Effect of Dynamic Imbibition on the Development of Ultralow Permeability Reservoir

Yafei Hu,Fei Ren,Junshi Li,Jianfeng Hou,Zhiying Wu,Huanhuan Peng,Baojun Bai

doi:10.1155/2021/5544484

Abstract

To explore the methodology for improving ultralow permeability reservoir recovery, cores of ultralow permeability reservoirs in China’s Ordos Basin were selected to study the dynamic imbibition micromechanism of crude oil in nanopore throat through core-flooding laboratory experiment and nuclear magnetic resonance (NMR) observation. In the meantime, the microimbibition characteristics and dynamic discharge of oil between matrix and fracture in partially closed boundary reservoirs were simulated to utmostly reflect the actual reservoir conditions. Our findings suggest that dynamic imbibition between fracture and matrix serves the core technology for improving the recovery of ultralow permeability reservoirs, while the main factors affecting dynamic imbibition efficiency include wettability, permeability, injection rate, fracture, water huff and puff cycles, and soaking time. Wettability, in particular, weighs the most, and imbibition can take place either on water-wet rocks or transformed oil-wet rocks with an imbibition agent added in during the waterflooding process. Meanwhile, the higher the permeability is in place, the greater the dynamic imbibition recovery might achieve. The experiments indicate that the dynamic imbibition recovery of a fractured core is 16.26% higher than that of a nonfractured core. Additionally, fractures can not only enhance imbibition recovery but also accelerate the occurrence of dynamic imbibition. The optimal water injection rate of dynamic imbibition is 0.1 mL/min; the reasonable huff and puff cycle of the ultralow permeability reservoirs tends to be two to three cycles; the optimal soaking time of ultralow permeability reservoir is speculated to be 30 days. Finally, the field practice shows that after Stimulated Reservoir Volume (SRV) and dynamic imbibition in 5 horizontal wells in An83 oilfield, there is a remarkable drop in water cut and a noticeable rise in oil production. This research underpins the significance of a dynamic imbibition effect in the development of ultralow permeability oilfield.

Highlights

As an important unconventional reservoir, ultralow permeability reservoir has emerged as a new “sweet spot” in recent years, drawing extensive attention
The imbibition law and characteristics of dynamic imbibition in ultralow permeability reservoirs are studied by the laboratory test and nuclear magnetic resonance method, and the influence of wettability, permeability, injection rate, and other factors on imbibition efficiency is analyzed
As required by the experiment design, the formation water was injected into sample 5-1 as a control group, and results showed that the dynamic imbibition recovery was 0% with formation water only (Figure 5), indicating that oil-wet rocks do not involve any imbibition

Summary

Introduction

As an important unconventional reservoir, ultralow permeability reservoir has emerged as a new “sweet spot” in recent years, drawing extensive attention. China has vast reserves of ultralow permeability reservoirs mainly located in Ordos, Songliao, Junger, and Qaidam basins and has huge untapped potential [1, 2]. Known as reservoirs with permeability ranging from 0.1 to 1 × 10−3 μ m2, are hugely tight and high in irreducible water saturation, which basically mean there is no natural production capacity [3, 4]. Oil exploitation in the matrix is hardly attainable since injected water tends to flow along the large fractures as a result of small pore throat and fracture development in the reservoir. The development effect is, not desirable with the oil recovery normally below 15% [5,6,7,8,9,10]

Methods

Results

Conclusion