A hydraulic fracture stimulation was monitored with borehole geophone arrays deployed in two observation wells. The resulting microseismic images showed that the first horizontal well stimulation was characterized by a diffuse cloud of microseismic events, and the production log indicated roughly uniform production along the length of the well. The second horizontal well stimulation was characterized by clustered microseisms, and the production log indicated significant production from the intersection of pre-existing fractures with the well. The resulting case study allowed a comparison of accuracies between a single observation well and two observation wells. A comparison was also made of the microseismic event location accuracy resulting from of the different configurations of observation wells. The dual well observations provide an opportunity for enhanced velocity model validation in addition to enhanced source imaging techniques to provide additional information about the fracture geometry.
The effectiveness of a hydraulic fracture stimulation is critical for optimal production in many tight gas fields1. Microseismic monitoring is a convenient technique used to image the fracture growth and provides unique information for improving the stimulation engineering. Deformation associated with the stimulation results in small magnitude micro-earthquakes can be used to image the growth and geometry of the stimulated fracture network2. Fracture networks are typically imaged based on the distribution of the microseismic event locations, although other seismic signal attributes can also be used to provide additional constraints on the fracture geometry3. For example, microseismic images can be used to infer fracture complexity resulting from interaction with pre-existing fractures. In these cases, the fracture network can be interpreted through alignment of microseismic events; although other aspects such as the magnitude of the microseisms4,5 and radiation pattern of compressional and shear waves can be used to constrain the fracture plane orientation3.
Microseismic hydraulic fracture imaging often uses geophones in a single wellbore; however, if more observation wells are available, improved seismic detection and imaging can be obtained. For example, two monitoring wells can be used to provide more accurate microseismic event locations6. Furthermore, improved sampling of the seismic radiation pattern can be achieved for better focal mechanism calculations. However, multiple well monitoring also provides enhanced microseismic detection, especially in cases of long laterals where a single monitoring well may not be able to detect microseisms over the entire well. In this paper, we present results of a dual monitoring well case study. In addition to interpretation of the interaction of the stimulation with pre-existing fractures during the stimulation, the project geometry also provides an opportunity to demonstration aspects of velocity model calibration.