WELL PERFORMANCE EVALUATION

Petroleum recovery from the reservoir may make use of the inherent energy of the system (Primary Recovery); Energy may however be added to die 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

Formation Evaluation Techniques

Different formation evaluation techniques are applied during the course of well
completion to:

1. Identify a potential pay zone – By well log and core analysis
2. The formation porosity, permeability and fluid saturation – By well log and core analysis
3. 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:

1. The well Inflow Performance – Flow from reservoir to wellbore
2. Vertical lift performance – Flow up the production string to surface
3. Choke or Bean performance – Flow through control systems to reduceflowing 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:

1. Damage around the near wellbore due to drilling/completion fluid invasion, sd
2. Damage due to perforations, sp
3. Damage due partial completion, spp
4. Damage due to perforation compaction, sc
5. Damage due to heterogeneity
6. 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,