Concept:
Total energy of a satellite in circular orbit:
\[
E=-\frac{GMm}{2R}
\]
Work done in shifting orbit equals change in total energy.
Step 1: Find initial total energy.
Initial radius:
\[
R=r
\]
So,
\[
E_i=-\frac{GMm}{2r}
\]
Step 2: Find final total energy.
Final radius:
\[
R=2r
\]
Therefore,
\[
E_f=-\frac{GMm}{2(2r)}
\]
\[
E_f=-\frac{GMm}{4r}
\]
Step 3: Compute work required.
\[
W=E_f-E_i
\]
\[
W=-\frac{GMm}{4r}+\frac{GMm}{2r}
\]
Taking LCM:
\[
W=\frac{-GMm+2GMm}{4r}
\]
\[
W=\frac{GMm}{4r}
\]
Hence,
\[
\boxed{\frac{GMm}{4r}}
\]