Wells are also divided into production and injection wells. The former is for production of oil and gas, injection wells is drilled to inject gas or water into the reservoir. The purpose of injection is to maintain overall and hydrostatic reservoir pressure and force the oil toward the production wells. When injected water reaches the production well, this is called injected water break through. Special logging instruments, often based on radioactive isotopes added to injection water, are used to detect breakthrough. Injection wells are fundamentally the same as production wellheads other than the direction of flow and therefore the mounting of some directional component such as the choke.