Influencing factors and fracability of lacustrine shale oil reservoirs

Abstract

Abstract Shale oil reservoirs need fracturing in order to achieve commercial productivity. A key factor influencing the fracturing process is shale fracability, which is strictly dependent on rock fabric. In this study, we focused on the lacustrine shale oil reservoirs located in Dongying Sag and in the Northern Jiangsu Basin. Through X-ray mineral diffraction, rock mechanics, and P- and S-wave velocity tests, we analyzed the mineral components and mechanics of different shale lithofacies, as well as the influence of these factors on shale fracability. Based on the results of these analyses, we proposed a new method for the evaluation of lake shale oil reservoir fracability. The experimental results showed that the macrostructure, mineral content, lithology, and the presence of microfractures in the shale are the main factors affecting its brittleness. Shales with lamellar and sheet-like structures have relatively high Young's modulous or lower Poisson's ratios; under these conditions, netted fractures are easily formed during fracturing. The fracture toughness of carbonate minerals in shales is strong. An appropriate amount of carbonate minerals seemed to increase the shale brittleness; however, an excessive amount increased its cohesiveness, hindering the initiation and propagation of fractures. Felsic minerals are characterized by high brittleness, a characteristic that favors shale fracability; on the other hand, clay minerals and organic matter are highly plastic, a characteristic that hinders fracturing transformations. Due to their macrostructure and composition, lamellar clayey limestones (dolomite), lamellar clayey siltstones, layered calcareous (dolomitic) siltstones, sheet-like clay-containing siltstones, and flaggy carbonate-containing siltstones are expected to be easily fractured. In addition, the presence of microfractures seemed to reduce the overall fracture toughness of the rock: they penetrated artificial fractures, enhancing rock fracability. Taking into account the fracture toughness of carbonate minerals and the plastic effect of organic matter, we proposed a new formula for the calculation of the brittleness index, based on the mineral component method. The brittleness indices obtained through the proposed formula were linearly correlated with those calculated through the rock mechanics parameters method. Overall, our results indicate that the silty lithological successions occurring in the study area should be the easiest to fracture, followed by the calcareous (dolomitic) and clayey or organic-rich lithological successions. This study is of guiding significance for evaluating the fracability of different lithofacies in lacustrine shale oil reservoirs. Especially, the new proposed brittleness index containing the plastic effect of organic matter can correctly evaluate the actual brittleness of organic-rich shale.