For the effective measurement of the reopening pressure in hydraulic fracturing, it is necessary to use the testing equipment with sufficiently small compliance. If not, there is no way to estimate the maximum stress in a plane perpendicular to a borehole, i.e. the maximum horizontal stress assuming a vertical borehole, from the reopening pressure. It may be very hard to reduce the system compliance drastically by just replacing any components of the hydraulic fracturing system. If the flow meter is emplaced as close as possible to a test section of a borehole isolated by straddle packers, the system compliance can be reduced drastically. To accomplish this idea, we have been developing a testing system with a downhole flow meter. We demonstrated the developed system in a field test successfully. Furthermore, we presented a new method which will allow us to do stress measurement at depths greater than a few km.
A typical setup for hydraulic fracturing test is shown in Fig. 1. A short test interval in a borehole is sealed off by use of two inflatable packers. The test interval is pressurized by fluid injection from a pump located on the surface to generate a tensile circumferential stress around the borehole. When the tensile stress exceeds the strength of a rock and the stress concentration by the borehole, fracture initiation occurs on the borehole wall. Assuming a vertical borehole which is free from pre-existing/natural fractures, the vertical fractures will be induced to be normal to the minimum horizontal stress Sh (parallel to the maximum horizontal stress SH). Further fluid injection drives fracture propagation away from the borehole. The stress concentration by the borehole is a function both of Sh and SH, and it affects the fracture behavior.