Borehole Deviation Surveys are Necessary for Hydraulic Fracture Monitoring

Abstract

Summary Significant errors in the calculated azimuth and other parameters of a monitored fracture can be caused by not performing accurate borehole deviation surveys for hydraulic fracture monitoring (HFM) and neglecting the effects of the deviating borehole trajectory. For common HFM geometries, a 2° deviation uncertainty of the positions of monitoring or treatment well surveys can cause more than a 40o uncertainty of the inverted fracture azimuths. Furthermore, if the positions of the injection point and the receiver array are not known accurately and the velocity model is artificially adjusted to locate perforations on assumed positions, several milliseconds discrepancies between measured and modelled SH-P-wave traveltime differences may appear along the receiver array. These traveltime discrepancies may then be misinterpreted as an effect of TI anisotropy, and use of such an anisotropic model may lead to the mislocation of the detected fracture(s). The uncertainty of the relative positions between the monitoring and treatment wells can have a cumulative, non-linear effect on inverted fracture parameters. Introduction Over the years, a large number of hydraulic fracture treatments have been monitored to determine fracture geometries.1,2,3 The fracture geometry is determined from microseismic events observed from a monitoring borehole. Usually, only a single (closest) monitoring borehole is used since other boreholes are too distant and signals at them are considerably attenuated. To drill a new borehole in a close vicinity of the treatment borehole is too expensive. Often, although the observation or the treatment well are not strictly vertical, they are assumed to be perfectly vertical and no borehole deviation survey (i.e. the measurements of the borehole inclination and azimuth, which are then used to calculate the borehole trajectory) is done. Commonly, monitoring geophones' orientation is determined from back-azimuths of P waves generated by the perforation shots located in the treatment well assuming isotropy and lateral homogeneity of the medium between wells. Thus, the orientation of the geophones in the monitoring well is determined relatively to the position of the treatment well at depths corresponding to the perforations. The absolute orientation (in a geographical coordinate system) of the monitoring array and of the observed microseismic event hypocenters can, however, be obtained only from the assumed positions of the receivers and the perforations. Therefore, any error in the positioning of the monitoring array or of the perforations is directly projected into error of the absolute fracture position. We show that the effects on the absolute fracture azimuth may be considerable even for wells only slightly deviating from expected positions. Fracture geometry (mainly its azimuth and length) resulting from such HFM is then used for "infill drilling", during which new wells are drilled into unfractured (and hopefully undrained) parts of the reservoir. Wrongly estimated fracture azimuth (and its position) can lead to financial losses. Furthermore, the borehole deviation are used not only the to determine fracture azimuth, but also the distance between the treatment and monitoring wells. Traditionally, in HFM, the initial isotropic velocity model is built from sonic logs and/or vertical seismic profiling (VSP) data4. Commonly, the velocity model is then adjusted to locate perforation shots to their assumedly correct positions4. The velocity model adjustment can be done in a number of ways but it usually involves fitting of S-P-wave traveltime differences. The S-P-wave traveltime differences, which cannot be explained by the isotropic model are then fitted by a VTI (Transverse Isotropy with Vertical axis of symmetry) velocity model. We show that such models can be completely artificial. Geophysical background: Wave properties in VTI medium do not depend on an azimuth, but depend on inclination. Typical example of such medium are horizontally layered shale formations, commonly occuring in present hydraulic fracture treatments. In such formations horizontal velocity exceeds vertical one.