where the casing contacts the borehole. These channels can provide a path for a later blowout. Even a thin crack can be pushed open by the enormous pressure of oil from below. Then erosion of the cement can occur from high-velocity sand particles in the oil. A hairline crack can thus become a wide-open gushing channel. Another cause of cement failure is not waiting long enough for the cement to solidify. This can be the result of a rushed drilling schedule, or it could happen if there is a leak causing the cement to creep during the time it is supposed to be setting. A "cement evaluation log" can be run after each cement job to provide a detailed, 360-degree check of the integrity of the entire seal. Sometimes these logs are skipped due to schedule pressures. Cement is also used to form permanent barriers in the annulus outside the production liner, and temporary barriers inside the liner. The temporary barriers are used to "shut in" the well after drilling and before the start of production. Figure 4 shows a barrier being tested by replacing the heavy mud above it with lighter seawater. If the cement plug is able to contain the pressure from the mud below, there will be no upward flow of seawater, and it can be replaced with mud for the final shut in. There are no cement barriers in the annulus in Figure 4. While there is no requirement for such barriers, adding them can minimize the risk of a blowout through a direct wide-open channel from the reservoir to the surface. Human factors See also Automated Data Inquiry for Oil Spills References External links U.S. Coast Guard and Environmental Protection Agency, Oil Spill Prevention, Control, & Countermeasure Regulations American Petroleum Institute, Oil Spill Prevention and Response NOAA, 2002. Oil Spill Prevention and Response: A Selected Bibliography on the Exxon Valdez Oil Spill Offshore Technology Resource Center. 2001. Comparative Risk Analysis for Deepwater Production Systems Oil & Gas UK, Oil Spill Prevention and Response Advisory Group (OSPRAG) International Oil Spill Conference (IOSC), 1969–present. Archives of over 3,000 papers and full-text conference proceedings covering spill prevention, planning, response and restoration processes, protocols and technology. oil spills are bad Drilling technology Energy in the United States Offshore drilling debate Environmental controversies Engineering articles needing expert attention Oil spill remediation technologies