Characteristics of heat transfer and secondary hydrate formation during depressurization with symmetrical well patterns in a pilot-scale simulator

Natural gas hydrate (NGH) is a promising clean energy resource; however, efficient and sustainable exploitation remains a bottleneck for large-scale commercial production. To overcome limitations of single-well systems, multi-well pattern optimization has become a pivotal research focus. This study compared a peripheral four-spot vertical-well pattern with a central single vertical well in sandy sediments using a 117.8 L pilot-scale hydrate simulator. Hydrate dissociation, gas and water production, and heat- and mass-transfer behavior were evaluated under depressurization. The four-spot pattern exhibited a double-edged effect. During the depressurization stage, its larger gas–liquid contact area and more complex flow field intensified secondary hydrate formation; the peak secondary-hydrate proportion reached 22.7%, compared with 6.1% for the single well. The resulting blockage delayed pressure propagation, reduced early gas production, and lowered the gas-to-water ratio. During the constant-pressure stage, however, peripheral dissociation reduced the thermal resistance near the reactor boundary and enhanced inward heat transfer. Consequently, the four-spot pattern reached a final dissociation ratio of 99.9%, compared with 95.3% for the single well, and shortened the constant-pressure production period by 25.7%. These results show that increasing well density can improve late-stage recovery but may aggravate early flow-assurance risks. Multi-well design should therefore coordinate well spacing and depressurization strategy to suppress secondary hydrate formation while preserving heat-transfer benefits.