Experimental and Numerical Investigation of Characteristics of Highly Heterogeneous Rock Mechanical Responses in Tight Sandy Conglomerate Reservoir Rock Under Tri-axial Compression

Bin Chen,Jiaying Lin,Xueliang Wang,Xuyang Guo,Jingqi Lin,Jiaqi Ji,Huayong Chen,Wentao Yang

doi:10.3389/feart.2021.735208

Bin Chen, Jiaying Lin + Show 6 more

Open Access

https://doi.org/10.3389/feart.2021.735208

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Abstract

Due to the use of horizontal wells and hydraulic fracturing, commercial tight oil production from some tight sandy conglomerate reservoirs has been achieved. Since the widely distributed gravels in the sandy matrix in conglomerate reservoir rocks are harder than the matrix, the rock mechanical response in conglomerates under compression is highly heterogeneous. This increases the complexity of understanding the hydraulic fracturing behaviors in conglomerate reservoirs. Previous tri-axial compression tests provided the stress-strain relationships of conglomerate samples as a whole, and the stress and strain in the gravels and in the sandy matrix were not investigated due to the limitation of the compression test lab. This study presents tri-axial test results for a conglomerate sample cored from a reservoir that has been economically developed. Lab results are then used to calibrate the numerical model for the simulation of the tri-axial compression process. Numerical results indicate that the elastic modulus and size of gravels have significant impacts on the axial stresses and axial strains in the conglomerate. Stress concentrations are observed in gravels due to the heterogeneous mechanical properties in the conglomerate. The reorientation of the maximum horizontal principal stress is quantified to study the mechanisms of the interaction types between hydraulic fractures and gravels embedded in the tight sandy matrix.

Highlights

Unconventional oil and gas resources in low permeability reservoirs have attracted tremendous attention in the upstream in the petroleum industry (Xie et al, 2020; Zhao et al, 2020)
Since the hydraulic fracturing quality is closely related to the complex mechanical behaviors in conglomerates, it is important to understand how the conglomerates deform under the compression induced by reservoir stimulation processes
The exertion of confining pressure and axial stress can establish the in-situ stress conditions in the subsurface. They help to obtain essential rock mechanical parameters and constitutive relationships using rock samples, which are especially critical for the optimization of hydraulic fracturing parameters

Summary

Introduction

Unconventional oil and gas resources in low permeability reservoirs have attracted tremendous attention in the upstream in the petroleum industry (Xie et al, 2020; Zhao et al, 2020). Based on a modified tri-axial compression test with changing confining pressure and axial stress, it is found out that stress paths of loading and unloading directly govern the brittleness, elastoplasticity, and tensile and shear failure mechanisms in tight rocks (Guo et al, 2019). Based on the literature review, it is noted that tri-axial tests have been widely studied, while the mechanical responses within the highly heterogeneous sandy conglomerates require further investigation.

Results

Conclusion