Quantitative Calculation Model for the Secondary Porosity of Sandstone Based on Mineral Dissolution Experiment and Its Application

The formation mechanism and quantitative evaluation of secondary pore is the key problem for deep reservoirs. The experiments on dissolution-precipitation behavior of formation water and minerals in reservoirs were carried out by using a high temperature and pressure geochemical experimental simulation system based on the present temperature and pressure conditions of Minfeng sub-sag in the Dongying depression and the corresponding formation water characteristics. The experimental results show that under the current formation water and temperature pressure conditions, quartz and plagioclase could undergo dissolution, and the solubility increased with the increase of temperature, while calcite underwent cementation, and its growth rate changed little with the increase of temperature, generally concentrated at around 70×10−3g/L. Based on the experimental simulation results and taking into account factors such as the permeability flow rate, precipitation velocity, burial time, and porosity of the formation, a mathematical model of secondary porosity in sandstone reservoirs due to dissolution was established. According to the mathematical model calculation, the CaCl2 water type at 171℃ in Feng 8 well had the maximum contribution value of 2.5235% to the physical properties of the reservoir, which was the most favorable water type and temperature for the development of a secondary porosity zone within the simulation depth range of this well. The model calculation shows that the seepage rate was the main factor affecting the development of secondary dissolution pores. Combined with the diagenetic phenomenon of actual reservoirs in Feng 8 well, it had good correlation and obvious dissolution of quartz and feldspar, development of carbonate minerals in the form of cementation and metasomatism, which was consistent with the experimental simulation results. Based on mineral solubility, mineral content, reservoir temperature and pressure conditions and formation water chemistry, the quantitative calculation model established here can be used to predict the deep secondary pore development zone.