Many studies have assessed microseismic (MS) interpretation, and its source mechanisms in hydraulically fractured shale wells. However, the ability to derive stimulated reservoir volume (SRV) and fracture geometry from MS data is still controversial. This is because MS events not only come from the induced main fractures of the current stage, but they also result from reactivation of natural fractures (NF) or faults, previous hydraulic fractures (HF), stratigraphic boundaries, or other operational noise. MS data of adjacent stages tend to overlap each other severely, masking the real SRV of the current stage. Simulated complex fracture networks using MS data always yield higher production results when compared to actual production. We address these issues with an efficient MS interpretation method.

This method divides the MS events into three windows using an Excel-VBA program: the Pad Window, the Proppant Window, and the Closure Window. These windows are based on the fracture stimulation job record for each stage. The Closure Window includes only the events from the end of pumping until the end of shut-in of the current stage. During the Closure Window, i.e., shut-in time before flowback, leakoff into the formation matrix can cause micropores to enlarge, resulting in MS events with compensated linear vector dipole (CLVD) and isotropic (ISO) sources (crack opening mechanisms). Leakoff can also cause slippage of pre-existing NF and result in MS events with double-couple (DC) source. These fractures are secondary fractures with potential to transport fluid and facilitate the induced major fractures. When all of the fluid leaks off at positions along the main fractures, the fracture will close, which may trigger MS events with CLVD and ISO source mechanisms.

We extracted the MS events in the Closure Window and eliminated those events induced by previous stages, NF, and pumping noise. Thus, we reduced the MS cloud overlap of different stages and increased the accuracy of the inferred fracture geometry and SRV. Case applications on several shale wells were used to test this method. On the basis of the results, we suggest continued monitoring MS events after the end of proppant pumping.

The method developed in this study can process any MS data and the associated fracture stimulation job data to segregate the three MS event windows. The Closure MS Window will indicate fracture geometry more accurately, and thus enhance optimization of hydraulic fracturing design and the prediction of hydrocarbon production.

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