We have modeled planetary perturbations on a sample of 106 Oort cloud comets for a maximum of 1000 perihelion passages. All the four giant planets are taken into account. In order to speed up the computation of each planetary kick, the planet orbits are circular, centred onto the Sun and coplanar. The first simulations were dedicated to the definition of the heliocentric distance rs from which the numerical integration required for the computation of each kick should start, and also below which perihelion distance qs a planetary kick should be computed. It appears that rs=100 au, and qs=37 au are suitable values for our goal. Then the transparency factor of our Solar System was investigated. It appears that it is above 80% only for perihelion distance q smaller than 5 au.For q > 10 au, it drops to value smaller than 10%. Our solar system seems to be more transparent that it is usually thought. Then we have studied the evolution of a sample of 1 Million of comets with perihelion in the planetary region of the Solar system affected by the planetary perturbations only. It appears that for q<5 au most of the comets are ejected from the Oort cloud in less than 10 perihelion passages. However, few of them, on retrograde orbits and with q < 1 au manage to stay for several tens of perihelion passage even at such small distance from the Sun. For q>10 au, many comets support more than 1000 planetary kicks without being ejected from the Oort cloud.