Understand that there is a difference between phreatic and phreato-magmatic eruptions... Wohletz and Heiken are referring to well blow-outs that can occur when the well casing or cement bond between the casing and the formation fails (it's also true that, if the well is cemented in poorly consolidated formation, the formation around the cemented well can fail and allow steam to rise to the surface). Those events could, arguably, be referred to as phreatic eruptions. But most geothermal wells are not drilled into molten magma - as I noted in my original comment, that has occurred rarely but, when it has, it has not triggered a phreato-magmatic eruption.