Hydrate Dissociation Evaluation and Stratum Subsidence Response Induced by Depressurization in Hydrate-Bearing Permafrost

Abstract

Ensuring formation stability is vital to realizing the commercial development of hydrate reservoirs. Therefore, studying the formation subsidence in the hydrate mining process is essential. This study proposes a method for predicting the maximum surface subsidence using the hydrate dissociation front. Firstly, to more realistically evaluate the hydrate dissociation characteristics and reservoir deformation during the hydrate mining process in permafrost, soil permeability experiments under different hydrate saturations were conducted, and the dynamic permeability model was established. On this basis, combined with the multifield coupling model, the depressurization process of the hydrate reservoir is simulated. The hydrate dissociation property and stratum subsidence response under different parameters are analyzed. In addition, the parameter sensitivity is evaluated, and the corresponding relationship between the hydrate dissociation front and the maximum surface subsidence is established. According to the research results, assuming that the safety limit of surface subsidence is set to 0.3 m, the safe hydrate dissociation range within soil permeability K = [20 mD, 120 mD] is about [0 m, 77.7 m]. For every 20% decrease in wellhead pressure, the hydrate dissociation range increases by 30%. It is worth noting that the three points A, B, and C (located at the upper, middle, and lower positions next to the well perforation end) show completely different deformation trends during the mining process.