<P> Tertiary recovery begins when secondary oil recovery is not enough to continue adequate extraction, but only when the oil can still be extracted profitably . This depends on the cost of the extraction method and the current price of crude oil . When prices are high, previously unprofitable wells are brought back into use, and when they are low, extraction is curtailed . </P> <P> The use of microbial treatments is another tertiary recovery method . Special blends of the microbes are used to treat and break down the hydrocarbon chain in oil, making the oil easy to recover . It is also more economical versus other conventional methods . In some states such as Texas, there are tax incentives for using these microbes in what is called a secondary tertiary recovery . Very few companies supply these, however, companies like Bio Tech, Inc. have proven very successful in waterfloods across Texas . </P> <P> The amount of oil that is recoverable is determined by a number of factors, including the permeability of the rock, the strength of natural drives (the gas present, pressure from adjacent water or gravity), porosity of the reservoir rock, i.e the rock storage capacity, and the viscosity of the oil . When the reservoir rocks are "tight", as in shale, oil generally cannot flow through, but when they are permeable, as in sandstone, oil flows freely . </P> <P> Although recovery of a well cannot be known with certainty until the well ceases production, petroleum engineers often determine an estimated ultimate recovery (EUR) based on decline rate projections years into the future . Various models, mathematical techniques, and approximations are used . </P>

How do you get oil out of the earth