Jets that emanate from high density porous liners are most widely used for penetration into Earth materials. These shaped-charges differ from those that contain solid copper liners in three major aspects: the porosity, the usually higher initial density, and the very short standoff in which they typically operate. Because penetration depth is commonly very difficult to increase by means of high density solid liners, it is important to understand the benefit of using porous liners in utilization of high density materials. The models published so far to describe the performance of jets formed by porous liners are based on the modification of the virtual origin model to suit this special case, which limits the accuracy of their predictions. Here we present a more general analysis that does not depend on the virtual origin assumption. Our study employs the SCAN semi-analytical code into which a new model for porous jet behavior is incorporated. It is used to explain the benefit of using this type of liner for oil-well penetration, hence penetration into low density targets, as opposed to penetration into hard steel.