Experimental research on hydraulic fracture evolution characteristics of multi-well pad fracturing in a tight conglomerate reservoir

Abstract

Natural gas is considered a cleaner and more efficient energy pillar than oil and coal, so it has become one of the focuses of global energy exploitation. The tight conglomerate gas reservoir has a broad development prospect. However, the previous fracturing experiments focus on single horizontal well fracturing research, and it is difficult to realize the economies of scale development. Therefore, the true triaxial fracturing experiment of three horizontal wells was carried out with the large-size specimen (400 mm × 400 mm × 400 mm) to study the hydraulic fracture (HF) propagation mechanism and the effects of geostress, fracturing sequence, vertical well spacing and fracture initiation position on the HF evolution characteristics during the multi-well pad fracturing in the conglomerate reservoir. The results show that under the stress shadow, the later fracturing HF tends to avoid the stress shadow zone of the preceding HF, and the breakdown pressure gradually increases. A higher horizontal stress difference helps the HF tend to cross the gravel and increases the proportion of tensile events to 73.2 %, but intermediate HF height propagation is inhibited, and the upward propagation distance has a decline of about 31.8 %. The preferential fracturing of the intermediate fracturing well can enable the intermediate HF to extend fully, and the upward propagation distance has an increment of approximately 140.3 %, which improves the reservoir reconstruction effect between the bilateral HFs. Meanwhile, it can intensify the induced stress interference between the HFs and increase the proportion of shear failure to 43.6 %. A shorter vertical well spacing seriously inhibits the upward propagation of intermediate HF height by around 57.9 % and increases the proportion of shear failure to 38.5 % under the induced stress interference. The preferential fracturing in the intermediate fracturing well can enhance the gas productivity, but it is necessary to control the pumping rate or pumping schedule to reduce the frac hit risk. This study can provide theoretical guidance and suggestions for multi-well pad fracturing in field fracturing.