: This paper describes the successful measurement of stress changes induced in a crystalline rock mass adjacent to a full size hydraulic fracture. A hydraulic fracture was initially formed using water and subsequently reopened using cross linked gel. The stress changes measured during these treatments are compared with the results of 2-D and axisymmetric models. The full three dimensional stress changes were measured using four ANZI stresscells installed and tested in situ prior to the start of hydraulic fracturing. The instruments were logged at 15 second intervals throughout the hydraulic fracture treatments to provide a time history of the complete three dimensional stress changes that occurred as each hydraulic fracture grew toward and then passed close to the instruments.
Analysis of the stress change data provides information about the fracture rate and mode of growth, orientation, and about the excess pressure acting inside the fracture to open it. The stress change data confirms that the hydraulic fractures grew as openingmode rather than shear mode fractures. Stress change calculated using a radial fracture geometry model with the fracture opened by a uniform pressure fit the measured stress change quite well. The fit obtained helps establish the orientation and growth rate for the fractures generated at the site.
Hydraulic fracturing has long been used to induce fractures in rock masses for reservoir stimulation and other purposes. However, it is rare to have the opportunity to measure the actual stress changes that are caused by a full scale hydraulic fracture as it grows. Recently, a study aimed at assessing the potential for using multiple hydraulic fractures to pre-condition a rock mass to increase caveability and reduce fragment size provided just such an opportunity.
The details of the site and the overall study results remain confidential as a condition of using the data, but the results of the stress change monitoring are of interest from a scientific perspective unrelated to the outcomes of the caveability study.
This paper describes the results of three dimensional stress change monitoring about two full size hydraulic fractures induced in a crystalline rock mass. An initial hydraulic fracture was made using water as the injection fluid. This fracture was later reopened using cross-linked gel.
2. STRESS CHANGE MONITORING
The stress change in the rock near the hydraulic fractures was measured using ANZI cells. These instruments are described in detail in Mills .
The ANZI cell is a soft, inflatable stress measurement instrument that measures the strain changes on the wall of a borehole induced by stress changes in the rock mass. In each instrument, eighteen electrical resistance strain gauges of various orientations are pressure bonded directly onto the borehole wall.
A multiple linear regression analysis of the strain changes measured by the instrument allows the complete three dimensional stress changes to be calculated. A high level of redundancy (only six of the eighteen strain readings are required to determine the full stress change tensor) provides a strong measure of the confidence that can be placed in the integrity of the result.