Heat oil in stockpot, add shrimp shells, & fry until they are bright pink. Add 8 cups water and bring to boil and cook for 5 minutes. Strain and return the broth to the pot discarding the shrimp shells. Add lemon grass, raw shrimp, Thai fish sauce and lime juice, and reduce the heat to medium. Simmer for 5 minutes and skim off any foam. Just before serving, add the coconut milk, red chilis, green onion and fresh cilantro. Taste and adjust seasoning-you may like more coconut milk or salt. Enjoy.