پایش مقاومت ویژه الکتریکی سنگ حین آزمایش مقاومت فشاری تک‌محوری در آزمایشگاه

نوع مقاله: مقاله تحقیقی‌ (پژوهشی‌)

نویسندگان

دانشکده مهندسی معدن و متالورژی، دانشگاه یزد

چکیده

ارتباط خواص مکانیکی سنگ‌‌ها با خواص فیزیکی (روش‌های ژئوفیزیکی)، موضوعی است که اخیراً مورد توجه محققان قرار گرفته است. در این میان روش‌های الکتریکی و لرزه‌‌ای بیشترین کاربرد را دارند. در این مقاله، عبور جریان الکتریکی در حین اِعمال تنش فشاری در آزمایشگاه بررسی شده است.
پس از نصب الکترودهای مخصوص روی 7 نمونه مغزه تقریباً اشباع، آزمایش مقاومت فشاری تک‌محوری و عبور جریان الکتریکی از نمونه‌‌ها به‌طور هم‌زمان صورت پذیرفت و تغییرات مقاومت ویژه الکتریکی در حین بارگذاری اندازه‌‌گیری شد.
ماسه‌‌سنگ‌‌ها افزایش مقاومت ویژه و سنگ آهک‌‌های فسیل‌‌دار کاهش مقاومت ویژه در سراسر محدوده افزایش کُرنش‌‌ نشان دادند. تراورتن‌‌ها و سنگ آهک با افزایش کُرنش‌‌ در ابتدا افزایش مقاومت ویژه و سپس کاهش مقاومت ویژه نشان دادند. رفتار مقاومت ویژه حین بارگذاری به بسته شدن منفذها (کاهش تخلخل) درکُرنش‌‌‌‌های کم و ایجاد درزه‌‌های القایی (افزایش تخلخل) در کُرنش‌‌‌‌های بیشتر ارتباط داده شد.
 

کلیدواژه‌ها


عنوان مقاله [English]

Electrical resistivity monitoring of rock samples during uniaxial compression test

نویسندگان [English]

  • Ahmad Ghorbani
  • Hossein Ali Ghari
  • Afshin Namiranian
چکیده [English]

The study of the physical properties (geophysical methods) of rocks associated with its mechanical properties has recently received lots of attention. Recent studies show that geophysical methods especially the seismic and geoelectric methods are able to estimate the mechanical parameters and recognize their spatial variations, including anisotropy. Meanwhile, electrical and seismic methods are the most used one.
Electrical measurement is one of the non-invasive geophysical methods commonly used by engineers working in various fields such as mining, geotechnical, civil, underground engineering as well as oil and gas mineral explorations. This method can be applied both in laboratory and in the field. Numerous scientists have focused on the relation between resistivity and porosity. However, there is a very limited study on the relation between the electrical resistivity and the rock properties apart from porosity.
In this paper, changes in the electrical conductivity of rocks during a uniaxial compression test were investigated in laboratory. The uniaxial compressive strength, elastic modulus, and density values of the samples were determined in laboratory. We installed special electrodes on seven nearly saturated core samples in order to measure the resistivity. Core samples had a 52-mm diameter and a 110-mm length. Two-electrode as well as four-electrode arrays were both used in resistivity monitoring in laboratory. Using a four-electrode array minimized the undesirable electrode polarization effects. In the four-electrode array, we used two non-polarizing Ag/AgCl electrodes mounted on the core sample. Our laboratory observations showed that there was not any electrode polarization effect. When we used a two-electrode array, the resistivity changes were less than 5 percent compared to a four-electrode array.  In our laboratory investigation, we used different sedimentary core samples including sandstone, fossilioferous limestone and travertine. Maximum resistivity observed for the travertine core sample was less than 12 kohm. During the uniaxial compressive test, deformation measurements were made and the stress–strain curves were plotted. Tangent Young’s modulus values were obtained from stress–strain curves at a stress level equal to 50% of the ultimate uniaxial compressive strength.
Sandstone core samples showed a resistivity increase in the whole strain range. On the contrary, the fossiliferous limestone samples (thin section showed that the sample was composed of tiny calcium fossils in a fine aggregate of micrite cementation) showed a resistivity decrease in the whole strain range. Travertine and limestone showed an intermediate behavior (resistivity increased in the lower strain and it decreased in the higher range). In other words, the onset of new crack formation occurs well inside the quasi-linear part of the stress-strain curve. The quasi-linear portion of the stress-strain curve was the result of a competition between closure of one population of cracks, and the growth of new propagation of the existing cracks.
Resistivity behavior during a uniaxial compression load is closely related to the pores in the lower strain ranges and then to the new induced fractures in higher strains. Our results showed that the electrical resistivity may be a representative measure of the rock properties. Additionally, the effect of certain minerals on the rock’s resistivity must be taken into account. The results indicated that the rock structure had an important effect on the resistivity behavior during a mechanical loading.
 

کلیدواژه‌ها [English]

  • Electrical conductivity
  • uniaxial compression test
  • elasticity modulus
  • Electrical resistivity
  • Petrophysics
فهیمی‌‌فر، ا.، وسروش، ح.، 1380، آزمایش‌های مکانیک سنگ (جلد اول: مبانی نظری و استانداردها)، مرکز نشر پرفسور حسابی، تهران.

