Minifrac Models

Appropriate analytical models are available when the test type has been specified as a minifrac test, and can be used to history match the after-closure data. The model interface is almost identical to conventional models and most of the features available in convention models, such as auto-history matching, are also available for minifrac models.

Minifrac models yield results consistent with the Soliman / Craig after-closure analyses when clearly defined flow regimes are developed.  However, as with conventional pressure transient analysis, the models provide more realistic estimates when clearly developed flow regimes have not developed, which highlight misinterpretation of data in straight-line or derivative analysis.

Minifrac models should not be used to match the early-time data because the models do not account for fracture closure, and are only designed for analyzing after-closure data. Changing wellbore storage will be incorporated at a later date to assist in matching the early time data.

The injected volume in minifrac theory is intended to represent the total volume of fluid injected into the reservoir. The value of Vinj, calculated in WellTest, is the volume injected from the Start of Injection line (located on the Data Chart on the Production Editor tab) to shut-in. In order for Vinj to equal the total injected volume, the Start of Injection line would need to be placed at the point-in-time when the wellbore had been pressured up to the initial reservoir pressure (see figure below). Any fluid injected beyond this time would pressure up the wellbore, generate a fracture, and eventually leak-off into the formation.  If the Start of Injection line is placed at the point where the wellbore had been pressured up to the initial reservoir pressure, a value for Vw is not required.

 

However, since the initial pressure is unknown prior to  interpretation of the minifrac test, it is common practice to set the Start of Injection line at or near the breakdown point (as shown below).

When the Start of Injection line is placed at breakdown, V_inj does not represent the total injected volume. To account for the added energy of pressuring up the wellbore, a fluid expansion term is added to V_inj, which requires an estimate of Vw. (V_T is the value used in the model calculations.)