During a recent international marketing trip we
have found that many operators of high rate gas and gas condensate wells
perform Static Gradient Surveys (SGS) and/or Flowing Gradient Surveys (FGS) as
opposed to Pressure Build-up or drawdown tests.The reason frequently given is that
operational procedures dictate that a well must be shut-in when running
wireline into the wellbore.By shutting
in the well, before running wireline into the wellbore, you have introduced a
major pressure transient into the reservoir that distorts the results from a
subsequent build-up test unless you are willing to extend the flowing and
shut-in times drastically.This is
something the operator is rarely willing to do and therefore he only performs a
SGS and/or a FGS.
When
surveillance work is limited to Static and Flowing Gradient Surveys they are losing
the opportunity to acquire critically important reservoir information such as
completion efficiency (skin), permeability and reservoir pressure (P*).The need to derive skin and permeability thru transient
testing is invaluable as you seek to understand not only the change in reservoir
pressure over time due to depletion effects but also understand the changing
nature of kh due to formation compaction, etc. and if skin is accreting over
time due to precipitates or fines migration that tend to restrict flow in gravel
pack completions.
Utilizing
surface pressure measurements for pressure transient analysis avoids the
concerns associated with running tools into a flowing well.The SPIDR well testing system has been
performing that function for almost 25 years in gas and high-yield gas
condensate wells all over the world.The
system consists of the data acquisition unit, the SPIDR, and a very
sophisticated computer model that converts well head pressures to reservoir
conditions.The computer model takes
into account frictional losses in the flowing wellbore, thermal effects in the
well bore during rate changes, and phase changes in the wellbore going from
flowing to shut-in conditions.
One
of the primary ways that DRC demonstrates effectiveness compared to downhole
gauges is to perform a simultaneous trial comparison.A downhole gauge is run to TD while the SPIDR
is simultaneously capturing pressure data at the wellhead.To maximize effectiveness and value, a PBU,
as a minimum should be performed.If the
SPIDR converted data proves to be acceptable to the customer and SPIDR Surface
Well Testing Technology is deemed as a NO RISK and LOW COST alternative to
running downholeÖthe SPIDR can be used for future pressure transient testing
surveyís WITHOUT operational concerns.
