Optimization of Combustion Improver Delivery Parameters in Confined Space of Wellbore for in Situ Methane Deflagration Fracturing

In situ methane deflagration fracturing technology requires the delivery of a combustion improver into an underground confined space. To achieve the required methane concentration for deflagration, the idea of delivering a combustion improver in a confined wellbore space is put forward. A three-dimensional mixing flow field model of the confined space in a wellbore has been established using computational fluid dynamics (CFD) numerical simulation methods. After the experimental validation of the numerical model, an investigation was conducted on three delivery parameters: the delivery pressure, wellbore pressure, and wellbore length. Based on the postdelivery methane concentration in the wellbore, the wellbore space was categorized into three areas: the strong deflagration zone, the weak deflagration zone, and the nondeflagration zone. The results indicate a characteristic concentration profile that is lower in the middle and higher at both ends, following a pattern of “nondeflagration zone-weak deflagration zone-strong deflagration zone-strong deflagration zone-weak deflagration zone-nondeflagration zone”. Variations in delivery pressure primarily affect the methane concentration in the midsection and upper end of the wellbore; higher delivery pressure increased the proportion of the strong deflagration zone, while the proportion of nondeflagration zone stabilizes after a certain threshold. Increasing the wellbore pressure does not alter the distribution pattern of methane but raises the overall concentration. Furthermore, extending the wellbore length leads to zones where the methane concentration does not reach the deflagration limit. Under constant delivery pressure and wellbore pressure, the range of impact on the methane concentration within the wellbore remains limited. To extend the wellbore length while ensuring a sufficiently affected range of methane concentrations, this can be achieved by increasing the delivery pressure or decreasing the wellbore pressure. The findings of this study are expected to provide a theoretical basis for the delivery of a combustion improver into the confined space of the wellbore during in situ methane deflagration.