The Longmaxi shale gas play in Sichuan, China is unique due to multiple tectonic deformations in the geological history, which makes the shale play highly heterogenous and hydraulic fracturing execution problematic. The hydraulic fracturing design has matured after ten years’ evolution. However, it could not ensure every well's success without considering heterogeneity. Screenout, fracture hits, and casing deformation are still very common because informed decisions cannot be made ahead of operation or in real time. These risks should be recognized and in turn mitigation measures could be proposed through deep understanding of the geological and geomechanical characteristics.
To address these challenges, a multidisciplinary team was formed to work on a pad. The geological and geomechanical (G&G) context was first determined by 3D G&G modeling. Then a survey was completed on the whole block to determine the root causes of screenout, frac hits, and casing deformation, and high-risk areas or stages were identified. Based on this, the proposed mitigation included adjusting perforation design, pump rate, fluid volume, and fracture sequence. The effectiveness of mitigation measures and the 3D G&G model were validated and iterated by onsite fracturing diagnostics.
Through the above efforts, the designed proppant volume achieved zero casing deformations, fewer screenouts, and weaker fracture hits. This was the first pad with zero casing deformations across the whole block. The key findings were proved to be effective and could ensure success of hydraulic fracturing in the study pad of ultradeep Longmaxi Shale:
The natural fracture corridors were the main cause of screenout. The possible bedding plane open and bad fracture initiation could be other causes of screenout.
The natural fracture was the main cause of fracture hit and the connection among hydraulic fractures was the second cause. Adjusting the zipper fracturing sequence to interfere with hydraulic fracture propagation could help weaken it.
Unstable natural fracture was the main causes of casing deformation in the Luzhou Block. Adjusting the fracturing sequence and fluid pumping intensity could mitigate casing deformation.
The key to effective mitigation was continuous 3D G&G modeling through ongoing iterations with field data.
Finally, a model-driven workflow was developed for hydraulic fracturing design and execution, which could be a resource for other shale plays with similar challenges worldwide.