Selective Plugging of Waterflood Input Wells Theory, Methods and Results

Selective plugging is a practical and economical method for preventing the wasteful circulation of water through barren zones and fractures. It is accomplished by injection into the wellhead of finely divided in a suitable liquid carrying agent, either oil or water. The paper describes several plugging agents now in use and the methods of application for such typical problems as open sand streaks natural and induced fractures, leaky packers or casing settings and thief zones. Field results are given for several typical selective plugging operational in Pennsylvania, Illinois, Oklahoma and Texas. These show how selective plugging reduced the amount of water injected and increased oil production. Introduction In most oil reservoirs the oil saturated formations have strata of varying permeabilities. In the process of water flooding the more permeable strata are depleted of oil first, and thereafter they continue to take large quantities of water even though they produce no more oil. Under such conditions the following will result:Large quantities of water are circulated through depleted formations making unnecessary expense in the way of water costs and pumping.The injection rate (pressure) may have to be reduced resulting in insufficient pressure on the less permeable strata causing the oil in them to be unrecoverable.High back-pressure in the vicinity of the producing well which will interfere with the flow of oil from the less permeable strata. Formation fractures natural or induced, will also by-pass large quantities of water. In fact, they are often the reason for failure of a flood right from the start. By-passing of water through fractures creates the same conditions as by-passing through highly permeable strata only worse. Loss of useful water will also result from leaky packers or casing settings and thief zones. These problem conditions were recognized soon after pressure flooding was started over a generation ago. Solutions were hoped for in the haphazard dumping in of nearly every granular solid imaginable along with portland cement, aqua gel and miscellaneous "gumbos." It wasn't until 1942, however, that a scientific approach was made to the problem through research sponsored by the Pennsylvania oil operators at the Pennsylvania State University. By 1944 the theory of selective plugging was well advanced and a practical plugging agent developed for the Bradford Third sand. Since then selective plugging agents and methods have been developed for conditions in most waterflooding areas of the country.