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The injection of water-based hydraulic fracturing fluids (HFF) into tight shale gas formations can significantly increase the possibility of fluids interacting with the reservoir rocks, causing damage to gas flow capacity of shale stimulated fracture network due to stress sensitivity after water invasion. As a result, it is significant to clarify the high-pressure fracturing fluids invasion within the shales, which helps to understand stress-dependent permeability alteration for the fracturing fluids-invaded shales. However, it is difficult to characterize the invasion and distribution of high-pressure fracturing fluids within shale reservoirs. In addition, the fracturing fluids invasion mechanisms for shale stimulated fracture network are not yet clear. Therefore, in this paper, by means of innovatively using low-field high pressure gradient nuclear magnetic resonance online displacement, the high-pressure fracturing fluids invasion within shale at different times and its controlling mechanisms were revealed. The results were presented as follows: it showed that only one clear single peak structure existed on the left side of NMR-T2 spectrum curves for shale matrix imbibing water, indicating that water absorption mainly occurred into shale matrix micro-nano pores during water intrusion process. The T2 spectrum surrounding area and the right-direction-shifting rate of left peak were positively correlated with matrix permeability, water injection pressure difference, and clay mineral content. It indicated that the filling of water into shale matrix would firstly occur in micro-nano pores, followed by large pores observed from NMR-T2 spectrum curves. With the increase in invasion durations, water saturation propagated in a progressive pattern of upward and downward fluctuations along the direction of water invasion within deeper matrix. It was found that the water invasion distance within shale matrix increased with invasion time, following the power function. The water imbibition distance generally could reach between 4cm~6cm within shale matrix cores of 0.000654mD~0.0068mD at high driving pressure of 15MPa~35MPa. The degree of high-pressure water invasion in shale matrix was linearly positively correlated with matrix permeability, water injection pressure difference, and clay mineral content. This paper gave insights into the high-pressure fracturing fluids invasion within shales. It provided scientific reference for reducing the stress sensitivity and alleviating reservoir permeability damage after hydraulic-fracturing process for shale gas wells.
| Country | China |
|---|---|
| Water & Porous Media Focused Abstracts | This abstract is related to Water |
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