The use of slug test to describe vertical variations in hydraulic conductivity

1. The use of slug test to describe vertical variations in hydraulic conductivity Journal of Hydrology 156(1994) James J. Butler,Jr.*,Geoffrey C. Bohling, Zafar Hyder, Carl D. McElwee 指導教授: 陳家洵 老師 報告者: 謝頤祥 報告日期: 2010.12.23

2. Purpose & Scope of Work Purpose:Using a multilayer model to evaluate the influence of vertical heterogeneity on the MLST (multilevel slug tests). Scope of Work: • To evaluate the effects of aspect ratio, partial penetration ratio,specificstorage, anisotropyandpacker length on the K(z) estimated from slug-test data. • To assess the nature of vertical averaging in slug tests in layered aquifers.

3. Li The Multilayer Model p,packer length b,test interval length rc,radius of well rw,radius of test interval rsk,radius of skin

4. Major Assumptions • Confined aquifer, low-Ki. (2x10-5m/s<Ki<2x10-4 m/s) • Layered: perfect stratification; each layer is homogeneous, anisotropic (Kr≠ Kz), and of constant thickness. • Skin zone of finite thickness (rsk); able to handle both positive (Ksk<Kaqf) and negative (Ksk>Kaqf) skin effects. • Multilevel slug test (MLST): partial penetration by a double-packer system.

5. Data Analysis Procedures 1.Using the multilayer model to generate H(t); pressure response to the MLST 2.Analyzing H(t) to estimate K values at different depths 3.Data analysis uses the shape factor given by Hvorslev (1951); KHV

6. layer K(z) layer layer 0 1x10-4 2x10-4 3x10-4 Influence of Layering on K(z) b=1.25m<LA=LB=5m 1.Inaccurate K(z) close to interfaces. 2.Due to vertical flow across the interfaces.

7. b=L K(z) K(z) K(z) K(z) K(z) (b=0.156m) (b=5m) Influence of the Aspect Ratio b/rw

8. K(z) K(z) K(z) Influence of the Partial Penetration Ratio L/b

9. Influence of Specific Storage Effect and Anisotropy • Specific storage has a considerable influence on K(z) estimated in layered systems, -in contrast to full screen slug tests in homogeneous conditions where SS has little influence (Cooper et al., 1967) • Anisotropy (Kr > Kz) does not introduce significant different results.

10. L=b=0.312m A*: no skin A: skin L=b=0.156m B*: no skin B: skin Influence of Low-K Well Skin K(z) K(z) K(z) K(z)

11. 6.25 0.22 3.12 0.11 6.25 0.11 b/rw=50 rsk=0.0m 50 0.11 25 0.11 Influence of High-K Well Skin 1.overestimate K(z) 2.the smaller the aspect ratio: -the less accurate of K(z) -the smaller range of K(z) 3.influence of rsk >> influence of b/rw K(z) K(z) K(z) K(z) K(z) K(z)

12. 3 5 5 2 3 5 3 15 5 3 3 5 3 2 5 5 3 Case 1 Case 2 Case 3 Time(s) Full-Screen Tests in Three Different Layered Conditions H(t)/H0

13. Layer-Thickness Weighted Mean • The possible vertical heterogeneity is averaged out in a full screen test. • When b/rw>200, the estimate of K is K.

14. Percent difference Effect of Packer Length case 1:layered skin case 2:homogeneous case 3:homogeneous no skin 0.75 case 4:layered

15. Conclusions (I) • For accurate estimation of K(z), b should be kept small: b/rw>200 K~K; b~L inaccurate K(z). • Neglecting the low-K skin will underestimate K(z) with less vertical variation in comparison to the actual. (3) Neglecting thehigh-K skin will overestimate the K(z) with less vertical variation in comparison to the actual. (4) Packer length > 0.75m.

16. Thanks you for your attention

17. Alternating High- and Low-K Filter Pack B* with no skin.

18. Recent Field Experiences • Permeameter analyses of cores from this same formation (Jiang, 1991; Butler and McElwee, 1992) have shown that repacked cores have considerably higher conductivities than the original sampled cores, indicating the collapsed zone would probably form a skin of higher conductivity than the formation as a whole. • The success of the Melville et al. (1991) program appears to be largely due to the thin well skin coupled with unremoved drilling muds that are apparently preferentially impeding vertical flow. • It is clear that well drilling and development procedures cannot be overemphasized in the planning of multilevel slug tests.