Some challenges related to geomechanical modeling at shallow depths

UDK: 622.
DOI: 10.24887/0028-2448-2017-8-99-102
Key words: minifrac, stress field, maximum horizontal stress, well caving-in
Authors: O.V. Salimov (TatNIPIneft, RF, Bugulma)

Diagnostic fracture injection tests have been conducted in two appraisal wells targeting Sheshminskian deposits of the Olkhovsko-Yuzhno-Chumachkinskoye uplift with the aim to determine formation fracture pressure. Fracturing fluid, namely, water containing 2% KCl, has been injected through the casing string (without packer) using Fidmash H-507 low-capacity pumping unit. No test proppant packs has been used.

According to results, conventional minifrac analysis in shallow wells does not always yield reliable information. It should be combined with fall-off, step-rate, and impulse fracture injection tests, hydraulic impedance measurements and other research methods. For shallow depth (up to 100 m) conditions, it is necessary to measure the full stress tensor; therefore, borehole imagers and scanners should also be applied.

Maximum horizontal stress based on minifrac test data has been evaluated.  The value derived from Haimson and Fairhurst equation is consistent with the stress field. It has been found that Ufimian rocks compressive strength cannot exceed 4.77 mPa, which is almost two times lower than the estimate for sandstones from McNally equation. In all cases, it is required to check if the resulting stress values fall within the stress field. Results that fall outside the field are considered gross errors. Moreover, stress field deployment helps make conclusions on the compliance of estimated stress values and rock strength properties. The estimates can provide a clear understanding of hydraulic fracture orientation and the tectonics of the area.


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