Well performance evaluation is the study of how much a well can produce and why. It combines reservoir flow equations (Darcy’s law), the productivity index, and the effect of formation damage (skin). This article explains the main ideas and how the three parts of the production system fit together.
Petroleum recovery from the reservoir may make use of the inherent energy of the system (Primary Recovery); Energy may however be added to the system in the form of injected fluid (Secondary recovery). Some of the residual hydrocarbon trapped during conventional production process may be mobilised (Tertiary or enhanced oil recovery). Likewise, the wellbore resistance to flow may be altered in terms of vertical lift performance through artificial lift technology.
Defining the performance of a reservoir requires an accurate knowledge of reservoir flow properties and the flow potential through formation evaluation techniques and well testing. Pressure data from a bottom hole pressure survey is a key input.
Formation Evaluation Techniques
Different formation evaluation techniques are applied during the course of well
completion to:
- Identify a potential pay zone – By well log and core analysis
- The formation porosity, permeability and fluid saturation – By well log and core analysis
- The fluid type – by core log and well testing.
One simple well test is the Drill stem test
The Drill stem test
This is a temporary completion of a well that enables the well to flow without a production flow string. Normally a drill string can be used or a special test tool assembly may be run on a tubing string.
There can also be production testing on an extended scale
Well Performance Evaluation
In general, the Darcy Law equation as applied to laminar flow is given as :
For linear Flow:
q = rb/day; qs = stock tank bbl/d
For radial flow
For steady state flow
For semi-steady state incompressible flow
k = darcy; T= 0R
(mz) is evaluated at ( Pe+Pw )/2
z = Compressibility factor
For such systems, the Production performance is measure in terms of three distinct aspects:
- The well Inflow Performance – Flow from reservoir to wellbore
- Vertical lift performance – Flow up the production string to surface
- Choke or Bean performance – Flow through control systems to reduce flowing pressure.
Well Inflow Performance
Well inflow performance is expressed in terms of productivity index. Productivity index (PI or J) is a measure of the capability of a reservoir to deliver fluids to the bottom of a wellbore for production. It defines the relationship between the surface production rate and the pressure drop (Drawdown) across the reservoir. Expressed mathematically, it is given as:
Effect of damage
The above equations have been based on the ideal situation of no damage to the reservoir due to completion, etc. Formation damage can be said to be a measure of the reduction in the production capacity of a reservoir. It is expressed in terms of skin factor, s.
Sources of damage include:
- Damage around the near wellbore due to drilling/completion fluid invasion, sd
- Damage due to perforations, sp
- Damage due partial completion, spp
- Damage due to perforation compaction, sc
- Damage due to heterogeneity
- Damage due to hole angle, etc.
The total skin =sum of all skins
ST =sd + sp +spp +sc +… .
Therefore these conditions create a change in the net flow bottom hole
pressure.
Therefore for an incompressible fluid steady state flow,
Example: productivity index
If a well produces 500 STB/d when the reservoir pressure is 3,000 psi and the flowing bottom hole pressure is 2,500 psi, the drawdown is 500 psi and PI = 500 / 500 = 1.0 STB/d/psi. A damaged well with the same drawdown would give a lower PI.
Production System at a Glance
| Part | What it describes | Main variable |
|---|---|---|
| Inflow performance | Flow from the reservoir into the wellbore | Productivity index, drawdown, skin |
| Vertical lift performance | Flow up the production string to surface | Tubing size, fluid density, friction |
| Choke (bean) performance | Flow through surface control equipment | Choke size, flowing pressure |
Frequently Asked Questions
What is the productivity index?
The production rate per unit of pressure drop between the reservoir and the wellbore (J = q / (Pr − Pwf)).
What is skin factor?
A number that describes extra pressure drop near the wellbore caused by damage (positive skin) or stimulation (negative skin).
Why are pressure surveys important here?
They give the reservoir and flowing pressures used in the inflow equations; see bottom hole pressure survey.
Key Takeaways
- Well performance has three parts: inflow, vertical lift and choke performance.
- PI links production rate to drawdown.
- Skin lowers productivity and is the sum of all damage sources.
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