  Assuming ALPs couple to gluons only, they can be produced via the $p p \to a
j$ process and decay into two jets at the LHC. When the coupling parameter,
$C_{\tilde{G}} / f_a$, is small, the lifetime of ALPs can be long enough
leading to displaced final state jets. In this paper, we consider the signal
including both the prompt and long-lived cases of ALPs by employing a
specialized Delphes module to handle displaced jets. Relevant background
processes are generated and simulated at the detector level, and multivariate
analyses based on machine-learning are performed to discriminate signal and
background events and achieve the best sensitivities. Based on the data
accumulated for this study, we forecast the expected upper limits on
$C_{\tilde{G}}/f_a$ for ALP mass $m_a$ in the range 5$-$2300 GeV at 2-, 3- and
5-$\sigma$ significances at the High Luminosity-LHC with 14 TeV center-of-mass
energy and 3 ab$^{-1}$ integrated luminosity. Vast previously unprobed regions
in the parameter space spanned by $C_{\tilde{G}}/f_a$ and $m_a$ are probed and
the best upper limits on $C_{\tilde{G}}/f_a$ at 2-$\sigma$ significance is
found to be around $1.0 \times 10^{-2} \,\, {\rm TeV^{-1}}$ for $m_a \sim 500$
GeV. The ALP mass is reconstructed from the kinematics of final state jets and
we find that it is measurable in this method when $m_a$ is below about 1 TeV at
the HL-LHC. The effects of systematic uncertainties and validation of the EFT
framework are also checked and discussed.
