Page 131 - Enhanced Oil Recovery in Shale and Tight Reservoirs

CHAPTER FIVE

Optimization of huff-n-puff gas
injection in shale and tight oil

reservoirs

Abstract

This chapter is to discuss the optimum huff-n-puff times, number of cycles, and soaking
time under practical operational and reservoir conditions. The operational and reservoir
conditions dictate the maximum injection and production rates and the maximum
injection pressure and minimum production pressure. Optimization principles and
optimization criteria for huff-n-puff gas injection are proposed. It is also found that
the benefits of soaking may not compensate the loss in injection and production
due to the time lost in the soaking period. Therefore, soaking may not be necessary
during the huff-n-puff gas injection in shale and tight oil reservoirs.

Keywords: Huff-n-puff gas injection; Maximum injection pressure; Minimum production
pressure; Optimization criteria; Optimization principle; Soaking time.

5.1 Introduction
Since Wan et al. (2013a) proposed huff-n-puff gas injection to
improve oil recovery in shale and tight oil reservoirs, many papers have
been published on the subject, as reviewed by Sheng (2015d). However,
Chen et al. (2014) showed that the final recovery factor from huff-n-puff
CO 2 injection is lower than that in the primary depletion because the
incremental oil recovery in the production stage is unable to compensate
the loss during the injection and shut-in stages. In their models, the
huff-n-puff time is from 300 to 1000 days; the bottom-hole injection
pressure is 4000 psi and the producing pressure is 3000 psi. Using his model,
Sheng (2015d) was able to repeat Chen et al.’s (2014) results (i.e., the
huff-n-puff oil recovery is lower than the primary oil recovery). However,
using the same model, by raising the injection pressure to 7000 psi,
Sheng (2015d) demonstrated that all the oil recovery factors from the
huff-n-puff process are higher than those from the primary depletion at
the end of 30, 50, and 70 years. In other words, Chen et al.‘s (2014) results

Enhanced Oil Recovery in Shale and Tight Reservoirs © 2020 James Sheng.
ISBN: 978-0-12-815905-7 Published by Elsevier Inc.
https://doi.org/10.1016/B978-0-12-815905-7.00005-0 All rights reserved. 117 j