Monitoring the Effects of Hydraulic Stimulation by Microseismic Event Location: A Case Study

Abstract

ABSTRACT One of the most promising techniques for monitoring the effects of hydraulic stimulation or fracturing is the use of the induced microseismicity to define the shape and orientation of the stimulated regions. The results presented in this paper are believed to be the first of their kind with an on-line location system mapping the stimulated zone during injection. The stimulation was undertaken as part of an investigation into the generation of hot dry rock geothermal reservoirs and consisted of a 3.5 million gallon injection at flow rates as high as 40 B/min. Many thousands of microseismic events were produced, located and presented as maps to enable the interpretation to proceed during the operation. There are very few techniques which have been applied to give an independent measurement of fracture height, length and orientation. The monitoring of microseismicity offers the prospect of an accurate definition of the geometry provided other observation wells are available. The techniques presented in the paper would be applicable in dense, brittle formations without a significant velocity anisotropy. They are of direct relevance to the stimulation of geothermal systems, tight gas sands and limestone reservoirs. The results from such case studies will enable the design of fracturing procedures to be evaluated on directly measured results. The results presented in the paper consist of a series of microseismic images relating to different stages in the hydraulic fracturing. The fracture front can be seen to grow with time and change orientation. The initial stimulated zone produced was 1.2 km long, 1.5 km high and 100 m thick. When the pressure was maintained on the system the stimulated zone grew to 400 m thick with the principal dimensions remaining the same. These results represent a considerable step forward in the state of the art of monitoring hydraulic stimulation and have been an essential tool in understanding the geothermal reservoir performance.