Introduction

The performance relationships presented were for single-phase oil wells and, while gas may come out of solution after oil enters the wellbore, the use of those relationships does not consider free gas to be present in the reservoir. Expansion of oil itself as a means of recovery is a highly inefficient mechanism because of the oil’s small compressibility. It is likely that even in the best of cases if the bottomhole pressure is above the bubble-point pressure, as is the case for heavy oil, a very small fraction of the original-oil-in-place would be recovered. Therefore, in most oilfields, oil will be produced along with free gas in the reservoir, either because the reservoir pressure is naturally below the bubble-point pressure (saturated reservoirs) or because the flowing bottomhole pressure is set below that point to provide adequate driving force. In terms of ultimate recovery, expansion of free or solution gas is a much more efficient mechanism than the expansion of oil.

Figure 3-1 is a schematic of a classic phase diagram, plotting pressure versus temperature and identifying the important variables. Depending on the initial and flowing pressures and the reservoir temperature, a diagram such as in Figure 3-1 can indicate whether single-phase deliverability relationships as presented apply, or if the two-phase equations.

Figure 3-1. Schematic phase diagram of hydrocarbon mixture. Marked are reservoir, bottomhole, and wellhead flowing conditions for an oil reservoir.

In Figure 3-1, the initial reservoir conditions for an undersaturated oil reservoir are marked as p_i, T_R. In this depiction, the reservoir is above the bubble-point pressure. The flowing bottomhole pressure, p_wf, is marked within the two-phase region. The flowing bottomhole temperature is taken as equal to the reservoir temperature, reflecting the typically isothermal flow in the reservoir (unless thermal recovery is attempted). The wellhead flowing conditions, with pressure p_tf and temperature T_tf, are also marked. Thus, the reservoir fluid would follow a path from the reservoir to the surface, joining these three points. In an initially saturated reservoir, all three points would be within the two-phase envelope. For comparison, the paths for the fluids in a retrograde gas condensate reservoir and in a single-phase gas reservoir are shown. In the gas cases there is a much more pronounced reduction in the temperature along the path, reflecting Joule-Thomson expansion effects associated with gas flow. Solid lines connote single-phase flow.