Large-scale experimental study of heterogeneity in different types of hydrate reservoirs by horizontal well depressurization method

The heterogeneity of hydrate distribution and decomposition in sandy reservoirs is an important issue. Because large hydrate simulators are scarce and visualization difficult, heterogeneity in large-scale hydrate reservoirs has been consistently underestimated. In this study, a pilot-scale hydrate simulator with an effective volume of 117.8 L was used to simulate the horizontal well depressurization decomposition of two different types of hydrate reservoirs: gas-saturated and water-saturated. The production performance and heterogeneity of these two hydrate reservoirs were analyzed during the hydrate decomposition process. For the first time, hydrate decomposition heterogeneity was classified into planar heterogeneity and interlayer heterogeneity based on the heterogeneous characteristics in the horizontal and vertical directions. Based on the pressure difference and duration, a dimensionless factor was proposed to quantify the intensity of the heterogeneity at a specific location. Furthermore, this study related the hydrate decomposition heterogeneity intensity variation process to the variations in the two-dimensional geometric model for hydrate distribution in the pore space. The experimental results showed that horizontal well extraction using the depressurization method was highly efficient. During the hydrate decomposition process, planar heterogeneity was stronger in the water-saturated hydrate reservoir than in the gas-saturated hydrate reservoir. In gas-saturated hydrate reservoir, hydrate saturation and heterogeneity increased from top to bottom. In water-saturated hydrate reservoir, hydrate saturation and interlayer heterogeneity increased first and then decreased from top to bottom. The study of heterogeneity was critical for optimizing the hydrate extraction scheme, and emphasis should be placed on the placement of production enhancement facilities in regions with significant heterogeneity.