A Simplified Capillary Bundle Model for CO2-Alternating-Water Injection Using an Equivalent Resistance Method

Wendong Wang,Yuliang Su,Fankun Meng,Yongmao Hao,Xueyu Geng,Lei Hou,Lei Li,Micòl Mastrocicco

doi:10.1155/2020/8836287

Abstract

CO2-alternating-water injection is an effective way of enhancing recovery for low-permeability oil reservoirs. The injection process is one of the essential issues that are facing severe challenges because of the low permeability and poor pore space connectivity. Previous researchers mentioned that water injection ability could be decreased by around 20% after the CO2-flooding; hence, it is necessary to quantify the water injectivity variation during an alternated injection process. In this paper, a CO2 convection-diffusion model is established based on the seepage law of CO2 and dissipation effect. The relationship between the width of miscible flooding and injection time is defined. Besides, an equivalent resistance method is introduced for developing a capillary bundle model for featuring an unequal diameter for CO2 water vapor alternate flooding. CO2-oil and CO2-water interactions are analyzed using the new model. The effects of oil viscosity, pore throat ratio, CO2 slug size, and equivalent permeability of the capillary bundle on water injection are analyzed. The result indicates that water injection ability increases with the rise of CO2 slug size and equivalent permeability of the capillary bundle and decreases with the increase of viscosity and pore throat ratio.

Highlights

The CO2-alternating-water injection method, used in the lowpermeability reservoirs, has the advantages of both CO2 flooding and water flooding
It is significanlty important to study the variation of injection ability with capillary bundle model during CO2-alternating-water flooding process
According to the principle of material balance, due to the constant total injection volume, CO2 will continue to diffuse into the crude oil during the water injection process, the length of the CO2 zone and the crude oil zone will decrease, the width of the miscible zone will increase, and the calculation formula of the miscible zone will be for the distribution mode

Summary

Introduction

The CO2-alternating-water injection method, used in the lowpermeability reservoirs, has the advantages of both CO2 flooding and water flooding. As microscopic flow mechanics gradually became the focus of investigation, some researchers began to pay attention to the complex seepage mechanism of pore space in porous media Some of those researchers have been focusing on the alternate injection of water and gas by pore structure by establishing a pore network model as a simulation platform. Yang et al used core samples collected from tight formations to conduct a series of wateralternating-CO2 flooding experiments with different wateralternating-CO2 ratios and slug sizes [19] They found that fluid injectivity is strongly dependent on slug size, water-alternating-CO2 ratio, and cycle time. Hu substituted the permeability of the capillary model into low-permeability equation and used superposition method to get total flow rate and threshold pressure gradient of the rock [23] This model gives a theoretical description of lowpermeability seepage characteristics. The distribution of oil, CO2, and water in the capillary model with different diameters has been analyzed, and the influence of geology and fluid properties on injectivity has been provided

CO2 Flooding Dynamics considering Mass Transfer

Unequal Diameter Capillary Beam Model for CO2-Alternating-Gas Flooding

CO2 flooding

Water Flooding Seepage Mode and Mathematical

Sensitivity Analysis

Conclusions