3D Integrated Fracture-Hit Mitigation and Fracture Complexity Enhancement Technology for Deep Shale: A Successful Story in the Southern Sichuan Basin

Abstract

With continuous development of mid-shallow shale gas reservoirs, the focus gradually shifts to deep shale reservoirs (depth>3500 m) that contribute to over 65% of total shale gas resources in the Southern Sichuan Basin. In this area, large-scale and high-intensity fracturing (pumping rate: >20 m3/min; sand loading intensity: 2-3.1 t/m) is the commonly used stimulation approach to achieve larger stimulated reservoir volume (SRV). However, the complex tectonic evolution generates well-developed and large-scale natural fractures/faults which can penetrate several horizontal wells and offer high risk of fracture hits, impairing well productivity. Besides, hydraulic fractures are frequently arrested by these natural fractures/faults, leading to overstimulation along them and reducing fracture system’s complexity. In this study, 3D integrated fracture-hit mitigation and fracture complexity enhancement technology is proposed to minimize the impact of fracture hits, avoid overstimulation along natural fractures/faults, and initiate fracture branches to increase fracture complexity. We first identify the stages with high risk based on geological interpretation data. Then, different sizes of proppants, degradable fibrous materials, temporary plugging agents, and the activators (specially-designed particles) are injected together to form a low-permeable interwoven structure with the help of viscosity-enhanced fracturing fluid. The plugging process involves four stages: (1) bridging, (2) bonding, (3) agglomeration, and (4) formation of the solid plugging pack. Real-time adjustment of fracturing fluid properties is conducted to satisfy the requirements the of plugging materials. Proppants, fibrous materials, temporary plugging agents, activators, and high-viscosity liquid completely plug the tips of multi-scale fractures, realizing 3D plugging of the whole SRV. Fracture branches are then created with the increase of net pressure. Indoor plugging capacity tests show that the plugging pressure difference between the two sides of the temporary plugging pack varies from 2.5 MPa to over 33 MPa, depending on injected materials’ fraction combination. Finally, by using a high-efficiency gel breaker, only proppants exist after degradation of the interwoven structure and the gelatinous plugging pack, leading to high conductivity at the fracture tips. This technology has been successfully applied to deep shale gas formations in the Southern Sichuan Basin. During fracturing operations, the pressure increments of adjacent wells are all less than 5.64 MPa. Compared with previous observations, some of the pressure increments are reduced by over 50% under similar stimulation intensity. Microseismic monitoring results indicate that the event locations of the treated stage are well-confined within a certain stimulated area, mitigating inter-well communications. The event density within the SRV is larger compared with that of adjacent stages. Production logging results show that the stages using this technology even provide higher contribution to well production compared with adjacent stimulated stages. This technology can be further improved and rolled out to other types of reservoirs for fracture hit mitigation.