Quantitative characterization of organic acid generation, decarboxylation, and dissolution in a shale reservoir and the corresponding applications-A case study of the Bohai Bay Basin

In order to study the generation of organic acids and their dissolution effects on reservoirs, the type, yield and stability of common organic acids in the stratum were analyzed by laboratory simulations of the organic acid generation from kerogen and the decarboxylation of these organic acids. A kinetic model for the generation of organic acids and thermodynamic models of the decarboxylation of organic acids and the dissolution reaction were established. Acetic acid was used as an example to characterize the integration of the organic acid generation, decarboxylation, and dissolution under geological conditions. In addition, the process of dissolution pore development in shale reservoirs of various ages was reproduced. The results show that among the organic acids produced in the kerogen thermal simulation, the most stable is acetic acid with a decarboxylation temperature higher than 250 degrees C, followed by formic acid, and oxalic acid is the least stable. A large amount of organic acids can be produced in the middle thermal maturation stage (Ro=0.89%-1.25%) of organic matter (OM). The modeling method combining kinetics and thermodynamics can achieve the quantitative characterization of the process of dissolution pore development. According to the calculation results, if the organic acids dissolve the reservoir as a whole, the porosity increment produced by the dissolution of the reservoir's minerals is not significant. In other words, the dissolution of the reservoir by organic acids does not have an overall significance. However, if the organic acids only move along the predominant pathway (such as fractures) after their generation and dissolved the surrounding minerals of this channel, obvious dissolution can be produced. Comparison of the mineral composition and crack development of shale reservoirs indicates that organic acids could preferentially dissolve the reservoir cracks, so as to improve the porosity and permeability of the reservoir.