Summary
When a gravel-pack completion fails, it usually results in loss of
production for the interval until a workover rig is available to recomplete the
well. This paper describes two recent successful rigless interventions
completed in offshore multizone completed gas wells in the Adriatic Sea. A key
factor in the success of these remedial treatments has been the application of
two different chemical technologies applied to the proppant while it is being
pumped into the well.
In the first well, production loss was caused by fines migration (fine silt
≤ 44 microns) into the 40/60 gravel-pack sand, which completely plugged the
screens and gravel. An intervention was performed which included sealing the
existing completion and then reperforating the premium screen using wireline
guns. The interval was then fractured with a tip screenout design and a
through-tubing screen was placed across the perforated screen. Critical to this
treatment was the use of a surface modifying agent to prevent fines migration
and plugging of the gravel pack. Production results have confirmed the
correctness of this technical choice.
A different problem existed with the second well, which had a frac-pack
completion; in this case, a screen failure resulted in formation sand and
proppant being produced to the surface. The well was refractured through the
hole in the screen and a newly developed resin was applied to the proppant to
lock the proppant into place and thus repair the damaged screen without
restricting the flow area with a secondary inner string. Unlike conventional
resin-coated proppant at low temperatures, this new type of resin does not
require confinement pressure and/or the use of chemical flushes to cause the
resin to set. Since the intervention, the well has been producing sand-free at
very close to the original productivity index (PI).
In both cases, these treatments have put back into production wells that in
the past could not have been restored to production except through recompletion
using a workover rig. This paper will describe the treatment design, well
operations performed, materials used, and production performance of each well,
comparing each aspect with the case of a conventional recompleted well.
Overview of Sand-Control Work in the Adriatic
Background. Sand control has been necessary in the Adriatic gas
fields since the first wells were brought into production. Initially, the best
option was conventional low-density gravel pack in the open hole (OHGP). With
pressure depletion, and as the reservoirs drilled became more complex,
cased-hole completions became necessary to provide isolation between the main
layers, to prevent water production, and to enable layers to be fully depleted.
These cased-hole completions were typically multilayer completions with dual
strings and three to five or more individual intervals completed per well.
Reservoir lithology consists of alluvial sands, turbiditic deposits that
were laid down by the Po River in the Pliocene era. The depth of the gas fields
varies from 800 m true vertical depth (mTVD) to over 3500 mTVD. These sands are
almost claylike in nature and are highly plastic, interbedded with stronger
shale bands which separate the main layers. Interval height varies from 2 to 30
m, with permeability ranging between 5 and 500 md to gas. Pore gradients also
vary widely, from overpressurized sand with a pore pressure of 1.6 sg to
partially depleted structures with pore pressures as low as 0.2 sg. As the main
area of the original fields has become increasingly depleted, the current new
completions are sidetracked from existing platform wells into smaller,
less-depleted sand lenses. These structures are siltier and contain sands of
lower quality. In most parts of the world, this type of formation would not be
considered for commercial extraction of the gas resource.
Originally, in these clayey formations, cased-hole gravel packs were
installed, with limited success. In 1995, the frac-pack technique was first
applied to reduce the skin effect created by the gravel pack. These first
treatments used brine or a linear gel system (Maroli et al. 1996). Gel
technology has improved in recent years, and the frac packs are now performed
using viscoelastic gel systems (Cobianco et al. 2005), which permit the use of
this technique at sand concentrations greater than 12 ppg. A significant
difference in fracturing practice in the Adriatic gas sands, as opposed to
other areas of the world, has been the use of low-efficiency fracturing fluids.
Adriatic-type sands are generally highly plastic formations which are easily
fractured at very low pump rates. Therefore, to prevent the fracture from
propagating out of the layer, the viscosity of the fluid, the leakoff rate, and
the pump rate need to be carefully controlled. As a large number of such
treatments have been completed in the past, the process has become highly
refined (Mathis et al. 2002).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
20 October 2005
- Meeting paper published:
15 February 2006
- Revised manuscript received:
29 March 2006
- Manuscript approved:
10 April 2006
- Version of record:
20 February 2007
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