Evaluation of underground hydraulic fracturing using transient electromagnetic method

The effective area of hydraulic fracturing is the core index to evaluate its effects. Through conducting transient electromagnetic tests, this paper deals with the influential range of the underground hydraulic fracturing as well as water-cut detection and gas extraction in the fracturing area. The resistivity response law of the coal seam in hydraulic fracturing process is explored, and the water-bearing area is determined. The obtained results from the tests show that the water-cut areas of the coal seam, measured by anti-interference transient electromagnetic instrument after fracturing, are commonly placed in the low-resistance area of the transient test. Further, the variations of amplitude of the low-resistance area in various directions of the test line are different. According to the variation law of the apparent resistivity of the coal seam before and after fracturing, the effective influential area of the hydraulic fracturing is defined, and the influence range is evaluated to be 35 m. The water cut and the gas extraction tests of the coal seam before and after fracturing are performed. The results reveal that the growth of water content in the coal seam is inversely proportional to the distance from the hydraulic fracturing borehole. The effective fracturing zone with the increment of the water content reaching 0.2% is the effective fracturing zone, and the effective fracturing zone of #9 and #10 is 38 m. After hydraulic fracturing, the gas extraction concentration would be in the range of 25.4–75.4%, with the average of 70.22%, which is 21.22% higher than that of the original coal body. The net amount of the gas extraction after fracturing is about eight times of that before fracturing. The effective fracturing range, which is determined by transient electromagnetic, is verified successfully. Exploring the effective fracturing regions of the hydraulic fracturing process would be very helpful in improving the evaluation system of the hydraulic fracturing effect.