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 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 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. Introduction This paper presents the results and preliminary interpretations of an experiment to map the growth of a hydraulically stimulated region during the actual injection. The development of direct methods to determine the geometry of such zones by independent measurements not relying on preconceived analytical models will help to remove some of the uncertainties inherent in the design of stimulation treatments. Microseismic events, generated by the stimulation, were detected on a three-dimensional sensor network and processed on-line to determine their location. During the full operation, some 30 000 events were detected and approximately 15% were good enough to be located accurately. This experiment was undertaken by the Camborne School of Mines as part of its continuing research programme into the feasibility of producing geothermal energy from hot dry rock. It took place in a pair of boreholes 7000 ft (2200 m) deep, drilled entirely in granite. The research programme is jointly financed by the United Kingdom Department of Energy and the European Economic Community as part of their geothermal research programmes. programmes. The hydraulic stimulation of the hot dry rock geothermal reservoir is the key component of the main research activity and the on-line mapping of the developing structure was a primary experimental objective. It was hoped that the measurement of stimulated volume by a technique independent of the hydraulic analysis would enable the correct production methods to be applied to the system.