NOVELTY - Method (m1) of removing geothermal scale from a wellbore comprises introducing a descaling agent comprising an N-(phosphonoalkyl)iminodiacetic acid or its salt into a wellbore of a geothermal well having geothermal scale present in it, contacting the geothermal scale with the descaling agent, and removing at least a portion of the geothermal scale from the geothermal well using the descaling agent.    USE - The method is useful for removing geothermal scale formed from a source of geothermal fluid.    ADVANTAGE - The method utilizes the N-(phosphonoalkyl)iminodiacetic acid which: is compatible for use in the high temperature environment of geothermal wells (which commonly exceed a temperature of 300 degrees F); has an even greater stability when dissolved in a fluid phase; is effectively used in a geothermal well, often without actively cooling the wellbore; reacts with geothermal scale in a controlled manner, again allowing geothermal descaling operations to take place at the native temperature of the geothermal well and/or without taking special precautions to control the chemical reactivity; allows treatment of geothermal scale to take place for a longer period of time than is possible with descaling agents having lower thermal stability (such as EDTA and propylenediaminetetraacetic acid), thus exhibiting high chemical stability; and is environmentally friendly and cost effective.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) method (m2) of removing geothermal scale from a wellbore by complexing a metal ion, comprising introducing a treatment fluid comprising the descaling agent into a wellbore of a geothermal well, where the geothermal well has a bottom-hole temperature of greater than or equal to 300 degrees F and having geothermal scale comprising a metal-containing compound present in it, dissolving at least a portion of the geothermal scale with the descaling agent to produce a metal ion, and complexing the metal ion with the N-(phosphonoalkyl)iminodiacetic acid;    (2) method (m3) of removing geothermal scale by complexing a metal ion, comprising contacting geothermal scale with the descaling agent, dissolving at least a portion of the geothermal scale with the descaling agent to produce a metal ion, and complexing the metal ion with the N-(phosphonoalkyl)iminodiacetic acid; and    (3) a system (1) comprising a pump fluidly coupled to a tubular (16), where the tubular contains a treatment fluid comprising the descaling agent.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the system that can deliver treatment fluids to a down-hole location.    System (1)    Tank (10)    Wellhead (14)    Tubular (16)    Subterranean formation (18)