A method of monitoring a nonhydrocarbon and nonaqueous fluid injected into the earth's subsurface through a first wellbore that involves positioning a fluid analysis tool within a second wellbore and determining the presence of the injected nonhydrocarbon and nonaqueous fluid by making a measurement downhole on the injected nonhydrocarbon and nonaqueous fluid using the fluid analysis tool. Also a related method of enhancing hydrocarbon production from a subsurface area having first and second wellbores that involves injecting a nonhydrocarbon and nonaqueous fluid into the subsurface through the first wellbore, positioning a fluid analysis tool within the second wellbore, and determining the presence of the injected nonhydrocarbon and nonaqueous fluid by making a measurement downhole on the injected nonhydrocarbon and nonaqueous fluid using the fluid analysis tool. Further, a related method of determining the relative or absolute quantity of a nonhydrocarbon and nonaqueous fluid injected into the earth's subsurface through a first wellbore that involves positioning a fluid analysis tool within a second wellbore, measuring the near-infrared spectroscopy signature of fluid downhole using the fluid analysis tool, measuring the downhole temperature and pressure using the fluid analysis tool, and estimating a relative or absolute quantity of the injected nonhydrocarbon and nonaqueous fluid within said downhole fluid using the measured near-infrared spectroscopy signature, the temperature, and the pressure to estimate a partial pressure of hydrocarbon constituents of the downhole fluid.