Summary
Pressure testing blowout preventers (BOPs) with synthetic-based mud (SBM)
requires lengthy testing times resulting from pressure/volume/temperature (PVT)
influences associated with SBM. PVT influences are especially pronounced in
deep water and high-pressure test environments.
A project was initiated in an effort to better understand the mechanics that
transpired during BOP testing. The primary objective was to demonstrate that
the pressure decline was quantified by the fluid-PVT behavior, mechanical
influences and thermodynamics of pressurization, and subsequent cool down. The
secondary objective was to demonstrate that the pressure decline resulting from
a leak could be reliably and efficiently detected with high-resolution pressure
data.
It was theorized that the gradual decay of pressure during the shut-in phase
was a result of the heat added to the system during pressurization. To pursue
this investigation, real-time PVT data were gathered at the cementing-unit (CU)
suction and discharge while testing the BOP. In addition, pressure/temperature
(P/T) gauges were placed in the drillstring—one at the drill floor, one at the
midpoint between the drill floor and the BOP, and one above the BOP stack. The
gauges confirmed significant heat up as the system was pressured up for each
test. The pressure decreased as the system cooled back toward the ambient
temperature. Modeling techniques were developed to understand the system
response.
This paper presents salient aspects of data acquisition, data
interpretation, and modeling techniques. Results demonstrate the potential
to significantly impact the industry, with respect to safety, time, and costs
for BOP testing.
Introduction
The challenges of obtaining valid BOP pressure tests in an efficient manner
have increased because of greater water depths, deeper drilling horizons, and
higher test pressures. Fig. 1 shows the important components involved in
testing a subsea BOP stack. The system is a pressure vessel comprising the test
line from the CU and the drillpipe from the surface of the rig to the BOP
stack. In this work, the capacity of the pressure vessel is referred to as the
test volume. The valves in the BOP stack are tested in sequence by closing each
valve and then pumping fluid from the CU into the test volume until a target
pressure is reached (the pumping phase). At the target pressure, pumping stops
and the test volume is closed until a test is deemed valid (the shut in phase).
In deepwater wells, the duration of the shut-in phase can be as long as 45
minutes when SBMs are used.
Regulations state that a test is valid when the required pressure is held
steady for 5 minutes.1 Data from a BOP test are typically recorded on a 4 hour
circular-chart recorder, as shown in Fig. 2. Test validation established by the
pressure trace on a chart recorder is based on individual judgment. Often, a
test must be repeated when visual inspection of the chart-recorder trace deems
it invalid. Frequently, test durations are longer than necessary to ensure a
valid test. The chart recorder (patented more than 100 years ago2) is still
used on the majority of the rigs today. However, recent advances in digital
technology and the relative ease of data processing with inexpensive
personal-computer technology create a clear opportunity for improvement in the
recording, analysis, and validation of BOP tests.
During a recent BOP test, the reasons for the gradual decay of pressure with
time were examined. Field experience and anecdotal evidence suggest that test
durations are considerably longer with SBM than with water based muds.
Discussions with rig personnel and engineers indicate that though “pressure
decay” is recognized as a characteristic deepwater phenomenon, it has not been
rigorously examined. Further discussions imply that the test duration can be
significantly optimized if the physical mechanisms that control the P/T
response of the test volume during the different phases of testing are
identified and quantified. As a result of the numerous benefits from a
reduction of test duration, a project to understand the physics of the BOP-test
process was initiated.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
19 May 2004
- Revised manuscript received:
7 July 2005
- Manuscript approved:
2 October 2005
- Version of record:
15 December 2005
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