Dual-Well Monitoring Enhances Microseismic Imaging of Hydraulic-Fracture Stimulation
In long laterals, a single monitoring well may not detect microseisms over the entire well. The use of two monitoring wells improves detection over the entire length of the lateral well.
To achieve optimal production in tight-gas fields, microseismic monitoring is used to monitor the effectiveness of hydraulic fracture stimulations. Deformation associated with the stimulation, results in small magnitude microearthquakes whose locations are used to image the growth and geometry of the stimulated fracture network. This technique provides unique information for improving the stimulation design. Generally, receiver arrays are placed in a single observation well to record the microearthquakes. In long horizontal boreholes, improved seismic detection and imaging can be obtained using data from additional observation wells.
Hydraulic fracture stimulations were performed in two horizontal tight-gas wells, and the fractures in both wells were mapped using two nearly vertical observation wells. Each of the simulations resulted in a large number of microseismic events but most of the recorded microseisms’ signals were detected in only one of the observation wells. In one well (left), the stimulations produced a diffuse cloud of microseisms and the associated production log shows relatively uniform production with no evidence of significant discrete fractures contributing to the flow into the well. In contrast, the stimulation in the second well (right) resulted in clustered microseisms with larger magnitudes and evidence of discrete fracture lineations, particularly in the dual-well event locations. These lineations were interpreted as pre-existng fractures intersecting the wellbore. This interpretation is consistent with the expected stress regime at the well location and was supported by the results of the focal-mechanism analysis. In this study, single- well monitoring would have indicated the regions of clustered deformation but a higher degree of uncertainty in the locations of the discreet fractures would have resulted in an elliptical pattern, in contrast to the linear pattern detected using dual-well monitoring. The direction of the maximum uncertainty is controlled by the geometry of the dual monitoring wells. In this case, if the fracture orientation had been at an oblique angle, the dual well locations would not have resulted in as linear of a feature.
Map view of the microseismic images from the two horizontal wells. For each well, the blue symbols are events recorded in monitoring well #1 and the red symbols are those recorded in well #2. The green symbols are the locations using both monitoring wells. The spinner logs are plotted beside the stimulated wells.