SNAPSHOT STANDBY:-

Introduced in 11g, snapshot standby allows the standby database to be opened in read-write mode. When switched back into standby mode,
all changes made whilst in read-write mode are lost. This is achieved using flashback database,
but the standby database does not need to have flashback database explicitly enabled to take advantage of this feature, thought it works just the same if it is.

Create a physical standby from my post How to create Physical Standby and then follow the below steps.

CONVERT PHYSICAL STANDBY DATABASE TO SNAPSHOT STANDBY DATABASE

• Make sure the standby instance is in MOUNT mode.

select open_mode, database_role from v$database;

• Make sure managed recovery is disabled.

ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

• Convert the standby to a snapshot standby.

SELECT flashback_on FROM v$database;
select name, open_mode, database_role from v$database;

ALTER DATABASE CONVERT TO SNAPSHOT STANDBY;
ALTER DATABASE OPEN;

SELECT flashback_on FROM v$database;
select name, open_mode, database_role from v$database;

 

CONVERT SNAPSHOT STANDBY DATABASE TO PHYSICAL STANDBY DATABASE

To convert it back to the physical standby, losing all the changes made since the conversion to snapshot standby, issue the following commands.

SHUTDOWN IMMEDIATE;
STARTUP MOUNT;
ALTER DATABASE CONVERT TO PHYSICAL STANDBY;
SHUTDOWN IMMEDIATE;
STARTUP NOMOUNT;
ALTER DATABASE MOUNT STANDBY DATABASE;
ALTER DATABASE RECOVER MANAGED STANDBY DATABASE DISCONNECT;

SELECT flashback_on FROM v$database;
select name, open_mode, database_role from v$database;