Yep, I agree with nibiru2012.
The image is just another folder after all.
In the scenario you mention the image size will be about 9 GB based on the fact that you reported ~18 GB used space.
This is with the default compression level set by Clonezilla.
Even tighter compression is available in Clonezilla but more compression take more time.
The default compression level is a good compromise between speed and ultimate image size.
Be aware that regardless of the image size the partition that you restore the image to must be the same size or larger than than the source partition.
Again using your example you stated that your drive\partition has 131 GB of free space and 18 GB used space that is roughly 150 GB total.
If you wanted to use Clonezilla to move your install to a new drive the new drive\partition would need to be exactly 150 GB or larger or Clonezilla would error out.
Indeed I used Clonezilla to migrate all 6 of my Operating System installs from a 320 GB drive to a 500 GB drive.
I prepped the new drive by creating corresponding partitions on my new drive (being careful to make them exactly the same size as the original).
The only difference being that the storage partitions on the new drive were grown to accommodate the greater drive capacity.