher in the carbonate which comprise a high percentage of micro to mesopores (0.1-10 μm) and small channels. On the other hand, in case that carbonate mainly comprise micropores of less than 1 μm and macropores greater than 10 μm such as flow channels, water does not significantly affect to the dissolution and precipitation due to less retention time. Finally, when carbonate rock has bimodal pore ranges of less than 1 μm and greater than 3 μm, the portion of macropores is increased due to the mineral dissolution. At the same time, plugging phenomena within the micropores and mesopores which are connected to macropore channels are simultaneously occurred. The results indicate that dissolution and precipitation occur in various ways depending on the initial pore structure during water injection into carbonate rocks and they change the properties such as porosity and permeability and also affect the oil flow.