Key Takeaways & Executive Findings
- •• A novel fracability evaluation model integrating improved brittleness index, rock strength, geostresses, and natural weakness characteristics was developed using a combined weighting method. • Rock brittleness and fracability are not equivalent for deep reservoirs; the proposed fracability index provides a more comprehensive assessment. • The fracability index shows a high correlation (84%) with pay zones and actual gas production, validating its practical significance. • The method offers theoretical guidance for selecting fracturing candidates and optimizing fracturing design in deep shale gas development.
Abstract
Fracability evaluation is critical for efficiently extracting deep shale gas using hydraulic fracturing to avoid blind drilling and fracking. However, existing fracability indices often fail to systematically consider the mechanical behavior of rocks at high temperatures and high pressures (HTHP), coupled with geostress distributions and heterogeneous reservoir characteristics. This critical omission limits their effectiveness in accurately identifying the optimal fracability sweet spots within deep reservoirs. In this work, a fracability evaluation model was proposed based on the combined weighting method, integrating the improved brittleness index, rock strength, geostresses and natural weakness characteristics. A fracability grading evaluation was carried out to determine the potential fracture characteristics corresponding to shales with different fracability levels. Additionally, the fracability index was used for field validation and applications. Results show that rock brittleness and fracability are not equivalent for deep reservoirs. The fracability index is closely related to the pay zones and actual gas production, with a correlation as high as 84%, implying that the proposed method has practical significance in both experimental and field applications. The above findings can provide theoretical guidance for the selection of fracturing candidates and the optimal design of fracturing in deep resource development.
1. Introduction
The development of shale gas as a clean and efficient energy resource, especially deep shale gas with huge reserves, is highly expected under the dual-carbon target. The focus of shale gas exploration is rapidly shifting to deep and ultra-deep formations. The deep shale gas in the Ordovician-Silurian strata of the Sichuan Basin, China, is a crucial area for future large-scale production increase. Segmented hydraulic fracturing of horizontal wells is currently an effective means of unconventional reservoir modification.
From the perspective of geological stratigraphy, the main gas-producing layers in the deep shale reservoirs of the Wufeng-Longmaxi formation can be divided into five sub-layers, i.e., L1-4, L1-3, L1-2, L1-1, and Wufeng. An engineering sweet spot assessment of multi-producing reservoirs is required to facilitate fracturing candidate preference (vertical well sections) and shot hole location optimization in horizontal well sections for efficient development of deep resources. Unfortunately, engineering practice has shown that many deep shale gas wells in the Sichuan Basin have failed to come on production due to improper deployment of wells and fracturing candidates. Therefore, fracability prediction of shale reservoirs in the Wufeng-Longmaxi formation is needed to screen engineering sweet spots for the commercial development of deep shale gas to avoid indiscriminate drilling and fracturing.
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ZHAO Guokai, GUO Yintong, YANG Chunhe, WU Mingyang, GUI Junchuan, TENG Shilong, WANG Lei, ZHANG Xinao (2025). A quantitative fracability evaluation method and its application to deep shale gas development in Sichuan Basin, China. SinoTechIntel Verified Research. https://doi.org/10.1016/j.ijmst.2025.08.007
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Frequently Asked Questions
What is fracability and why is it important for deep shale gas development?
Fracability refers to the ease with which a reservoir can be effectively stimulated by hydraulic fracturing to form a complex fracture network and achieve a large stimulated reservoir volume. It is crucial for selecting optimal fracturing candidates and well placement, especially in deep shale gas reservoirs where improper deployment can lead to well failures.
How does the proposed fracability evaluation method differ from existing indices?
The proposed method integrates an improved brittleness index, rock strength, geostresses, and natural weakness characteristics using a combined weighting approach. Unlike existing indices, it systematically considers the mechanical behavior of rocks under high temperature and high pressure (HTHP) conditions, along with heterogeneous reservoir characteristics, providing a more accurate identification of fracability sweet spots.
What were the key findings of the study?
The study found that rock brittleness and fracability are not equivalent for deep reservoirs. The proposed fracability index showed a high correlation (84%) with pay zones and actual gas production, demonstrating its practical significance in both experimental and field applications.
How was the fracability evaluation model validated?
The model was validated through field applications in the Sichuan Basin, where the fracability index was used to assess deep shale gas reservoirs. The high correlation with actual gas production confirmed the model's effectiveness in identifying optimal fracturing candidates.
What are the practical implications of this research?
The research provides theoretical guidance for the selection of fracturing candidates and the optimal design of fracturing operations in deep resource development. It helps avoid blind drilling and fracking, thereby improving the efficiency and commercial viability of deep shale gas extraction.
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