Mini-frac analysis performed prior to a main fracture treatment is a common procedure for many companies involved in well stimulations. The purpose is to determine instantaneous shut-in pressures (lSIP), fracture closures, and leak-off. If the value for leak-off is high (greater than 0.005 ft/vmin), it can cause early sand-off where only 20–80% of a fracture design is placed. This high leak-off can be attributed 10 two factors:
high reservoir permeability or.
induced shear fracturing near the wellbore. Reservoirs with permeabilities greater than 100 md have been successfully fractured using fluids with good sand carrying capabilities, high viscosities, and/or low wall-building coefficients.
Shear fractures, created dry by bending stress off a hydraulically induced tensile fracture and opened wide by hydraulic fracturing fluids, can show apparent permeabilities in the hundreds of millidarcies, and are capable of easily sanding-off any well. It is the authors' opinion that most shear fractures are created in brittle rock, i.e. sandstone reservoir rock high in quartz (greater than 80% concentration). Most oil sands reservoirs (quam rich) in northern Alberta have shown signs of shearing during high-rate cyclic steam injection and have been responed in the literature. Deeper formations in central Alberta, such as the Basal Quam and Gething show results similar to minifrac analyses performed previously in the McMurray oil sands and can be used to identify the presence of shear fractures. Once characteristic shearing has been detected fracture design is modified to increase sand placement and importantly, to increase gas and/or oil production from the stimulated well.
Well testing procedures have been available for years1–2 and existing methods will be applied here. More specifically, traditional well test analyses are performed in oil and gas reservoir studies to evaluate the potential for improving economics and/or increasing recoverable reserves. A well-planned mini-frac program7,8 can determine with reasonable certainty, the Initial Shut-in Pressure (ISIP), fracture closure (minimum stress), fracture height, formation permeability, and leak-off coefficient.
A propped fracture treatment is designed using this data and for many reservoirs, the odds are still 50/50 whether the well will sand-off (plug with sand in the wellbore), with only part of a planned treatment placed. The capability of technology to guarantee a fracture treatment going to completion has yet to be developed.
Lately, mini-fracs are nearly always performed on problem reservoirs as most of the high-quality reservoirs have already been tapped. A well that sands-off and has not had any previous mini-fracs provides little information. Fracturing by trial and error is costly and does not pass today's economic hurdles. Therefore mini-fracs are a stepwise necessity not a research project.
There are good sand-offs and bad sand-offs. A well may sand-off because the formation permeability has been under-estimated and leak-off exceeds fracture growth. Such wells tend to sand-off from the fracture tip backwards to the wellbore (tip screen out). While this type of sand-off is usually undesirable, the propped fracture width is maximized, resulting in maximum fracture conductivity. High permeability wells minimize costs with short fractures.
