It is useful to map hydraulic fractures in order to improve the rate of success of such operations and to optimize the pattern of stimulated wells. The proposed method can be used during a minifracturing or prefracturing operation and necessitates a downhole 3D acoustic detector clamped in the well. A few to a few dozen cubic meters of gel being injected during fracturing phases, the acoustic activity is recorded after every injection, then signals containing apparent P and S waves are analyzed to determine the directions and distances of the emitting sources. A number of arrivals cannot be easily interpreted, in particular those corresponding to refractions along the casing. Two field examples corresponding to a vertical and an inclined fracture are given. Knowing the P and S wave velocities in the different layers and a careful choice of the tool position are essential to the success of the method.
Hydraulic fracturing is a very useful technique for stimulating production wells. This operation consists of the development of a high permeability area in the producing layer. To perform such an operation it is necessary to inject a large volume of gel and proppant which create and maintain the fracture open. Bearing in mind that the major risk is for the fracture to extend into the adjacent layers, it is important to evaluate the development of the fracture and, if possible, to control it.
The creation of a small hydraulic fracture (" minifrac") helps towards getting a good estimate of the direction and the dimensions, i.e. length and height corresponding to the fracture. With this type of test and with the use of a numerical model of fracture propagation, it is possible to estimate the dimensions of the main fracture. This will lead to a decision on the feasibility of such an operation. The orientation of the fracture is very important too for establishing a field production pattern.
For mapping a hydraulic fracture, the most promising method is the interpretation of the associated acoustic emissions. The aim is to locate the emitting sources whose spatial distribution is supposed to be representative of the fracture plane (or a part of it). This method has been investigated since the '70's. The most famous site is Fenton Hill, New Mexico, where is being performed the Hot Dry Rock Geothermal experiment.
Principles of acoustic measurements and characteristics of the tool
Hydraulic fracturing induces an acoustic activity in the fractured rock. This activity is mainly characterized by burst emissions and has been detected in different ways: at the surface, in an observation well located in the vicinity of the treated well, or directly in the treated well.
As attenuation increases with distance surface observations become less interesting, even assuming ideal conditions (see Batchelor et al.)
For two major reasons, the best method would be to place the detection tool in an observation well :
It is then possible to record the acoustic activity during the injection phases, whereas this is not possible when the tool is placed in the treated well.
The calibration of both compressional and shear wave velocities can be done between the wells using appropriate sources (explosives, electromagnetic shots, etc.).