Burial depth interval of the shale brittle–ductile transition zone and its implications in shale gas exploration and production

Abstract

Brittleness and ductility of shale are closely related to shale gas exploration and production. How to predict brittleness and ductility of shale is one of the key issues in the study of shale gas preservation and hydraulic fracturing treatments. The magnitude of shale brittleness was often determined by brittle mineral content (for example, quartz and feldspars) in shale gas exploration. However, the shale brittleness is also controlled by burial depth. Shale brittle/ductile properties such as brittle, semibrittle and ductile can mutually transform with burial depth variation. We established a work flow of determining the burial depth interval of brittle-ductile transition zone for a given shale. Two boundaries were employed to divide the burial depth interval of shale brittle/ductile properties. One is the bottom boundary of the brittle zone (BZ), and the other is the top boundary of the ductile zone (DZ). The brittle-ductile transition zone (BDTZ) is between them. The bottom boundary of BZ was determined by the overconsolidation ratio (OCR) threshold value combined with pre-consolidation stress which the shale experienced over geological time. The top boundary of DZ was determined based on the critical confining pressure of brittle-ductile transition. The OCR threshold value and the critical confining pressure were obtained from uniaxial strain and triaxial compression tests. The BZ, DZ and BDTZ of the Lower Silurian Longmaxi shale in some representative shale gas exploration wells in eastern Sichuan and western Hubei areas were determined according to the above work flow. The results show that the BZ varies with the maximum burial depth and the DZ varies with the density of the overlying rocks except for the critical confining pressure. Moreover, the BDTZ determined by the above work flow is probably the best burial depth interval for marine shale gas exploration and production in Southern China. Shale located in the BDTZ is semi-brittle and is not prone to be severely naturally fractured but likely to respond well to hydraulic fracturing. The depth interval of BDTZ determined by our work flow could be a valuable parameter of shale gas estimation in geology and engineering.

Differential Characteristics of the Upper Ordovician‐Lower Silurian Wufeng‐Longmaxi Shale Reservoir and its Implications for Exploration and Development of Shale Gas in/around the Sichuan Basin

“…Therefore, the shale in the brittle-ductile transitional zone with moderate burial depth has both suitable preservation conditions and brittleness values, which is conducive to the efficient development of shale gas. The bottom boundary depth of the brittle zone of the Wufeng-Longmaxi shale in the eastern Sichuan Basin is generally between 2195~2763 m, and the top boundary depth of the ductile zone is approximately 4470±230 m (Yuan et al, 2017). The depth interval of the brittle-ductile transitional zone is generally consistent with the favorable depth interval of physical properties and overpressure (Fig.…”

“…At the same time, higher in-situ stress and formation temperatures (commonly greater than 110°C) result in higher requirements for wellbore stability and fracturing stimulation. According to Yuan et al (2017), the depth intervals of shale mechanical properties are vertically divided into the brittle zone, the brittle-ductile transitional zone and the ductile zone with increasing burial depth. Although shale in the brittle zone with shallow burial depth is characterized by suitable brittleness, extensional and open fractures may occur under tectonic stress during uplift and erosion; thus, the overpressure and preservation conditions may have been destroyed.…”

Determination of the red bed soft rock’s apparent preconsolidation pressure

“…Nygard et al (2004 and2006) carried out confined compression tests which were used to obtain soil preconsolidation pressure on KWC Shale and KBC Shale, and got a characteristic stress named apparent preconsolidation pressure. Subsequently, Gutierrez et al (2008), Liu et al (2015), Yuan et al (2017) carried out the same tests on different kinds of soft rock to obtain the apparent preconsolidation pressure, and these test results all indicate that the characteristic pressure equals to the soft rock brittle-ductile transition pressure. So it is essential to take full consideration of the apparent preconsolidation pressure in practical engineering design for getting more rational and practical results of such soft rock.…”

The growth of faults and fracture networks in a mechanically evolving, mechanically stratified rock mass: a case study from Spireslack Surface Coal Mine, Scotland

“…Finally, several of the N-S-trending joints become reactivated (maroon) and are interpreted as faulted joints (cf. Zhao and Johnson, 1992). Calcite mineralisation at Spireslack SCM (Fig.…”

“…Both fault sets abut against favourably orientated Phase 1 or Phase 3 joints, indicating they formed later. Abutting relationships of Phase 3 joints against NNW-trending faults suggesting Phase 2 joints were reactivated as faulted joints (following Zhao and Johnson, 1992) during the first phase of faulting. Phase 5 and 6 joints, which display variable orientations in Fig.…”