Study on Rock Mechanics Microscopic Heterogeneity Characterization, Fracture Propagation Simulation and Frac-Ability Prediction of Complex Mode Conglomerate in Mahu Block
Authors: Liu S. K. , Zou Y. S. , Zhang S. C. , Ma X. F. , Liu L. , Zhang X. H. , Zhao Z. W. , Zhang Z. P. 
Published: August 2024

The conglomerate reservoir in Mahu Sag is highly heterogeneous, with an unclear understanding of fracture propagation. The reservoir contains a mix of sand, gravel, and mud. Previous studies have not fully grasped the fracture propagation patterns in conglomerate reservoirs due to a focus on outcrop observations and finite element simulations. There is an urgent need for research on the morphology of fracture propagation in complex conglomerate formations and the characterization of micro-heterogeneity in rock mechanics. By conducting mechanical tests and discrete element analysis, researchers are exploring how the size of gravel particles impacts rock mechanics test results. Through fracture propagation simulation studies, researchers have identified key factors influencing fracture behavior in conglomerate formations and have developed a model to predict the complexity and Frac-ability of fracture. The research findings suggest that the significant variation in gravel size contributes to the severe heterogeneity of conglomerate formations. Larger gravel particles in poorly consolidated rock samples exhibit plastic properties, reducing their brittleness. The unique development index of gravel influences fracture complexity, with medium-sized gravel leading to the most complex fracture morphology.

1. INTRODUCTION

Because gravel and matrix have distinct mechanical characteristics, the presence of gravel will cause the rock's local heterogeneity to be particularly apparent.[1] The propagation behavior of the hydraulic fracture tip on gravel will be impacted by this local heterogeneity. Consequently, in order to properly comprehend the rock mechanical characteristics of conglomerate reservoirs, it is imperative to investigate the multi-scale propagation law of hydraulic fractures. To address the issue of complex fracture propagation in conglomerate reservoirs, numerous researches have been conducted.

Guo Xianmin et al. [2] carried out a true triaxial hydraulic fracturing physical simulation experiment with full-diameter downhole cores of gravelly conglomerate, gravelly conglomerate and conglomerate lithofacies for conglomerate rock mass in Sha4 member of Shengli Oilfield, and found that the rupture pressure of gravel conglomerate with different lithofacies was dominated by rock sample cementation strength and pore structure. The fracture growth morphology is affected by gravel content, gravel size and natural fracture development degree. Ma et al. [3] used natural outcrops of conglomerate to carry out fracturing experiments, and used fracture width observation equipment to observe local fracture morphology, focusing on the effects of gravel particle size, horizontal stress difference, fracturing fluid viscosity and displacement on hydraulic fracture expansion mode and fracture width distribution. On this basis, Liu et al. [4] combined CT scanning and acoustic emission monitoring technology to further analyze the focal mechanism of conglomerate hydraulic fracturing. Nie et al. [5] studied the effect of alternating loads on the expansion path of conglomerate hydraulic fractures through variable displacement. The above preliminary exploration on the law of hydraulic fracture growth in conglomerate shows that the hydraulic fracture growth morphology of conglomerate is not only affected by factors such as ground stress, fracturing fluid viscosity and displacement, but also dominated by gravel properties (gravel content, gravel particle size, gravel strength and gravel boundary cementation strength), that is, gravel is the main source of characteristic structure and heterogeneity of conglomerate.