With all that horsepower sitting on location, shaking the ground while pumping a frac job, it is hard to imagine that the treatment is doing anything but creating a linear elastic mode-one fracture. However, hydraulic fracturing is far from an exact science because it is difficult to see what the fracturing treatment is doing downhole. Whether it is breaking into new rock in a linear elastic mode or just opening pre-existing planes of weakness in the reservoir to the wellbore, both situations could be beneficial. If the rock is failing down pre-existing planes of weakness, two cases have been observed: (1) a situation where natural fractures that are open and critically stressed, which open below closure pressure and (2) a situation where planes of weakness are open as pressure is exerted on the formation by the frac treatment. The results of both of these cases cause misleading fracturediagnostic interpretation, high leakoff, narrower than expected fractures, frac screenout, and/or complex fracture geometry. When the frac treatment is breaking new rock, the frac treatments pump to completion as designed. In reality, many frac jobs are a combination of these two scenarios, as witnessed by post-frac radioactive tracer logs and by downhole microseismic surveys during the frac treatment. Tracer surveys have limited depth into the formation, but microseismic surveys, which cover a much larger area around the borehole, tend to see a rather large stimulated reservoir volume with greater width and shorter lengths than what is modeled in hydraulic-fracture simulators. If the fracture treatment is breaking new rock, the created fracture will tend to be more like what is modeled in current fracture-simulation models. A novel concept using the diagnostic fracture-injection test or the main frac treatment to integrate the geomechanically determined stress state of the rock with hydraulic-fracture diagnostics has been developed. By comparing the estimates of minimum horizontal stress from both disciplines, one can achieve a better understanding of what the frac treatment is doing downhole and improve the post-stimulation analysis or trouble-job analysis.
The fundamental principle for this investigation is to show the integration of geomechanically determined stress state with closure pressure determined by the diagnostic fracture injection test (DFIT) [1]. The DFIT test is conducted by pumping a small volume of non wall-building fluid (water) to determine leakoff type, pore pressure, permeability, and closure pressure. If the numerical value of “closure pressure” determined with the DFIT test is near the same value as the value or magnitude of minimum horizontal stress (Sh) determined through geomechanical analysis, the hydraulic-fracture treatment will most likely be pumped as modeled. However, if the DFIT “closure pressure” has reopened pre-existing planes of weakness in the rocks, the closure pressure could be lower than the actual minimum horizontal stress (Sh). This underestimation of Sh could have a detrimental effect on the main frac treatment. This is one cause of a screenout resulting from uncontrolled leakoff. Another condition that is often observed on DFITs is the process zone stress (PZS) [2].
