Influence of Migration and Plugging of Nanoparticles on Coal Permeability in Coal Reservoirs

Xiaopeng Zhai,Zhiyang Guo,Shuhong Xu,Yuanchang Chen,Hongyong Hao,Yishan Lou,Weihong Chen,Lihui Zheng,Yun Xu,Jianfang Jiang,Nicolò Colombani

doi:10.1155/2021/8886426

Abstract

The plugging of nanopores in low-permeability coal reservoirs is an important factor that affects productivity reduction. However, the mechanism of plugging of the nanopores in coal reservoirs remains unclear. In this study, the coal samples from the Anze coalbed methane block of the North China Oilfield are used as the research object. Experiments are conducted on the mechanism of nanopore plugging by the variation of nanopore permeability based on the pressure oscillation method and the nanopore (scanning electron microscope) method. The research shows that the foreign working fluid invades a coal sample; the sample changes from being hydrophobic to being water absorbent within a certain period. The instability caused by the expansion of coal clay mineral particles promotes the dispersion and shedding of particles, and the migration of particles is accelerated under the shear stress of the working fluid. In addition, the viscosity and pressure difference of the working fluid are important factors that affect particle plugging. The viscosity of the fluid increased by two times, and permeability decreased by 1.21 times. As the pressure difference increases by two times, permeability can be reduced by up to two orders of magnitude. The findings of this study can help for better understanding of the mechanism of plugging of the nanopores in coal reservoirs and the reasons of production reduction in low-permeability coal reservoirs. Such findings provide theoretical support for the selection of the working fluid, and reasonable production pressure difference can effectively reduce the damage on coal permeability in a low-permeability coal reservoir.

Highlights

Most of the coalbed methane resources in China have low porosity and low permeability
Through the method of macroscopic permeability damage rate test and microscopic observation and analysis, this study proved that nanoparticle erosion and plugging are important factors of coal reservoir damage
(1) When the working fluid entered a coal sample, a time limit exists for the water absorption of coal from the hydrophobic phase

Summary

Introduction

Most of the coalbed methane resources in China have low porosity and low permeability. Studies on the pore structure of coalbed reservoirs have focused on the analysis of microstructure characteristics, establishment of pore quantitative evaluation methods, and determination of the water absorption and production mechanisms of coalbed methane; these studies provide a Geofluids theoretical foundation for research on the permeability damage of coal seams with nanopores [8,9,10,11,12,13] Their results showed that coal pores are mostly nanoscale pores and vulnerable to reservoir damage caused by external fluid pollution [14, 15]. With a large number of experimental results, the laws of nanopore plugging and permeability reduction were explored, the main factors that affect the permeability of coal reservoirs were determined, and drill fluid systems for lowporosity, low-permeability coal reservoirs were optimized

Microanalysis of Test Samples and Mineral Components

Main Experimental Methods

D-1 D-2 D-3 D-4 D-5

Results and Discussion

Summary and Conclusions