Page 36 - Well Logging and Formation Evaluation

26 Well Logging and Formation Evaluation

doesn’t produce as it was supposed to, as per calculations. Either it fails
to produce at all, it dies quickly, or water or gas breakthrough occurs. Very
often the petrophysicist is called to provide an explanation for these
phenomena. When a well fails to flow or dies very quickly, the first thing
to look at is the perforation operation. Check the following:

• Is the petrophysical interpretation completely reliable? What was the k h
as derived from the logs?
• Could formation damage have occurred during the cementation and
completion operation?
• Is there proof that the guns detonated, and were they on depth?
• What was the over/underbalance at the time of perforation? What draw-
down is currently being applied?
• Are there any other mechanical factors (sliding side doors, safety
valves) that could prevent flow?

When water breakthrough occurs sooner than expected, the following
should be considered:

• How close are the perforations to any water leg as logged in the well?
Could water coning be occurring?
• What is the quality of the cement bond, and could there be flow behind
casing?
• Where does the dynamic model of the reservoir predict the encroach-
ing water front to be?
• Are neighboring wells already starting to water out, and are TDT/GST
data available for any?
• Could the water be entering from elsewhere in the wellbore (other
producing zones, leaks in the tubing, etc.)?
• What does the relative permeability data from the core indicate the
relative permeability to oil and water to be?
• Is the well in the transition zone, and how sure are you that any S w
calculated on the logs is capillary or clay bound?

When gas is produced unexpectedly, the following should be considered:

• How close are the perforations to any gas leg as logged in the well?
Could gas cusping be occurring?
• What is the quality of the cement bond, and could there be flow behind
the casing?