Page 183 - Machine Learning for Subsurface Characterization

158 Machine learning for subsurface characterization

Volume fraction of miscible oil
V o
Volume fraction of movable water
V w
Volume fraction of bound fluid
V b
Volume fraction of kerogen
V k
Subscripts
i Formation (i)
j Formation parameter (j)
o Oil, optimum
w Worst

1 Introduction

1.1 Geology of the shale formation
The shale formation under investigation is a 200-ft-thick formation divided into
three distinct sections: upper, middle, and lower shale formations. The upper
and lower sections are black shales, and the middle section is sandy
siltstone. The upper and lower shale formations are hydrocarbon source
rocks with total organic carbon (TOC) ranging from 12 to 36 wt%. The clay
mineral content in these two sections is dominated by illite and quartz. The
middle shale formation is the hydrocarbon-bearing reservoir and has a low
TOC content ranging from 0.1 to 0.3 wt%. Below these three shale
formations, there exists dolostone interbedded with clay-rich conglomeratic
dolomudstone. The shale formations are characterized by ultralow matrix
permeability that constrains oil mobilization [1]. Current recovery factor in
such shale formations is around 3%–6% of the oil in place [2]. High oil-in-
place estimates with low primary recovery mandates enhanced oil recovery
(EOR) projects based on light, miscible hydrocarbon injection.

1.2 Literature survey
Due to the ultralow permeability and nanoscale pore sizes, the mechanisms of
EOR in tight reservoir are different from those in conventional reservoir. In
tight oil formation, EOR efficiency is controlled by the combined effects of
miscibility, diffusion, sorption, dissolution, and capillary condensation, to
name a few, out of which diffusion is the dominant mechanism [3]. Large
permeability contrast between the organic-rich porous matrix and fracture
can lead to fracture-dominated flow in fractured shales. In tight shale
formations, the net displacement due to the injection of light hydrocarbon
generally includes four steps: (1) injected light hydrocarbon flows through
fractures, (2) injected hydrocarbon goes into the porous organic-rich shale
matrix by diffusion and imbibition; following that, the injected hydrocarbon
may interact with connate oil due to miscibility, (3) connate oil migrates into
fractures via swelling and reduced viscosity, and (4) injected light
hydrocarbon achieves equalization inside the organic-rich matrix [2]. Our