Inflow, Outflow and Nodal Analysis

A well's rate is set by two things acting together: how readily the reservoir delivers fluid into the wellbore, and how readily the wellbore and surface system carry it away.

Inflow performance

The inflow performance relationship describes the rate a well produces for a given flowing bottom hole pressure. The lower the flowing pressure, the greater the drawdown and the higher the rate.

• Above the bubble point, with single-phase liquid flow, the relationship is essentially a straight line and is described by the productivity index — barrels per day per unit of drawdown.
• Below the bubble point, gas comes out of solution in the reservoir, the relative permeability to oil falls, and the relationship curves downward. Additional drawdown yields progressively less extra oil.

The inflow relationship is determined by permeability, thickness, fluid properties, drainage geometry and skin. Skin is the one a production engineer can change, and removing damage is often the cheapest production gain available.

Outflow performance

The tubing performance curve describes the bottom hole pressure required to lift a given rate to surface. It is the sum of the hydrostatic head of the fluid column and the friction losses, and it has a characteristic U shape:

• At low rates, the flow is too slow to carry liquid efficiently, liquid holdup increases, the column becomes heavy, and the required pressure rises as rate falls. This is the unstable region where wells load up and die.
• At high rates, friction dominates and the required pressure rises with rate.

Nodal analysis

Plotting inflow and outflow on the same axes, the intersection is the operating point — the rate and pressure at which the well will actually produce. This simple picture is the most useful tool in production engineering because it shows immediately where the constraint lies.

It answers practical questions directly: will a bigger tubing help or hurt; what rate would result from lowering wellhead pressure; how much would removing skin gain; at what point will the well need artificial lift; and what will happen as water cut rises and the column gets heavier.

The critical insight it provides is where the bottleneck is. If the well is inflow constrained, no amount of lift or tubing change will help much and the answer is stimulation. If it is outflow constrained, stimulation gains little and the answer is lift, tubing or reduced back pressure. Money is routinely spent on the wrong half of this because nobody drew the curves.

Well testing and monitoring

Performance is measured by periodic well tests — routing the well to a test separator to measure oil, gas and water rates — supplemented by flowing wellhead pressure and temperature, downhole gauges where installed, and multiphase or virtual metering. The value is in the trend: a rate that is falling, a water cut that is rising, or a productivity index that is dropping each point to a different cause and a different remedy.