The use of nwell "ground plane" under the pad reduces of noise coupling to and from a p substrate. The nwell is usually connected to a quiet vdd for lowest p n diode capacitance.
It is usually better to use n+ return path shield the advantge is lower resistance which makes for a better quality parasitic c.
Other foundries allow poly or m1 shields which also work well, but at the cost of higher parasitic c value.