Fracture quality images from 4D VSP and microseismic data at Jonah Field, WY.

Abstract

Natural and induced fractures provide the only means of production in tight gas sand reservoirs. The objective of this work is to locate and characterize natural and induced fractures from the analysis of scattered waves recorded on 4-D (time lapse) VSP data in order to optimize well placement and well spacing in these gas reservoirs. Using synthetic seismic data, we have previously shown that it is possible to characterize the quality of fracturing based upon the amount of scattering of seismic energy from hydraulic fractures. We show this concept applied to a field dataset from the Jonah Field in Wyoming. The time lapse (4D) VSP data from this field are imaged with a migration algorithm using shot travel time tables derived from the first breaks of the 3D VSPs and receiver travel time tables based on the microseismic and perf shot arrival times. We create images of the fracture planes through two of the hydraulically fractured wells in the field. The scattered energy shows correlation with the locations of the microseismic events and azimuthal scattering which is different from the azimuthal reflectivity of the reservoir. This gives us more confidence that we have separated the scattered signal from simple formation reflectivity. Variations of the scattered energy along the image planes suggest differences in the quality of the fractures in three distinct zones.