The thick gravel layer is widely distributed in the Tarim oilfield in western China. Gravel content and cementation in strata vary greatly. The gravel layer's rock mechanics characteristics are difficult to determine. The damage of bit and the sticking due to the broken gravel often occurs, greatly impacting the rock breaking and hole stabilization. Therefore, it is essential to study the influence of gravel block content on rock mechanical properties. In this paper, through drilling coring and indoor processing to obtain different block content of underground gravel core, through X-ray diffraction and core scanning device, determine the cementation degree and core block content. The acoustic and mechanical parameters of conglomerate under different gravel content are obtained by the rock mechanics testing machine. A log interpretation model for calculating gravel content was established based on the experimental results, which can better evaluate the gravel layer's mechanical characteristics.

1. Introduction

The geological conditions of the BZ Block in the Tarim oilfield in western China are extremely complicated. Due to the gravel layer's long deposition time, the thickest gravel layer's thickness can reach more than 5000 meters, which leads to extremely poor drillability and low mechanical drilling speed of the gravel-bearing section in the region. Moreover, the drill bit's wear is severe, and downhole complex situations such as drill tool sticking and well leakage often occur, seriously affecting the whole project's progress. Therefore, it is imperative to understand the conglomerate's rock mechanical properties for drilling and completion projects.

The conglomerate is one of the bimrocks (block in matrix rocks), bimrocks are solid blocks or geological mixtures of rocks surrounded by the weak matrix (Medley, 1994). Many scholars have studied its physical and mechanical properties, deformation, and failure mechanism for this particular geological body. Miller et al. (1957) studied the effect of coarse sand content on the mixture's cohesion and internal friction angle by triaxial compression test of the mixture composed of clay and coarse sand. Savely et al. (1990) studied the effect of volume fraction of pebble gravel on shear strength through large-scale in-situ tests.

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