Laboratory Investigation Fracture Penetration in Multi-Interlayers with the Cyclic Hydraulic Fracturing

Abstract

ABSTRACT: In the tight reservoirs of Ordos Basin, shale interlayers are abundant. The mechanical properties of interlayers are different greatly from the adjacent sandstones, affecting the vertical propagation of hydraulic fractures, and resulting in a poor reservoir simulation with conventional hydraulic fracturing. The cyclic fracturing is an effective method to improve the efficiency of stimulation in such reservoirs. To verify the penetrating feasibility of hydraulic fractures with the cyclic fracturing and the conventional hydraulic fracturing, stimulation tests with true triaxial stresses are used to simulate the cyclic hydraulic fracturing in outcrop blocks of 300 × 300 × 600 mm. Hydraulic fractures form in the shale interlayer and penetrates both neighboring layers of sandstones. From visual observations and 3D scanning of the fractures, hydraulic fracture penetrations shows that: (1) Hydraulic fractures in the shale interlayer are complex, and a series of horizontal fractures along the bedding exist, which makes it difficult for fractures penetrate into the adjacent sandstones with conventional hydraulic fracturing; (2) Different from conventional hydraulic fracturing, hydraulic fractures can penetrate the neighboring sandstones from shale interlayer with the cyclic fracturing. 1. INTRODUCTION In recent years, the production of unconventional oil and gas has increased rapidly (EIA, 2021). The Ordos Basin has tremendous reserves of unconventional oil and gas resources. Shale oil in the Ordos basin are over 10 billion tons (Zhou et al., 2020). The production of shale oil in Chang 7 is of great significance for alleviating China’s overly dependence on importing oil and gas and guaranteeing national energy security (Fu et al., 2019). The shale oil in Chang 7 formation can be divided into three types (Yang et al., 2016) according to lithology, the thickness of sand body and other factors. Type I shale oil has been developed a lot (Fu et al., 2020), and the Qingcheng Shale oil field with an annual output of one million tons of crude oil has been built. However, for Types II and III shale oil, with many interlayers, the thickness of sand body is generally less than 5 m. The reservoir changes rapidly laterally and is strongly heterogeneous. In horizontal well drilling, it is easy to drill into interlayers, resulting in frequent encountering interlayers (Jiao, 2020). As a result, many sections are abandoned with conventional hydraulic fracturing.