Hydraulic fracturing has become a state-of-the-art stimulation technique. It
has been proved over the years that significant production increase can be
obtained by applying the right fracturing technique. Now, the most advanced
techniques of geothermal-energy-recovery systems widely use hydraulic
The following paper presents the experimental results of the tests carried
out on four different compounds using the improved "grooved-plate"
method. The tests have shown a large variation of the tested-thread-compounds
sealing capacity. Starting from the experimental results and the theoretical
analysis of the American Petroleum Institute (API) connection, a useful chart
was built to determine the real connection resistance, on the basis of its
initial makeup torque. The chart offers to engineers involved in the design of
a fracturing process the possibility to estimate the maximum pressure that may
lead to a connection leak.
Most of the published data show that a long fracture is the key to optimum
well stimulation. The desired length of the fracture can be achieved by use of
equipment capable to deliver the right pressure and fluid volume. Because the
hydraulic-fracturing technique can also be applied to old wells, equipped with
standard API connections, the high pressures that are achieved during the
pumping phase require the understanding of leak resistance of API connections.
It has also been proved that during the injection phase, the high pump rate may
lead to additional pressure increase into the well tubulars. The time and
pressure values are two key parameters that may affect the sealing capacity of
the API connection.
Testing the sealing capacity of a casing connection is not an easy task
because it depends on many factors: thread type and form, thread compound,
aging of the thread compound, and makeup-induced stresses. Actually, there are
no standards to evaluate the seal capacity of a thread compound. To date, three
approaches have been found in the literature:
- The fixture designed during the project PRAC 88-51 that consists of two
circular-steel plates having a spiral grove from the center to the exterior
(Wood et al. 1990).
- Full-scale testing of threaded assembly using a high load press (ISO
13678:2000 2000) in which not only the thread compound but the entire
sealing capacity of the assembly is tested.
- Small-sized connections as described by Hoenig and Oberndorfer (2006).
There are many advantages and disadvantages for each one of the methods, but
testing thread compounds separately requires removing all inconsistent
parameters that may affect the evaluation process. The main parameters that may
affect the thread-compound evaluation are the stress/strain state induced
because of makeup and thread tolerances.
The fixture proposed by project PRAC 88-51 offers the advantage of comparing
only the threaded compounds, by neglecting the makeup- and tolerance-induced
errors. This is why it has been considered the use of the same experimental
setup as the one described by Wood et al. (1990). The experimental setup will
be presented in detail in this paper.
© 2009. Society of Petroleum Engineers
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- Original manuscript received:
31 January 2007
- Meeting paper published:
31 March 2007
- Revised manuscript received:
3 March 2008
- Manuscript approved:
17 April 2008
- Published online:
16 March 2009
- Version of record:
1 March 2009