  The possibility to unambiguously determine the equation-of-state of the
cosmic dark energy with existing and future supernovae data is investigated. We
consider four evolution laws for this equation-of-state corresponding to four
quintessential models, i.e. i) a cosmological constant, ii) a general
barotropic fluid, iii) a perfect fluid with a linear equation-of-state and iv)
a more physical model based on a pseudo-Nambu-Goldstone boson field. We
explicitly show the degeneracies present not only within each model but also
between the different models : they are caused by the multi-integral relation
between the equation-of-state of dark energy and the luminosity distance.
Present supernova observations are analysed using a standard $\chi^2$ method
and the minimal $\chi^2$ values obtained for each model are compared. We
confirm the difficulty to discriminate between these models using present SNeIa
data only. By means of simulations, we then show that future SNAP observations
will not remove all the degeneracies. For example, wrong estimations of
$\Omega_m$ with a good value of $\chi^2_{min}$ could be found if the right
cosmological model is not used to fit the data. We finally give some
probabilities to obtain unambiguous results, free from degeneracies. In
particular, the probability to confuse a cosmological constant with a true
barotropic fluid with an equation-of-state different from -1 is shown to be 95%
at a $2 \sigma$ level.
