Lost bore wells need to be located for secondary oil extraction or ecological reasons when there is risk of ground or water contamination with oil or methane. In cases when bore well hardware is visible on the surface, locating it presents no trouble. In cases when coordinates of the bore well are approximate and it is not visible on the surface, the bore well is considered lost. Considering that only in the US there are 3 to 8 million bore wells, locating lost bore wells presents an important geophysical task.
Magnetic prospecting is most effective for locating old bore wells. Magnetic field penetrates through any kind of rocks and magnetic prospecting can be performed in any weather – through snow, ice, water, concrete, etc. Since the casing of the bore wells is made from strongly magnetized types of steel, locating the orifice of the pipe 108-132 mm in diameter on depths of 2-3 meters does not present any difficulty. Naturally, the larger the diameter of the pipe (and therefore its magnetic mass), the higher the depth at which it can be located. As a rule, the land site, which is to be searched, has about 20-40 meters in cross-section. On such sites, search is done along approximately parallel traverses 3-6 meters apart. In more complicated cases when the bore well couldn’t be located, magnetic prospecting can be performed. Such situation can come up when the orifice of the bore well is 4-6 meters deep. In such cases the size of the magnetic anomaly will be small and can be “masked” by the interferences from metal debris on the surface or underground. Magnetic field map built as a result of magnetic surveying will give a conclusive answer whether a bore well is located on the site.
Fluxgate gradiometer is very efficient for building magnetic field maps. It allows to build a very detailed grid (1 x 0.25 meter cell) of a 20 x 20 meter site in one hour. The map of the covered area can be viewed on display in real time.
In all cases we came across, bore wells located on depths 0.2 to 1.65 meters, gave anomalies that ranged between several hundred and 8 thousand nT (nanoTesla) when working with magnetometer MMPOZ-1 and up to 20 thousand nT when working with fluxgate gradiometer MG-400. There was no obvious correlation between depth of the bore well and strength of its anomaly: while the spread of the depths was minimal, the anomaly size varied from -400nT to +6000 nT. This means that the foremost factor determining the intensity of the anomaly is magnetization level of the bore well casing. Secondary factor – depth of the orifice.
Picture 1 and Foto 1, 2 show that anomalies above bore wells have complex shape, high amplitude, and their linear size reaches several meters in cross section. This allows modern magneto-metric equipment with sensitivity 0.1 nT easily locate orifice of bore wells on depths of up to several meters.
Fluxgate gradiometers are most effective magneto-metric devices for locating lost bore wells and can easily do it in motion with precision of 10-20 centimeters. It is possible due to frequency of taking and updating measurements on display – 10 times per second.