0.6 0.4 0.2 0
The flow test on a core sample from the Heidrun field was typical of the procedure.14
Al Si 0 100 Time, min
0.6 0.4 0.2 0
Al and Si without secondary and tertiary reactions
Al Si
0 100 Time, min
> Reaction time. Longer reaction time increases aluminum [Al] concentration in the ef uent, but silicon [Si] concentration rst increases from zero, then decreases, for both mud-acid (top) and ClayACID treatments (bottom). The model curves show that excluding secondary and tertiary reactions, as could happen in a short core test, could lead to incorrect predictions.
200 300
Al and Si without secondary and tertiary reactions
200 300
A
small formation-core plug, 3.73 by 6.4 cm [1.47 by 2.5 in.], obtained from a well near the one to be treated, was saturated with simulated forma- tion brine and flushed alternately with laboratory oil and brine until the effluent was clear. A laboratory engineer heated the core to reservoir temperature and flowed prefiltered test fluids through the core with a 1,000-psi [6.9-MPa] backpressure. This pressure kept any generated carbon dioxide [CO2] in solution. The Heidrun field study used a 9/1 mud-
acid— 9% HCl and 1% HF— and a ClayACID treatment. Flow rate and differential pressure data recorded every 30 s allowed calculation of permeability throughout the test. The engineer collected effluent in 10-mL plastic tubes on a regular schedule and noted any fi nes in the sam- ple. After fi ltering and diluting with nitric acid (continued on page 64)
6. Thomas RL and Crowe CW: “ Matrix Treatment Employs New Acid System for Stimulation and Control of Fines Migration in Sandstone Formations,” paper SPE 7566, presented at the 53rd SPE Annual Technical Conference and Exhibition, Houston, Texas, October 1– 3, 1978; also in Jou rnal of Petroleu m Technology 33, no. 8 (August 1981): 1491– 1500.
7. Al-Dahlan MN, Nasr-El-Din HA and Al-Qahtani AA:
The new Virtual Lab simulator overcomes the problem of unrepresentative geometry and pro- vides guidance for successful matrix acidizing in sandstone reservoirs. It is the foundation of a system for designing acid treatments that prop- erly accounts for the cylindrical geometry around a wellbore (see “ A New Stimulation Process,” page 62).11
In addition, Schlumberger
has created a large, proprietary database of reaction kinetics and thermodynamics to use with this simulator. This database saves clients time and money because additional tests are necessary only when a formation or a new acid formulation contains compounds that are not in the database. The need for new tests has become less common as the database has fi lled with reaction parameters.
Formation mineralogy can be obtained from either whole core or sidewall cores. A short-core
fl ow test gives an estimate of the surface area of the reacting minerals in a formation. This test also provides information about core permeabil- ity and the effect of an acid on permeability as pore-blocking material dissolves. Short-core tests alone do not provide suffi cient information for determining an acid treatment, but a short- core test provides data necessary to model
reactions using the Virtual Lab simulator.12 Numerous treatment designs can be tested in the simulator, and Virtual Lab results will indi- cate the best design for fi eld conditions.
From Laboratory to Field
Central to any successful acidizing treatment is accurate information about
the reaction
chemistry relating to formation minerals. The literature contains much of the relevant thermo- dynamic-equilibrium data. However, most publicly available reaction-kinetics data are from tests obtained at temperatures below fi eld matrix acidizing conditions. Schlumberger labo- ratories performed batch-reactor tests at a wide range of temperatures to create an extensive proprietary database.13 The database of reaction-kinetics data reduces the number of fluid formulations that it is necessary to test. However, usually at least one core-fl ow test is recommended to determine the reactive surface area of minerals in the formation represented by the core. More than 50 core-flow tests have been performed to validate the Virtual Lab software. This database also provides analogs for future cases in which core material is not available.
“ Evaluation of Retarded HF Acid Systems,” paper SPE 65032, presented at the SPE International Symposium on Oil eld Chemistry, Houston, Texas, USA, February 13– 16, 2001.
8. Nasr-El-Din HA, Hopkins JA, Shuchart CE and Wilkinson T:
“ Aluminum Scaling and Formation Damage Due to Regu- lar Mud Acid Treatment,” paper SPE 39483, presented at the SPE International Symposium on Formation Damage Control, Lafayette, Louisiana, USA, February 18– 19, 1998.
9. Thomas et al, reference 5.
10. Gdanski R: “ Fractional Pore Volume Acidizing Flow Experiments,” paper SPE 30100, presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, May 15– 16, 1995.
11. Ziauddin M and Robert J: “ Method of Optimizing the Design, Stimulation and Evaluation of Matrix Treatment in a Reservoir,” U.S. Patent No. 6,668,992 B2 (December 30, 2003).
12. Ziauddin M, Gillard M, Lecerf B, Frenier W, Archibald I and Healey D: “ Method for Characterizing Secondary and Tertiary Reactions Using Short Reservoir Cores,” paper SPE 86520, presented at the SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, February 18– 20, 2004.
13. Ziauddin M, Frenier W and Lecerf B: “ Evaluation of Kaolinite Clay Dissolution by Various Mud Acid Systems (Regular, Organic and Retarded),” presented at the 5th International Conference and Exhibition on Chemistry in Industry, Manama, Bahrain, October 14– 16, 2002.
Hartman RL, Lecerf B, Frenier W and Ziauddin M: “ Acid Sensitive Aluminosilicates: Dissolution Kinetics and Fluid Selection for Matrix Stimulation Treatments,” paper SPE 82267, presented at the SPE European Formation Damage Conference, The Hague, The Netherlands, May 13– 14, 2003.
14. Ziauddin M, Kotlar HK, Vikane O, Frenier W and Poitrenaud H: “ The Use of a Virtual Chemistry Laboratory for the Design of Matrix Stimulation Treatments in the Heidrun Field,” paper SPE 78314, presented at the SPE 13th European Petroleum Conference, Aberdeen, Scotland, October 29– 31, 2002.
Spring 2004
61
Concentration, mol/kg
Concentration, mol/kg
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