  We have analysed Chandra/High Energy Transmission Gratings spectra of the
X-ray emission line gas in the Seyfert galaxy NGC 4151. The zeroth order
spectral images show extended H- and He-like O and Ne, up to a distance $r
\sim$ 200 pc from the nucleus. Using the 1st order spectra, we measure an
average line velocity $\sim -230$ km s$^{-1}$, suggesting significant outflow
of X-ray gas. We generated Cloudy photoionisation models to fit the 1st order
spectra. We required three emission-line components, with column density,
log$N_{H}$, and ionisation parameter, log$U$, of 22.5/1.0, 22.5/0.19, and
23.0/-0.50, respectively. To estimate the total mass of ionised gas and the
mass outflow rates, we applied the model parameters to fit the zeroth order
emission-line profiles of Ne~IX and Ne~X. We determined the total mass of
$\approx 5.4 \times$ 10$^{5}$ M_sun. Assuming the same kinematic profile as
that for the [O~III] gas, the peak X-ray mass outflow rate was $\approx 1.8$
M_sun yr$^{-1}$, at $r \sim 150$ pc. The total mass and mass outflow rates are
similar to those determined using [O~III], implying that the X-ray gas is a
major outflow component. However, unlike the optical outflows, the X-ray
outflow rate does not drop off at $r >$ 100 pc, which suggests that it may have
a greater impact on the host galaxy.
