Effect of mineral dissolution by organic acids on pore structure of shale reservoir

Key words:  organic acids  dissolution  ions release  shale pore structure  porosity enhancement effect

Abstract:

In order to know the dissolution of common mineral ions in shale dissolved by organic acid, the impact of dissolution on secondary pore structure of shale was investigated, in which the dissolution experiments were designed on shale samples from the member 3 of Shahejie Formation of Paleogene in Nanpu sag with different types of organic acids (acetic acid and oxalic acid) and different temperatures. The plasma-atomic emission spectrometry was conducted on the solution after the reaction. Also, the X-ray diffraction, scanning electron microscopy and low-temperature nitrogen adsorption analysis were conducted on dissolved samples. The results show that dissolution of common mineral ions in shale with increasing reaction temperature has two situations, which are "increase, decrease, compound and little" by acetic acid, and “increase, decrease, compound” by oxalic acid respectively. And different ions show different dissolution coefficients. In general, the organic acid dissolution of shale shows obvious positive correlation with temperature. And different types of organic acids show different dissolution mechanisms on shale significantly. In the dissolved shale, the size and distribution of intragranular pore in shale increase. It is also found that the small scale of erosion occurs in the contact part of intergranular pore, resulting in net-pore phenomenon and increased width of micro crack. Also the accompanied new-born clay minerals are formed, such as illite, kaolinite and montmorillonite. The acetic dissolution promotes the formation of microporeand mesoporein the shale, which increases the surface area. Meanwhile oxalic acid dissolution results in the formation of mesopore and macropore, which increases the average pore size of the shale. The preliminary determination of shale porosity enhancement by organic acid dissolution shows the limited capacity of enhancing porosity, which is not as significant as expected.