Page 291 - Fundamentals of Enhanced Oil and Gas Recovery

Enhanced Oil Recovery (EOR) in Shale Oil Reservoirs 279

the oil recovery rate increased by 44 bbl/day. However, this incremental oil recovery
was not completely attributed to CO 2 injection; this is because the workover has been
done in the well as well as some CO 2 effects in near wellbore region [40,41].
The third CO 2 huff-n-puff injection pilot test in Bakken formation was done in
2014; in this scenario, a vertical well drilled and completed in the Bakken formation.
It was planned to inject CO 2 for 20-30 days by 300-500MSCF/day and the soaking
time was 20 days. Also, an offset well was used to evaluate the CO 2 breakthrough
time; after 1 day from injection started CO 2 broke through the offset well and the
injection process was shut down [40]. This was mainly because huff-n-puff mode
could not be successful when CO 2 find the way to break through offset well.
Another field test of CO 2 injection scenario was performed on the Parshall field,
Mountrail County. In this pilot the huff-n-puff injection model was used in a hori-
zontal well which completed in the Bakken formation. Injection time in this pilot test
was 11 days and after this time, the oil production rate increased [1]. It is worth to
mention that this field is a naturally fractured one and controlling the conformance is
quite challenging in designing any type of EOR scenarios; mobility of carbon dioxide
in such a reservoir was 304. Also, this reservoir has a local fracture network; it means
that the connectivity of the fractures will affect the performance of the EOR scenario
[41].

9.3.2 Water Injection
In US and Canadian shale formations, there have not been many water injection field
projects. However, in China, large-scale water injection is carried in tight formations.
This section provides descriptions for ongoing projects and pilots throughout the
United States, China, as well as Canada.

9.3.2.1 Continuous Waterflooding
In shale and tight reservoirs, one main concern is water injectivity. It can be under-
stood that water injection may have more injectivity issue than gas injection. Thus,
the first objective of a water injection pilot is to check water injectivity. Surprisingly,
the field tests conducted so far did not have the injectivity problem in shale reservoirs
[40] and in many tight reservoirs in China.
It is commonly accepted that water rock interaction causes permeability
impairment. However, it was observed that the water may help to generate microfrac-
tures or open existing microfractures in shale formations if no confining pressure is
applied [42 46].
However, Behnsen and Faulkner [47], Duan and Yang [48], and Faulkner and
Rutter [49] reported that with isotropic confining pressure, a significant reduction was
observed on clay-bearing rocks or montmorillonite sample permeability measured
with water. In conventional propped hydraulic fracture treatments, water fractures rely