The requirement for screened and gravel packed completions in horizontal wells has led to the development of alternative stimulation fluids and placement techniques.
This paper describes a common damage mechanism in this type of completion and the remedial cleanup procedures developed.
While horizontal wells do not normally require stimulation for primary production reasons, it is often necessary to clean up the drill in fluid (DIF) filtercake.
A placement technique was developed that includes a coiled tubing, controlled injection technique (CIT) in combination with a true fluidic oscillator (TFO) to aid in placing a modified propriatory scale and DIF filtercake solvent (PSDS) fluid behind the screen into the gravel packed annulus.
The placement technique is supported with surface testing results of two standard jetting tools vs. a TFO for the placement of the stimulation fluid. This PSDS fluid was specifically modified to improve the effectiveness of removing filtercakes layed down with a drill solids-laden DIF mud sample from the field. The presence of formation-drilled solids results in a much more tenuous filtercake than one created with a clean DIF.
The technique was developed for a long horizontal gravel- packed North Sea well, however the procedure has global applications.
The Alba field is located in block 16/26 of the UK sector of the North Sea. The Alba reservoir (Eocene sand) is a highly permeable, unconsolidated sand body with lenses of reactive shales within the reservoir sand. It is overlain by a bed of impermeable, highly reactive shale.
The nature of the reservoir dictated that development would be best achieved by openhole completions and highly deviated or horizontal reservoir sections with the productive interval being sited near the top of the sand body. It was expected that drilling in the reservoir section would be through reactive shales and unconsolidated sand, thus three high priority requirements were perceived to be shale inhibition and borehole stability while drilling and sand exclusion while producing. These aspects have continued to be of great importance throughout the evolution of the development of this reservoir.
Since 1994, three drill-in fluid systems have been used in this field.
Synthetic oil-based mud (SBM)
Sized sodium chloride water-based system
Sized calcium carbonate water-based system
Also, three main sand-control techniques have been used for production:
Dual wire-wrapped, prepacked, stand-alone screens
Premium stand-alone screens
Premium screens and gravel pack. To protect the screens in the wells from significant moving shales a pre-drilled liner is installed prior to running the screens.
In respect of open-hole gravel pack two variations have been used:
Water-based drill-in fluid and water-based gravel- carrier fluid
SBM drill-in fluid and water-based gravel-carrier fluid
Well A-44 was drilled and completed with a gravel pack (inside the pre-drilled liner) on December 28, 2001 and brought onto production on January 4, 2002. The well had an initial productivity index (PI) of 75 to 80 bbl/D/psi.
The well flowed dry oil for six days but on January 10 began cutting water at 1.8%. The water-cut steadily increased over the next three days to 9% and on January 13 it jumped to 18%. Another rapid increase was noticed again on January 15 when the water-cut increased from 18% to 28%.