Overpressure retardation of hydrocarbon generation: a new kinetic model considering the effects of pressure and its application

The traditional kinetic model reflects the impact of fixed pressure on hydrocarbon generation in thermal simulation experiments, but the pressure in experiments differs from the overpressure in formation, affecting the evaluation of hydrocarbon generation. In this study, a new parallel first-order reaction kinetic model for hydrocarbon generation in relation to formation overpressure retardation is proposed and its application is illustrated.

According to the impact of pressure on activation energy (E) and pre-exponential factor (A), the pressure factor is introduced into the Arrhenius formula:

k=A·exp(p/a)·exp⁡(-(n⋅p+E)/RT)

Where k is the reaction rate at 1 bar, R is the universal gas constant, T is the absolute temperature (K), p is pressure (MPa), a and n are the impact factors of pressure on A and E respectively.

By calculating n and a, the new model can simulate hydrocarbon generation under any temperature and pressure and is no longer limited by experimental conditions.

Sample from Baiyun Depression in the Pearl River Mouth Basin were selected to carry out a gold-tube thermal simulation, and the kinetic parameters of natural gas generation were calculated by using the new model. Since the traditional model reflects the impact of experimental pressure, the new model calculates the kinetic parameters without the impact of pressure, therefore, the average activation energy (Ea) calculated by the traditional model is greater than that of the new model, and the impact of pressure is reflected by impact factors (n and a)(Fig. 1).

Figure 1 Comparison of kinetic parameters of natural gas generation calculated by traditional model and new model

The new model reflects the inhibition effect of overpressure on natural gas generation (Fig. 2). According to the information on fluid inclusions, The kerogen began to generate mass gas at 23 Ma. The calculation results of the traditional model show that kerogen starts to enter the large-scale gas generation stage at 32 Ma, which is inconsistent with the time of overpressure formation. When the formation pressure coefficient is 1.8, the mass hydrocarbon generation time calculated by the new model is about 24 Ma, which is more consistent with the geological reality. The new model proves that natural gas generation is retarded under the impact of overpressure.

Figure 2 Comparison of the conversion rate of natural gas of the traditional model and the model in relation to overpressure under geological conditions.The black line reflecting the influence of pressure under experimental conditions; the blue line is the history of natural gas generation without the impact of overpressure; the orange line is the history of hydrocarbon generation under formation overpressure.