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CHAPTER TWO




              Huff-n-puff gas injection in oil
              reservoirs




              Abstract

                 This chapter discusses huff-n-puff gas injection in shale and tight oil reservoirs. The
                 effects of matrix size, pressure and pressure depletion rate, soaking time, gas compo-
                 sition, diffusion, water saturation, stress-dependent permeability on EOR potential are
                 discussed. The EOR mechanisms are discussed. The minimum miscible pressure in
                 huff-n-puff injection is found to be higher than estimated from the conventional slim-
                 tube tests. The gas penetration depth is strongly related to natural fracture density.
                 Some field projects are presented.


              Keywords: Diffusion; Gas composition; Gas penetration; Huff-n-puff; MMP; Pressure
              depletion rate; Soaking time.


                   2.1 Introduction

                   In shale and tight reservoirs, because of the ultralow permeability and
              high injectivity of gas, it is intuitive that gas injection is preferred. Gas
              injection can be carried through flooding and huff-n-puff. Again, because
              of ultralow permeability, most of the pressure drop occurs near the injection
              well. It will take a long time for the injected gas to drive oil to the produc-
              tion well. Therefore, the flooding mode loses its advantages. By contrast, in
              the huff-n-puff mode, as gas injection and fluid production are performed at
              the same well, the pressure near the well can be quickly built up during the
              huff period, and fluid (gas, oil and water) can be produced immediately after
              the well is put in the puff mode (Sheng and Chen, 2014). The benefits of gas
              injection can be quickly returned. And the process of huff-soak-puff can be
              repeated (cycled). Thus, the benefits can be extended for a long time.
              Therefore, the huff-n-puff gas injection is a preferred mode. In this chapter,
              the huff-n-puff injection is discussed in detail, including mechanisms, field
              projects and experimental and numerical studies of the factors that affect
              the performance.




              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/B978-0-12-815905-7.00002-5  All rights reserved.  7 j
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