The Conversion of Wellhead Pressures on Subsea Wells
By Mike Redman and Chris Floyd |
Thu, 7 Aug 2008
Frequent, accurate bottomhole pressures (BHP) are critical
for a reservoir engineer. They enable the engineer to forecast reserves,
evaluate reservoir properties, optimize production or identify production
problems. For subsea wells, because of the large capital investment and high
productivity, the importance of bottomhole pressures is even greater. Because
of this, almost all subsea wells have both downhole and wellhead permanent
gauges installed. Unfortunately, oftentimes downhole permanent gauges do fail,
and until recently this left the engineer with little recourse in gathering the
BHPís necessary for reservoir analysis.
To solve this problem, DRC has introduced a new service: the
conversion of non-SPIDR wellhead pressures from subsea wellheads to bottomhole
conditions. This service has two limitations, the well must be a valid
candidate for surface testing, and the wellhead gauge data must be of
sufficient quality for interpretation. A brief phone call with a DRC engineer
can determine if the well is a good candidate for surface testing. In general,
the following types of wells are valid candidates:
- Dry
Gas Wells
- Gas
Wells in Mist or Annular Flow producing above the critical unloading
velocity
- Undersaturated
Oil Wells (above the bubble point)
If the well is a good candidate for surface testing, then a
data set can be submitted to DRC for evaluation. DRC will evaluate the data
quality based on a number of factors. Among these are:
- Accuracy
- Repeatability
- Resolution
- Sampling
Frequency
If the data is interpretable, DRC will process the file to
bottomhole conditions. There are many advantages gained by this service. First,
the engineer has access to DRCís 23+ years of experience converting and analyzing
pressure-transient data from wellhead measurements. In this time, DRC has
developed proprietary frictional loss models for calculating pressure drop and
a proprietary wellbore cooling model to account for changing fluid density
during a pressure build-up. These models were developed in conjunction with
hundreds of downhole data sets on a wide range of wells.
By introducing this service, DRC hopes to provide value to
its customers by enabling them to continue to make informed decisions to best
optimize productivity and reservoir performance from their subsea wells even
after their downhole permanents have failed. Additionally, subsea wells provide
a unique opportunity for fine-tuning DRCís models. Concurrent data files of
subsea wellhead and downhole permanent pressures can be used to calibrate DRCís
model for the specific well. This calibration can be applied going forward to
data sets recorded after the downhole permanent fails as well as to other
similar wells in the same field.
