In North Africa oil fields, numerous old wells have been completed with 4.5- or 5-in. slotted liners. These wells have been producing oil for several years and have a history of stimulation activity performed to clean up near-wellbore damage caused by scale and asphaltene deposits and workover-fluid invasion.
The prevalent technique used for removing near-wellbore damage has been to jet the scale buildup with a jetting nozzle run on coiled tubing (CT) and use an appropriate solvent (i.e., acid or aromatic solvents) for dissolving it. The results from these routine stimulation treatments have been good, with an increase in production two to three times pre stimulation production rates. However, a recent change in stimulation technique involves use of a "fluidic oscillator" (FO) tool. Results from using the FO tool instead of the conventional jetting tool have been significant; the post-stimulation production has improved several times over the previous stimulation gains seen in the same wells.
The advantage the FO tool provides is that it generates pulsating pressure waves. The pressure waves radiate spherically from the point of origin (from the FO tool) and provide 360° coverage. When the pressure wave contacts the formation, the energy is absorbed by the rock matrix, and the continuous cycling of these pressure waves causes the damage to reach its fatigue-failure point. As the damage is removed and the original permeability is restored, the pressure waves can penetrate more deeply into the formation matrix, for more complete damage removal. Benefits are quite evident from the comparative results provided by several wells in the North Africa oil fields. The FO tool technique should help other operators optimize their treatments and achieve these benefits.
One North Africa oil field covers about 2000 km2 with more than 1,000 wells. It has two thick plays, separated by a fault zone, producing from two formations, the Cambrian and Ordovician, at depths up to 10,500 ft. The porosity is low and ranges from 5 to 10% while the average permeability ranges from 0.5 to 1.0 mD but can reach up to 1000 mD in cases where opened fractures are encountered. The oil density ranges from 43.7 to 45° API with the average gas:oil ratio (GOR) of about 200 m3/m3 and formation temperature of approximately 248°F.
Well completions in this field can be openholes, slotted liners, and mixed liners, and cemented and perforated liners with or without concentric pipes. Slotted liners are used in about 80% of the completions (Allouti et al. 1998). They contribute to development of an environment conducive to scale buildup (Gholineezhadm 2006) and the accumulation of clays and fine particles that can choke and plug the liner slots.
Wells in this field have been producing for several years, normally requiring remediation from workover, recompletion, and snubbing operations. In most cases, the well is killed for safe operation using oil-based mud or workover-fluids. These fluids usually invade the formation during workover operations and are incompatible with formation fluids, causing severe damage to the near-wellbore matrix. The most common formation-damage modes in this field are salt deposition, scale deposition, asphaltene deposition, clay and fines migration, emulsions, perforations, and reservoir-pressure evolution.
Near-wellbore treatment procedures have been standardized and include
Tube cleaning and perforation wash with 7.5 to 15% HCl
Preflush containing HCl (7.5% or 10%) or brine
Main treatment with HCl and HF acid for matrix acidizing or aromatic solvent for asphaltene removal
Overflush containing HCl (7.5 or 10%), brine, or light hydrocarbon
Diversion with foam (most preferred technique to help ensure maximum zonal coverage)