Summary
This paper deals with a case study of a high-rate sour-gas field in
Pakistan. The paradigm shift from sand control to sand management boosted
production rates by more than 100%. At the same time, higher wellhead pressures
and less drawdown will extend field life and ultimate recovery. Major
investments such as wellhead or field front-end compression could be delayed
and, thereby, appreciable cost savings could be generated.
After initial well test results, it was concluded that all wells in the
Miano gas field will need gravel-pack installations to protect completion and
surface equipment. Only after 2 years of production at high rates in excess of
50 MMscf/D was it found that the installed gravel packs reduce flow rates
severely. The paper will demonstrate how, through reinterpretation of data that
formed the basis for the previous decision to gravel pack the wells and a
thorough evaluation of sand-production potential and risks, a successful
sand-management strategy was deployed. Existing gravel packs were perforated,
and later production wells have been completed without gravel packs beforehand.
Rigorous monitoring of surface equipment by means of erosion probes, ultrasonic
wall-thickness measurements, and choke-performance charts has been introduced
to ensure safe production operations.
Introduction
From drillstem tests (DSTs) on exploratory and appraisal wells in the Miano
gas field, it was predicted that sand production would occur during field
production. In consequence, a sand-control strategy was formulated on the basis
of complete sand exclusion, because zero sand-production tolerance was believed
to be mandatory in high-rate gas wells. To achieve this goal, all wells were
completed with gravel packs inside the 7-in. liners. These internal gravel
packs (GPs) consisted of two joints of 4½-in. wire-wrapped screens, of either
12- or 20-gauge slot width based on grain-size analysis from produced-sand
samples, packed with 20/40 US mesh and 12/20 US mesh gravel sand, respectively.
A strong focus was maintained on applying best practices during perforation and
gravel-pack operations. These included underbalanced perforations [800 to 1,000
psi underbalance, 4½-in. tubing-conveyed perforation guns, 12 shots per foot
(spf)], cleanout flow before gravel-pack operations, pickling of the
gravel-pack string, pregravel-pack acidizing of the perforations, filtering of
all fluids down to 4-µ particle size, and ensuring proper packing by use of
squeeze and circulating modes. A maximum value for differential pressure
resulting from damage and turbulence skin across the gravel-pack completion was
set at 500 psi for the anticipated maximum rate of 50 MMscf/D, on the basis of
findings from literature research. Initial production rates were as expected,
but soon after start of production, the first well-performance problems
occurred. The flowing wellhead pressures (FWHPs) declined faster than expected
from material-balance calculations. It was quite evident, and confirmed by
pressure-buildup analysis, that skin was increasing in the downhole completion.
This was attributed to reservoir fines being mobilized and produced into the
gravel pack and thereby plugging it. The safe limit of 500-psi maximum drawdown
was soon exceeded in an effort to maintain production at the committed
level.
Acid-stimulation treatments of gravel packs worked well to reduce the skin,
but the effects were not long-lasting. Finally the original sand-control
strategy of complete sand exclusion was abandoned and a sand-management
strategy formulated. In principle, this allows the production of certain
quantities of sand within the limits of safe production operations.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
3 January 2006
- Revised manuscript received:
30 October 2006
- Manuscript approved:
16 January 2007
- Version of record:
20 June 2007
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