ولی، ج.، کاظم‌‌زاده، ع.، آلوکی بختیاری، ح. و اصفهانی، م.ر.، 1389؛ تاثیر شکل هندسی منافذ بر سرعت امواج لرزه‌‌ای در سنگ‌های کربناته مخازن هیدروکربوری، مجلة فیزیک زمین و فضا، دوره 35(3)، 35-49.

Ara, T., Bjorndalen, N., Talabani, S., and Islam, M. R., 2004, Predicting oil reserve in carbonate reservoirs: EEC Innovation, 2, 20–43.

Archie, G. E., 1942, The electrical resistivity log as an aid in determining some reservoir characteristics: Trans. Am. Inst. Min. Metall. Pet. Eng., 146, 54–62.

Bilim, N., Ozkan, I., and Gokay, M. K., 2002, Determination of discontinuities at rock materials by electrical resistance method, in Sensogut., C., and Ozkan, I., eds., Proceedings of the seventh regional rock mechanical symposium Ankara: Kozan Ofset, 121–127.

Brace, W. F., Orange, A. S., and Madden, T. R., 1965, The effect of pressure on the electrical resistivity of water-saturated crystalline rocks: J. Geophys. Res., 70, 5669–5678.

Ghorbani, A., Cosenza, Ph., Revil, A., Zamora, M., Schmutz, M., Florsch, N., and Jougnot, D., 2009, Non-invasive monitoring of water content and textural changes in clay-rocks using spectral induced polarization: A laboratory investigation: Appl. Clay Sci., 43, 493–502.

Glover, P. W. J., Gomez, J. B., and Meredith, P. G., 2000, Fracturing in saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: experimental study: Earth and Planetary Science Letters, 5621, 201-213.

Glover, P. J., Gomez, J., Meredith, P., Hayashi, K., Sammonds, P. R., and Murrell, S. A. F., 1997, Damage of Saturated Rocks Undergoing Triaxial Deformation Using Complex Electrical Conductivity Measurements: Experimental Results: Phys. Chem. Earth, 22 (1-2), 57-61.

Inoue, M.; and Ohomi, M., 1989, Relation between uniaxial compressive strength and elastic wave velocity of soft rock, Proceedings of the International Symposium on Weak Rock: Tokyo, 9–13.

Kahraman, S., 2001, Evaluation of Simple Methods for Assessing the Uniaxial Compressive Strength of Rock: Int. J. Rock Mech. Min. Sci., 38, 981-994.

Kahraman, S., and Albert, M., 2006, Predicting the physico-mechanical properties of rocks from electrical impedance spectroscopy measurements: Int. J. Rock Mech. Min. Sci., 43, 543–553.

Kate, J. M., and Gokhale, C. S., 1998, Electrical

 

 

     resistivity behaviour of sandstone during compression, in Moore, D. P., and Hungr, O., eds., Proceedings of the eighth International Congress IAEG, vol. 1. Rotterdam: Balkema, 543–550.

Kate, J. M., and Rao, K. S., 1989, Effect of large overburden stress on geophysical behaviour of sandstones, in Maury, V., and Fourmaintraux, D., eds.,. Proceedings of the ISRM-SPE International symposium on rock at great depth, vol. 1. Rotterdam: Balkema, 171–178.

Kate, J. M., and Sthapak, A. K., 1995, Engineering behaviour of certain Himalayan rocks, in Daemen, J. J. K., and Schultz, R. A., eds., Proceedings of the 35th US symposium on rock mechanics, Rotterdam: Balkema, 783–788.

McNally, G. H., 1987, Estimation of coal measures rock strength using sonic and neutron logs: Geoexploration, 24, 381-395.

Patnode, W. H., and Wyllie, M. R. J., 1950, The presence of conductive solids in reservoir rocks as factor in electric log interpretation: Petrol. Trans. AIME, 189, 47–52.

Sharma, P. K.; and Singh, T. N., 2008, A correlation between P-wave velocity, impact strength index, slake durability index and uniaxial compressive strength: Bulletin of Engineering Geology and the Environment , 67, 17-22.

Schon, J. H., 1998, Physical properties of rocks, fundamentals and principles of petrophysics: 2nd. ed., Pergamon, Oxford, 583p.

Slater, L., 2007, Near Surface Electrical Characterization of Hydraulic Conductivity: From Petrophysical Properties to Aquifer Geometries-A Review: Surveys in Geophysics, 28, 169–197.

Sousa, L. M. O.; dei Rio, L. M. S.; Calleja, L.; de Argandona; V. G. R.; and Rey, A. R., 2005, Influence of microfractures and porosity on the physico-mechanical properties and weathering of ornamental granites: Eng. Geol., 77, 153-168.

Turgrul, A.; and Zarif, I., 1999, Correlation of mineralogical and textural characteristics with engineering properties of selected granitic rocks from turkey: Eng. Geol., 51, 303-317.

Yasar, E.; and Erdogan, Y., 2004, Correlating sound velocity with the density, compressive strength and Young’s modulus of carbonate rocks: Int. J. Rock Mech. Min. Sci., 41, 871-875.