  This paper aims to develop numerical approximations of the Keller--Segel
equations that mimic at the discrete level the lower bounds and the energy law
of the continuous problem. We solve these equations for two unknowns: the
organism (or cell) density, which is a positive variable, and the
chemoattractant density, which is a nonnegative variable. We propose two
algorithms, which combine a stabilized finite element method and a
semi-implicit time integration. The stabilization consists of a nonlinear
artificial diffusion that employs a graph-Laplacian operator and a shock
detector that localizes local extrema. As a result, both algorithms turn out to
be nonlinear.Both algorithms can generate cell and chemoattractant numerical
densities fulfilling lower bounds. However, the first algorithm requires a
suitable constraint between the space and time discrete parameters, whereas the
second one does not. We design the latter to attain a discrete energy law on
acute meshes. We report some numerical experiments to validate the theoretical
results on blowup and non-blowup phenomena. In the blowup setting, we identify
a \textit{locking} phenomenon that relates the $L^\infty(\Omega)$-norm to the
$L^1(\Omega)$-norm limiting the growth of the singularity when supported on a
macroelement.
