Characteristics of Shale Gas Reservoir in Jiyang Depression and its Significance in Drilling and Exploitation

Abstract

Physical and geochemical characteristics of shale play conclusive role in confirming operation measures during drilling and stimulation. The properties of shale samples from Jiyang depression were investigated through X-ray diffraction, scanning electron microscope, adsorption isothermal, high pressure mercury intrusion, methylene blue trihydrate, pressure pulse decay, tests of specific water wettability and shale stability index. Correlations, geological and engineering significances of them were discussed. Results show that shale reservoir in Jiyang depression has exploitation value corroborated by good characteristic parameters: 2.86% TOC, 69.9% brittle mineral, 26.14% clay mineral, high permeability of 0.011 × 10^-3μm², large Langmuir volume (5.82 cm³/g) and Langmuir specific area (0.91 m²/g), effective porosity (3.77%) and thickness (130.66m). Langmuir specific area is the key control on methane adsorption and storage verified by its moderate positive relativity with Langmuir volumes rather than TOC. High illite content (69.29%) may lead to instability of borehole and velocity sensitivity damage. Microfractures provide channels for filtration, invasion and loss of drilling fluid. Large specific water wettability (4.36 × 10^-7g/m²) and smaller shale stability index (19.99 mm) dispalyed that shale formation were unstable once contacting with fresh water. Countermeasures must be adopted during drilling and fracturing to reduce reservoir damage and complex downhole conditions.