Incremental Oil Recovery Through the Implementation of Gas Huff and Puff

In November 2002, as part of its SHARP programme, the DTI initiated a review of the gas Huff and Puff technique to investigate its potential for increasing late life recovery from North Sea reservoirs. In particular, small fields that have been shut in due to pressure depletion and where additional investment in secondary recovery or artificial lift is not justified. As Sally Knudson (s.knudson@senergyltd.com) of Senergy Ltd reports, for fields of this type the study findings have indicated that there is potential for additional economic recovery of oil through the application of Huff and Puff.

Background

The gas Huff and Puff technique is a well treatment to improve productivity late in a well’s life. It is an immiscible gas flood of the near-wellbore region (ca 250 ft). The candidate well acts as both producer and injector. The treatment process comprises three phases;

• A Huff : Injection of a slug of gas
• A soak : Shut in period
• A Puff : Production period

Experience of Huff and Puff is predominately US based but its success is very well documented in industry literature. Evolving from the steam Huff and Puff, the gas Huff and Puff technique was first practiced in 1960 and commercial application was achieved by 1981. The following range of conditions for treatments is reported in the literature:

It is encouraging that conditions applicable to the North Sea environment are captured within this table.

The gas utilisation factor (GUF) is the most commonly used measurement of treatment success;

GUF = Gas Slug Size Volume /Incremental Oil Volume (Mscf/stb)

However, this factor fails to capture the benefit of recovering the majority of the injected gas and because the volumes are at standard conditions the efficiency at reservoir conditions is not clear.

Figures 1 and 2 summarise slug volume versus incremental recoveries, at standard and reservoir conditions, that are reported in the US literature.

Mechanisms

The Huff and Puff treatment involves complex phase behaviour and dynamic changes in the near wellbore region. The many mechanisms involved, from hysteresis to wettability changes, are discussed, largely qualitatively, in the US literature. Simplistically, the benefit of Huff and Puff treatment can be seen by considering the well inflow equation;

Where kro is potentially increased by dynamic effect and µoBo is potentially decreased by phase behaviour effects. It is the phase behaviour effects are most commonly discussed in the literature. When the injected gas dissolves into the under-saturated oil in the near wellbore region the oil viscosity is reduced. PVT data illustrating this effect for a synthetic low bubble point oil representative of North Sea reservoirs are shown in Figure 2. The benefit of a “window of saturation”, with a low bubble point pressure and high bottomhole pressure limit, is illustrated in Figure 3.

Figure 3: Window of under-saturation (Click image for larger view)

Parametric study

A numerical parametric study was undertaken to identify the key criteria for a favourable Huff and Puff treatment under natural depletion conditions. Uncertainties relating to relative permeability and PVT descriptions were captured by sensitivity analysis. The results, Figure 4, were found to be comparable to US figures at reservoir conditions. Incremental oil can be achieved for a range of conditions but, is greatest under the following;

• Well lift constrained (i.e. no artificial lift)
• Thin producing zone
• Good permeability
• Presence of wellbore damage

Phase behaviour sensitivity analysis indicated that the most favourable responses were achieved for a reservoir oil with a bubble point of ca 1500 psig.

Mechanisms such as re-pressurisation, diffusion, viscous fingering and gas segregation to form a secondary gas cap were not investigated in detail in this study. However, they are likely to occur and will have a beneficial effect on the Huff and Puff performance.

Conclusions

The Huff and Puff review undertaken has identified that:

• It is a proven well stimulation technique
• It is an application that can be used to increase late life recovery both on a field wide or individual well basis.
• Incremental oil volumes are low. However multiple cycle applications and a low cost gas supply may overcome economic hurdles
• Field trials would improve understanding and assist in minimising the uncertainties associated with the process
  

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