Mutual inductance and coupling coefficient in 3D environment

if there are two solenoids d(cm) apart from each other (in 3D environment[x y z]); in other words, they are not laid on the same axis and the difference between the value of x and y and z (for S1 and S2) is not zero (see Figure please)
what would be the mutual inductance (M)?
or the coupling coefficient (k) equation?

Thanks
SS.bmp

It's hard to answer..
You should simulate those coils in 3D environment..