10 8 6 4 2
0
400 Damaged zone 300 Fluid invasion
75 gal/ft 100 gal/ft 125 gal/ft 150 gal/ft
0 12 Radius, ft 3
3.6 ft 4.2 ft 4.7 ft 5.2 ft
4 200 100
10% acetic- acid preflush
0 0 50 100 Clay-stabilizing acid 150 200 Volume, gal/ft
> Optimizing treatment volumes. The geochemical model accounts for acetic acid spending, or weakening, as it interacts with formation carbonate minerals. Far from the wellbore, the carbonate is 7% of the formation volume. The radius of formation that is cleaned of carbonate material is much smaller than the invaded radius. Injecting 100 gal/ft [1.2 m3/m] of perforated height cleared carbonate to a greater radius than did a volume of 75 gal/ft [0.9 m3/m] (left). However, additional injection did not signi cantly increase the cleared radius. Using a 100 gal/ft pre ush, the model indicated an optimal treatment using clay-stabilizing acid of 75 gal/ft (right). Beyond that quantity of injected clay-stabilizing acid, skin increased because permeability was destroyed.
250 NH4CI brine 300 350 400
The geochemical simulation tested pre-
fl ushes of both 10% acetic and 5% formic acid to remove near-well carbonate minerals from the formation. The two formulations provided simi- lar skin reduction, so acetic acid was used because it was more readily available at the time. The model indicated the optimal prefl ush volume and the optimal clay-stabilizing acid vol- ume (above). The simulation showed that the borosilicate coating that stabilizes clays extended about 1 ft [0.3 m] into the formation with the optimal clay-stabilizing acid treatment, but that additional clay-stabilizing acid did not extend the protected zone signifi cantly. ChevronTexaco planned the second phase of clay-stabilizing acid stimulations based on optimized acid volumes from the Virtual Lab simulator. Stimulation jobs on one oil-producing well and three gas producers were successful and showed significant production increases (right). This use of the new stimulation design process increased profitability from the stimu- lated wells. The fl uid system, customized for the specific lithology in the Gulf of Thailand wells, provided a lasting solution.
Reacting to the Future
The new stimulation process, including the Virtual Lab simulator, provides a tool to improve well performance in sandstone formations. Sandstone matrix acid treatments are complex, and the success rates are historically low. The new process with the software and proprietary databases as its basis assures a much higher ratio of successful matrix acid treatments.
500 450 400 350 300 250 200 150 100 50 0
B-1 B-2 B-3 B-4
> Production improvement in Gulf of Thailand wells. Matrix acidizing increased oil production in Well B-1 from 0 to 442 B/D [70 m3/d] (green). A comparison of pretreatment (pink) and posttreatment (red) production from three gas wells also shows signi cant improvement.
5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
Determining formation mineralogy is an
important fi rst step in the process. If data such as the ELANPlus Elemental Log Analysis are available, they can be used with the Virtual Lab software. In addition, the growing databases for geochemistry and flow properties will provide more analogs for locations lacking core material. The Virtual Lab software is a general- purpose geochemical simulator and is not
restricted to solving for matrix acidizing in sandstones. The tool could be used for carbonate acidizing, carbon dioxide sequestration and water-compatibility testing. Schlumberger continues to expand the reaction database, increasing the variety of problems
that
this geochemical software can solve for the industry.
— MAA
68
Oilfi eld Review
Carbonate volume, % Wellbore
Oil production, B/D
Skin
Gas production, MMcf/D
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