Abstract
Hydraulic stimulation treatments are standard techniques to access geologic resources which cannot economically be exploited with conventional methods. Fluid injection into unproductive formations may increase their permeability by forming new fractures and activating existing ones. A major risk of this process is a possible occurrence of seismic events that can potentially be felt on the surface or even cause minor damage. In this paper, an advanced fluid injection scheme is proposed that aims to mitigate these unwanted events and to improve the permeability enhancement process. Amongst other procedures, it involves different types of cyclic injection and a traffic light system specifically designed for cyclic injection schemes. The concept is applied to develop a stimulation design for the Pohang enhanced geothermal system site in Korea, where it was first deployed in the field in August 2017.
Highlights
Economic production of a variety of geological resources, such as shale gas, tight oil, coal bed methane and geothermal heat using enhanced geothermal systems (EGS), relies on hydraulic stimulation treatments
We introduce an advanced fluid injection protocol with the aim to effectively reduce the risk of inducing seismic events above a given threshold, called cyclic soft stimulation (CSS)
A second part followed with an extended longterm cycle (LTC) because still no seismicity was observed before the base injection rate (BIR) phase of the first part
Summary
Economic production of a variety of geological resources, such as shale gas, tight oil, coal bed methane and geothermal heat using enhanced geothermal systems (EGS), relies on hydraulic stimulation treatments. These are reservoir enhancement methods where fluid is injected into a reservoir to increase its productivity by a combination of developing new tensile and shear fractures, and tensile opening and shearing of pre-existing fractures. Details about hydraulic fracturing and other reservoir stimulation methods can be found in an abundance of textbooks and other publications (e.g., Bunger et al 2013; Economides and Nolte 2000; Economides and Martin 2007; Huenges and Ledru 2010). While improving the hydraulic reservoir performance, hydraulic stimulation treatments cause fluid-injection-induced seismicity.
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