Physical simulation experiments on non-uniform extension of multi-cluster hydraulic fractures under complex fracturing conditions
TAN Peng1，2，3，XING Yuekun2，HAN Taisen2，CHEN Jinlong2，XU Hang2，CHEN Zhaowei1，3
(1. CNPC Engineering Technology R&D Company Limited，Beijing 102206，China；2. State Key Laboratory for Fine Exploration and intelligent Development of Coal Resources at China University of Mining and Technology，Xuzhou，Jiangsu 221116，China；
3. National Engineering Research Center for Oil & Gas Drilling and Completion Technology，Beijing 102206，China)
Abstract：Achieving uniform propagation of multi-cluster hydraulic fractures within shale gas reservoir containing natural fracture zones remains challenging. This study developed a physical simulation methodology to model multi-cluster fracturing in specimens containing prefabricated natural fracture zones. The experimental setup for sample preparation and pumping allows for the distribution of liquid into multiple cluster fractures according to a predetermined sequence of extension，with independent monitoring of flow rates for each cluster. Leveraging the engineering-experimental similarity in fracture propagation characteristics，in-situ stress，and geometry，analogous relationships were derived between experimental and engineering parameters. We experimentally investigated the effects of pumping rates(0.825 m3/min and 3.3 m3/min per cluster)，cluster spacing(6 m and 12 m)，and spatial arrangement positions(middle and side clusters) of natural fracture zones on the propagation of multi-cluster hydraulic fractures in field applications. We find that (1) low pumping rates combined with large cluster spacing enhance the uniform propagation of multi-cluster hydraulic fractures in both samples with and without natural fracture zones. Fractures adjacent to natural fracture zones tend to propagate preferentially，thereby inhibiting uniform multi-fracture propagation. (2) High pumping rates and small cluster spacing facilitate the formation of single-cluster branch fractures and promote inter-cluster fracture connectivity. Conversely，low pumping rate and large cluster spacing result in simpler，non-interfering multi-cluster fracture morphologies. When the initiation cluster is adjacent to a natural fracture zone，the morphology of hydraulic fractures exhibits connectivity between hydraulic fractures and natural fracture zones. (3) During multi-fracture propagation，each cluster of fractures sequentially receives fluid inflow，indicating that multiple hydraulic fractures extend in an alternating sequence rather than simultaneously. Clusters situated within fracture zones tend to receive fluid inflow preferentially and connect with natural fracture zones. Consequently，the pump pressure curve exhibits fluctuations prior to reaching the break pressure. (4) Based on the theories of fracture instability extension and stress shadow，this study discusses the mechanisms of single-cluster fracture instability propagation and multi-fracture complexity enhancement. A high pumping rate increases the driving force for fracture propagation，while natural fracture zones decrease fracture resistance，thereby promoting single-cluster fracture instability. Namely，these factors inhibit the uniform extension of multi-cluster hydraulic fractures. Furthermore，a combination of high pumping rate and small cluster spacing intensifies interference between fractures，leading to the generation of complex fractures. Finally，optimization suggestions for the fracturing design were proposed，focusing on achieving uniform propagation of multiple hydraulic fractures near the wellbore while promoting the formation of complex hydraulic fractures in more distant regions.
