Probing the mechanism of external fluids invasion of nanopores in fractured tight sandstone reservoir

Abstract

The oilfield development process is accompanied by reservoir damage, which affects the reservoir permeability and reduces production. In this study, the mechanism of external-fluid invasion was analyzed to maintain oil-well productivity. The characteristics of the damage caused by external fluids to tight sandstone reservoirs were examined via scanning electron microscopy and high-pressure mercury-invasion experiments. Then, seepage models for fluids in the nanopores of tight sandstone reservoirs were established to investigate the effects of the natural fracture parameters and fluid properties on the seepage characteristics and damage mechanism of fractured tight sandstone reservoirs. The results indicated that the damage rates of water-based drilling fluid, oil-based drilling fluid, guar-gum fracturing fluid, and slick-water fracturing fluid for the natural core of tight sandstone are 53.20%, 33.87%, 23.64%, and 19.10%, respectively. The proportion of nanopores in fractured tight sandstone reservoirs ranges from 44.62% to 62.45%. The invasion of external fluids leads to a reduction in the permeability as well as the number of microfractures and their lengths. The changes in permeability barely affect the seepage characteristics of the fluid in the nanopores. The reductions in the lengths and number of microfractures change the connectivity of the entire flow space, affecting the seepage characteristics. The effect of the lengths of the microfractures is more significant than that of the number of microfractures. Additionally, the invasion speed of the external fluids has the most significant effect on the seepage characteristics. A higher viscosity of the external fluids corresponds to less damage to the nanopores. Drilling fluid causes less damage to nanopores than fracturing fluid does. The recovery of fractured tight oil reservoirs with nanopores can be enhanced by reducing the degree of external-fluid invasion.

Graphical Abstract

Keywords:

• Fractured tight sandstone reservoir
• nanopore structure
• external-fluid invasion

Acknowledgments

This research was conducted at the Key Laboratory of Enhanced Oil & Gas Recovery of the Ministry of Education at Northeast Petroleum University (Daqing, China). The authors gratefully acknowledge the support of the National Natural Science Foundation of China（No. 51574089 and Heilongjiang Provincial Department of Education (TSTAU-R2018018) and the Innovative scientific research project for Postgraduates of Northeast Petroleum University (YJCX2016-013NEPU). We would like to thank Editage (www.editage.cn) for English language editing.

Disclosure statement

We would like to submit the enclosed manuscript entitled “Probing the Mechanism of External Fluids Invasion of Nanopores in Fractured Tight Sandstone Reservoir,” which we wish to be considered for publication in “Journal of Dispersion Science and Technology.” No conflict of interest exists in the submission of this manuscript, and the manuscript has been approved by all authors for publication. I would like to declare on behalf of my coauthors that the work described is original research that has not been published previously, and is not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the enclosed manuscript.