Propped hydraulic fractures are key to producing tight reservoirs, and knowledge of fracture geometry is fundamental for a proper field development plan. To this end, hydraulic fracture propagation characterization in tight gas reservoirs, pioneered by Amoco during the 1980s, is obtained by combining a dynamic closure test, fracture height determination, and pumping net pressure behavior. The result provides an estimated propped fracture geometry that can then be compared with pressure or rate transient analysis to validate the effective propped geometry.
Although surface pressure is generally sufficient to establish bottomhole (BH) pressure decline behavior by correcting for a constant hydrostatic pressure, the inherent uncertainty of friction pressure of crosslinked gel in the tubular does not establish the correct pumping net pressure trend. BH sensor pressure recordings are operationally simple and inexpensive when a fracturing string is required and recovered after the operation (frac-pack operation, well with mechanical pump), but that is not the case with casing fracturing. To remediate this, a high-speed, high-accuracy, and miniaturized BH pressure and temperature sensor, based on microelectromechanical systems (MEMS), was developed under a project funded by the Mexican Secretary of Energy and combined with an economical and practical way to deploy a BH sensor hung from a slickline or wireline cable thereby enabling pressure recording during the entire operation including, the main proppant fracturing (patent pending).
This paper presents the results and analysis of two case studies in Mexico where BH pressure and temperature were recorded with this novel sensor. Both jobs were conducted in vertical wells in tight sandstones. In the first field-test job, a post-fracture wellbore propped height determination using dipole sonic logs and radioactive tracer were used. In addition, the entire operation was monitored with microseismic monitoring. The results of these three measurements enabled validating the fracturing model for an increased confidence for field development. In the second field-test job, the BH gauges enabled visualization of the different events that occurred when proppant of different mesh sizes was used during the treatment, containing the fracture eliminating premature screenouts.
Only direct measurement of the fracture behavior net pressure and its associated dimension allow the proper calibration of fracturing models required to correctly predict and optimize propped fracture and thus reduce the cost of hydrocarbons produced.
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