Page 45 - Petroleum and Gas Field Processing

Determining Tubing Size
Determining the size of production tubing is the starting point for
completion and drilling design, as it directly affects the sizes of all casing
strings and, accordingly, the design and cost of the drilling program. The
tubing size must be selected to handle the desired production rates under
the varying producing conditions for the life of the well. To properly
determine the tubing size, the whole production system (from the
formation to the surface separator) must be considered. The ability of the
formation to produce fluids by the natural drive and improved recovery
methods from the start of production until depletion need to be
considered. The flow of produced fluids through the production tubular,
the wellhead restrictions, and the surface flow line over the life of the well
need to be analyzed taking into account possible means of artificially
lifting the fluids.
The flow from the formation to the bottom of the well (bottom hole)
is governed by what is known as the inflow performance relationship of the
well, whereas the flow from the bottom hole to the surface is represented
by the outflow performance relationship.
1. Inflow Performance Relationship (IPR): This is the relationship
between the flow rate (q) and the flowing bottom-hole pressure
(P wf ). The relationship is linear for reservoirs producing at
pressures above the bubble point pressure (i.e., when P wf is
greater than or equal to the bubble point pressure). Otherwise,
the relationship takes the shape of a curve, as illustrated in
Figure 16. When the IPR is linear, it can be represented with
what is known as the productivity index (PI), which is the inverse
of the slope of the IPR. The PI is basically the production rate
per unit drawdown (the difference in pressures between the
average reservoir static pressure, P R , and P wf ). The IPR depends
on the reservoir rock and fluid characteristics and changes with
time, or cumulative production as illustrated in Figure 17.
Methods exist for determination of the IPR and for predicting
future IPRs.
2. Outflow Performance Relationship (OPR): Outflow performance
involves fluid flow through the production tubular, the wellhead,
and the surface flow line. In general, analyzing fluid flow
involves the determination of the pressure drop across each
segment of the flow system. This is a very complex problem, as it
involves the simultaneous flow of oil, gas, and water (multiphase
flow), which makes the pressure drop dependent on many
variables, some of which are interdependent. There is no

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