Effect of Brittle Mineral Size on Hydraulic Fracture Propagation in Shale Gas Reservoir

Wei Gao,Ruilin Hu,Dan Xu,Haoyue Sui,Javed Iqbal

doi:10.1155/2019/9147048

Abstract

The properties of brittle minerals have great effect on the morphology of postfracturing network in shale reservoirs in the southeastern Ordos Basin, China. In order to study the effect of brittle mineral size distributions on the fracture parameters, the concrete cubes of 300 mm × 300 mm × 300 mm in size with four distinct brittle mineral sizes of 2.36 mm, 0.425 mm, 0.15 mm, and 0.075 mm were investigated under large-sized triaxial hydraulic fracturing test. The effect mechanism of aggregate on the fracture properties of shale was studied using ultrasonic technique, photosensitive electron microscope, and numerical simulation. The test results obtained for each specimen (both disturbed and undisturbed conditions) indicate that brittle mineral size has significant effect on the fracture extension. The tensile strength, fracture toughness, and fracture pressure were found to decrease with a decrease in maximum brittle mineral size when the maximum brittle mineral size is smaller than 0.425 mm. In addition to this, the degree of attenuation difference also follows the similar trend. Observed fracture morphology reveals that with an increase in maximum size of brittle mineral specimen, the tortuous and complicated cracking path generation increases. These findings would be very helpful in order to better understand the behavior of shale under hydraulic fracturing test.

Highlights

Shale is a fine-grained sedimentary rock composed primarily of clay, organics, and brittle minerals such as quartz, feldspar, and pyrite
Resulted from pores, preexisting cracks and brittle mineral that exist in the shale may impact the mechanical properties and the fracture pattern of the formation which can potentially affect the performance of hydraulic fracturing operations and the production rate [8,9,10]
It can be seen that both tensile strength and fracture toughness increase with increase in maximum brittle mineral size when the maximum brittle mineral size is smaller than 0.425 mm

Summary

Introduction

Shale is a fine-grained sedimentary rock composed primarily of clay, organics, and brittle minerals such as quartz, feldspar, and pyrite. It is the most common sedimentary rock and is of particular interest with respect to shale gas production in China [1,2,3,4,5,6,7]. Recent studies showed that strong heterogeneity of the lithology exists within the Upper Triassic continental shales in the southeastern Ordos Basin in China, especially the different sizes of detrital grains which combine mud matrix or organic matter [26]. Ultrasonic technique, photosensitive electron microscope, and numerical simulation were used to detect the fracture process in a real-time setting

Experimental Details

Results and Discussion

Numerical Simulation for Hydrofracturing Crack Initiation and Propagation

Conclusions