well intervention to replace gas lift system components. The scale deposits on the gas lift mandrels were so severe that a 71-mm [2.8-in.] gauge cutter could not pass below 875 m [2,870 ft]. Faced with concerns about corrosion and possible damage to the formation using conven - tional acidizing, the operator chose to treat the scale problem with an HACA chelant. The treatment goal was to use a mild fluid that would remove carbonate scale and not damage the sandstone formation. The well was treated with the HACA chelant using coiled tubing with a rotating jet to spray and soak the areas containing the gas lift components. Following treatment, the fluids used in the operation were displaced with water and the gas lift system was restarted. A gauge cutter was run through the entire length of the wellbore and encountered no obstructions. After treatment, oil production increased to 402 m3/d [2,528 bbl/d], indicating removal of scale and possible stimulation of the sandstone.
As illustrated by the treatment in this West African well, using chelants in sandstones with conventional fluid placement plans is often quite effective. Schlumberger has extended the utility of these new chemicals in sandstones with its OneSTEP technology. This technology uses a unique chelant fluid and simplified placement techniques to stimulate production with less risk of damage and precipitates. This fluid substantially reduces the number of required stages during acidizing. Petronas Carigali recently employed this technology to stimulate one of its offshore wells in Southeast Asia. The Tiong field lies off the western coast of Malaysia in 77 m [253 ft] of water (previous page, top). Discovered in 1978, the field began producing oil and gas in 1982. Tiong is a sandstone formation with a high bottomhole temperature—109°C [228°F]. After experiencing declining production and noting a high skin value for the formation, Petronas evaluated several Tiong wells as candidates for acidizing treatment.25
Tests on core samples from the 24. Frenier et al, reference 10.
25. Skin is a dimensionless factor calculated to determine the production efficiency of a well by comparing actual conditions with theoretical or ideal conditions. A positive skin value indicates some damage or influences that are impairing well productivity. A negative skin value indicates enhanced productivity, typically resulting from stimulation.
26. Tuedor FE, Xiao Z, Fuller MJ, Fu D, Salamat G, Davies SN and Lecerf B: “A Breakthrough Fluid Technology in Stimulation of Sandstone Reservoirs,” paper SPE 98314, presented at the SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, February 15−17, 2006.
Displacement
candidate wells indicated formation damage from kaolinite fines and calcite. Petronas selected a well for the acidizing tests and chose the OneSTEP system for its operational simplicity and use of chelants (below).26
This combination
marries a low risk of secondary and tertiary reactions that might cause precipitation with fewer fluid stages and simplified logistics. Other benefits accrue from low corrosion rates and a good health, safety and environmental footprint.
Prior to carrying out the treatment, Schlumberger calibrated the Virtual Lab model using results from well testing before running simulations. The well tests determined formation dissolution kinetics, measured physical proper ties of the rock and compared treatment options in radial-flow tests. The final choice for the treatment fluid at Tiong was a chelant plus other additives. With this chelant fluid, the OneSTEP treatment was carried out at the Tiong well in April 2007. No operational problems were encountered and the test was successful—oil production increased by a factor of four and gas production by a similar amount (previous page, bottom).
For Petronas, stimulation of oil and gas production was not the only benefit of the OneSTEP technique. This simplified acidizing operation saves significant rig time, resulting in lower cost. In the Tiong treatment, the operational time saved was measurable—
Treatment Stage Stage 1
Conventional Fluid Placement Step
Stage 2
1 2 3 4 5 6 7 8 9
Stage 3 Stage 4 Stage 5
10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Fluid Type
Brine preflush Acid preflush Main treatment Overflush Diverter
Brine preflush Acid preflush Main treatment Overflush Diverter
Brine preflush Acid preflush Main treatment Overflush Diverter
Brine preflush Acid preflush Main treatment Overflush Diverter
Brine preflush Acid preflush Main treatment Overflush Brine
Treatment Stage
Stage 1 Stage 2 Stage 3 Stage 4 Stage 5
Displacement
OneSTEP Fluid Placement Step
1 2 3 4 5 6 7 8 9
10
Fluid Type
Main treatment Diverter
Main treatment Diverter
Main treatment Diverter
Main treatment Main treatment
Diverter Brine
> OneSTEP technique. Conventional sandstone acidizing—usually with HF—is a complex process involving several pieces of equipment and many sequential steps (left). As many as six acid tanks and two brine tanks may be employed, and five stages with 25 steps may be carried out, depending on the type of diversion technique. In conventional treatment, brine preflush removes and dilutes acid- incompatible components. Similarly, HCl preflushing removes calcites prior to the main HF treatment. In contrast, OneSTEP treatment typically uses only two acid storage tanks and one brine tank and requires significantly fewer treatment steps (right). This treatment simplicity is a result of two factors—use of a chelant instead of HF and employment of Virtual Lab predictive software before the job is started. The chelant eliminates problems with secondary and tertiary reactions, while Virtual Lab testing ensures that any potential problems are addressed before the job begins.
Winter 2008/2009 61
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