Evolution Mechanisms of Carbonate Reservoirs Based on Dissolution Rates and Multifractal Analysis of Microscopic Morphology

Abstract

Carbonates dissolution processes and their effects on the development of secondary pores play important roles in oil/gas exploration and development, which have been controversial for a long time. Dissolution experiments of carbonates are still challenging for studying the formation of micro-spaces evolution in carbonate reservoir rocks. In this paper, the chemical water–rock reaction experiments, microscopic morphology and multifractal analysis were used to study the dynamic dissolution of carbonate rocks on the Middle and Lower Ordovician of Tarim Basin, NW China. Measurements were made at temperatures of 40–150 °C and pressure of 40 atm in silicic acid solution and in NaCl solution at pCO2 during dissolution processes based on the rotating disk apparatus. The results show that the dissolution rate of carbonates varies with the temperatures during the dissolution process, corresponding to the early dissolution in the formation and the post-reformation of fluid of carbonates reservoirs. The range of “dissolution window” for each sample was calculated, and the burial depth of high-quality reservoirs was speculated. Compared to limestones, dissolution processes for dolomites could create more micro-spaces under deeper burial. Except regional geological conditions, the micro-spaces produced in dissolution processes of carbonates are also affected by the local topography of mineral arrangements and the element distribution singularities on the mineral surface. In addition, the phenomenon that burial dissolution created a significant number of secondary pores is conducive to the formation of high-quality reservoirs. Investigation of both the mechanism of dissolution and controlling factors of typical carbonates on the Middle and Lower Ordovician of the Tarim Basin, NW China, in this paper, provides new insights into the evaluation of carbonate reservoirs.