A Synthetical Fracability Evaluation Method for Continental Shale Oil Reservoir

Abstract

ABSTRACT In China, jurassic continental shale oil resources are rich in Sichuan Basin. Jurassic continental shale oil resources has high clay mineral content, multiple limestone interbeds and variable longitudinal stress, which is barely considered in existing evaluation method. A synthetical fracability evaluation method was established considering six geological characteristic parameters including clay content, quartz content, interlayer thickness proportion, gas content, natural fracture, pressure coefficient, and five engineering characteristic parameters including Poisson's ratio, Young's modulus, brittleness index, horizontal principal stress difference coefficient, interlayer and layer average stress difference. The grey system theory method was selected to calcualte the weight of each parameter considering the correlation degree between multiple parameters and production capacity. Case study demonstrats that the synthetical fracability index has a good positive correlation with the test oil and gas equivalent in the target region, providing guidance for the evaluation of jurassic continental shale oil reservoir. JURASSIC CONTINENTAL SHALE OIL CHARACTERISTIC In China, jurassic continental shale oil resources are rich in Sichuan Basin, which is a high clay shale with inorganic pores and extremely low porosity and permeability. The longitudinal lithology of jurassic reservoir varies greatly, the layer heterogeneity is strong and the fluid property is complex. The crude oil is rich in light components and is a high pressure near-saturated condensate gas reservoir (Jia et al, 2012; Zou et al, 2015; Li et al, 2022; Cai et al, 2022). Compared with marine shales, jurassic continental shales have higher clay content ranging from 48% to 61% and strong plasticity, with mechanical brittleness index 0.383. The shale is also rich in limestone interlayers and dense laminated structure less than 3cm. Based on the mechanical study of the shale, shell limestone has the characteristics of high stress, i.e., 3.8∼4.5 MPa higher than shale, high compressive resistance from 191∼358 MPa, high elastic modulus 44.88∼55.99 GPa, leading to difficult initiation and extension. As the jurassic continental shale is generally characterized by multiple interbeds in layers and variable longitudinal stress (Fig. 1), the results of physical simulation experiments show that longitudinal penetration for fractures to propagate through limestone interlayers is more difficult (Fig. 2).