  Aims. We present simulations of the perfomances of the future SPHERE IFS
instrument designed for imaging extrasolar planets in the near infrared (Y, J,
and H bands). Methods. We used the IDL package code for adaptive optics
simulation (CAOS) to prepare a series of input point spread functions (PSF).
These feed an IDL tool (CSP) that we designed to simulate the datacube
resulting from the SPHERE IFS. We performed simulations under different
conditions to evaluate the contrast that IFS will be able to reach and to
verify the impact of physical propagation within the limits of the near field
of the aperture approximation (i.e. Fresnel propagation). We then performed a
series of simulations containing planet images to test the capability of our
instrument to correctly classify the found objects. To this purpose we
developed a separated IDL tool. Results. We found that using the SPHERE IFS
instrument and appropriate analysis techniques, such as multiple spectral
differential imaging (MDI), spectral deconvolution (SD), and angular
differential imaging (ADI), we should be able to image companion objects down
to a luminosity contrast of ? 10-7 with respect to the central star in
favorable cases. Spectral deconvolution resulted in the most effective method
for reducing the speckle noise. We were then able to find most of the simulated
planets (more than 90% with the Y-J-mode and more than the 95% with the
Y-H-mode) for contrasts down to 3 \times 10-7 and separations between 0.3 and
1.0 arcsec. The spectral classification is accurate but seems to be more
precise for late T-type spectra than for earlier spectral types. A possible
degeneracy between early L-type companion objects and field objects (flat
spectra) is highlighted. The spectral classification seems to work better using
the Y-H-mode than with the Y-J-mode.
