Evaluation of fracturability of shale reservoirs in the longmaxi formation in southern sichuan basin

Abstract

Major breakthroughs have been made in the exploration and development of shale gas in the Longmaxi Formation in the southern Sichuan Basin, China. The previous have adopted various methods to study the fracturability of shale, but the quantitative characterization of the comprehensive properties of shale during hydraulic fracturing is still difficult. For this reason, in this study, taking the Longmaxi Formation shale in the Changning and Luzhou Blocks in the southern Sichuan Basin as an example, the mineral composition, petrophysical properties, rock mechanical properties and in-situ stress of the shale were systematically studied using X-ray diffraction, pulsed porosity-permeability analysis, rock mechanics and in-situ stress tests. Furthermore, the brittle mineral content, elastic modulus, and in -situ stress parameters were calculated, and the Analytic Hierarchy Process (AHP) method was adopted to establish a comprehensive evaluation index of shale fracturability. The research results show that the shale of the Longmaxi Formation in southern Sichuan is dominated by mixed shale facies, and it has the characteristics of high content of brittle minerals, low porosity, large compressive strength and Young’s modulus, and small value of in-situ stress difference coefficient. Therefore, the Longmaxi Formation shale has good fracturing conditions. TOC and quartz contents have important effects on the fracturability of the Longmaxi shale. The analytic hierarchy process was adopted to determine the weight coefficients of the modulus brittleness index, mechanical brittleness index, in-situ stress difference coefficient, rock compressive strength and TOC content. Furthermore, a comprehensive evaluation index of fracturability was constructed. Calculations show that the comprehensive fracability indices of shale in the Changning and Luzhou Blocks are 0.49 and 0.59, respectively. Moreover, the quality of shale in the Luzhou Block is better than that in the Changning Block, which is related to the organic matter and quartz content and the microscopic pore structures inside the shale.

The influence of basal detachment strength on formation of the southwestern Sichuan fold-thrust belt: insights from discrete-element numerical simulations

“…During Late Triassic to Early Jurassic, a thrust movement occurred in southwest Sichuan due to the collision of the South China Plate and the North China Plate. In this stage, the SWSB did not experience thrust folding, but rather developed a wedge-shaped sedimentary structure underlain by Middle Triassic evaporite rocks, with forebulge located between the Longquan mountains and Weiyuan anticline (Guo et al, 1996;Jia et al, 2006;Hubbard et al, 2010;Wei et al, 2019;Chen et al, 2020a;Wang et al, 2020;Wang et al, 2022). From the Mid-Jurassic to the Cretaceous, the southwestern Sichuan basin entered a phase of intracontinental subsidence, during which the wedge-shaped structure here was strengthened, while the forebulge migrated further to the east.…”

Characteristics and Key Controlling Factors of the Interbedded-Type Shale-Oil Sweet Spots of Qingshankou Formation in Changling Depression

“…The laboratory rock mechanics experiment shows that silty-laminated shale has a higher Young's modulus (22.82 Gpa) and a lesser Poisson's ratio (0.12), and this is followed by felsic shale. Clayey shale has the lowest Young's modulus (4.84 GPa) and the highest Poisson's ratio (average 0.2), corresponding to the worst fracturability [42]. Therefore, it is clear that both silty-laminated shale and argillaceous-laminated felsic shale have better physical properties, mobility, and fracturability, and they are the most favorable shale lithofacies of the Qingshankou I Member in the Changling Depression.…”

Characteristics and Evolution of Tectonic Fractures in the Jurassic Lianggaoshan Formation Shale in the Northeast Sichuan Basin

“…Among these deposits, the Jurassic system is widely distributed, and the strata are well developed. The system is a set of typical terrigenous clastic rock deposits dominated by delta-lake phases, with thicknesses of 1500 m-4000 m [37]. In the Northeast Sichuan Basin, the Jurassic system is in parallel unconformity with the underlying In the Middle-Late Triassic period, with the closure of the Mianlue Ocean and the uplift of the Qinling orogenic belt, the marine sedimentary history ended in the main body of the Sichuan Basin, and the basin began to receive continental clastic deposition in the Late Triassic [36].…”

“…Among these deposits, the Jurassic system is widely distributed, and the strata are well developed. The system is a set of typical terrigenous clastic rock deposits dominated by delta-lake phases, with thicknesses of 1500 m-4000 m [37]. In the Northeast Sichuan Basin, the Jurassic system is in parallel unconformity with the underlying Triassic strata, gradually transitions to an angular unconformity towards the basin margin and is in unconformable contact with the overlying Lower Cretaceous strata; the Cretaceous and above strata in the region are severely eroded and less preserved.…”