On abandonment plugging of exploratory wells in Kuzbass
Abstract Generated abstract
The paper examines the reliability of abandonment plugging practices for exploratory wells in the Kuznetsk coal basin, where inadequately sealed boreholes may create pathways for groundwater inflow into future mine workings. Based on field observations and analysis of caliper survey materials, it argues that standard plugging methods often fail because borehole walls are not properly cleaned, actual diameters are much larger than assumed, wooden plugs do not wedge securely, and unsuitable cement is sometimes used. The author recommends preparing borehole walls, accounting for enlarged and irregular well diameters, selecting stable rock intervals near coal seams, using proper plugging cement, and considering local hydrogeological conditions to determine where plugging barriers are actually necessary.
Full Text
On Abandonment Plugging of Exploratory Wells in Kuzbass
G. A. Plevako
(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)
In the Kuznetsk Basin and within other coal-bearing provinces of the country, all exploratory wells, after they have been tested, must be reliably plugged so that, during the exploitation of deposits, sudden inrushes of groundwater into mine workings through wellbores do not occur.
The method of abandonment plugging of exploratory wells, in brief, is as follows. All coal seams intersected by the well are sealed off in the roof and floor by wooden meter-long plugs, which are held in the specified interval by special ruffs made from pieces of steel cable inserted into the holes provided in the plug. After the plug, which is lowered into the well with the aid of a core barrel, a cement-concrete mortar is forced out of the latter in the volume of the core barrel. Thus, the plugs are cemented, and the interval between them is filled with clayey mortar. Plugging operations are carried out by the drilling crew under the supervision of the senior foreman.
An analysis of the method of plugging work and the results of field investigations of its execution indicate the low quality of the plugging, which proves to be very poor for a number of reasons.
Before performing abandonment plugging, it is necessary to prepare the wellbore, which consists in removing from the walls of the well the clay cake formed in the process of washing the bottomhole with clayey mud. This requirement, as a rule, is not fulfilled. Bonding of the cement stone with the rocks in the walls of the well through clay cakes practically does not occur (Volokhiterkov, Volkov, 1965).
To install the plugging bridge, one must know the actual diameter of the well in the interval being plugged, since from its value the volume of the cement-concrete mixture and the dimensions of the wooden plug are calculated. In practice, the diameters of the plugs correspond to the diameters of the core barrels.
As has been established, the diameter of a well after drilling is always greater than the calculated one because of the selective destruction of rocks during the drilling process. As a result of analysis of well caliper-survey materials in various areas of Kuzbass, it was found that, beginning from the ground surface and down to depths of approximately 100 m, the diameter of wells increases almost twofold. In the intervals 100–150 m the diameter of wells
Leninskoye
Well 9874
Kinerskoye
Well 6683
Belovskoye
Well 4554
Fig. 1. 1 — coals; 2 — sandstones; 3 — carbonaceous argillites; 4 — siltstones; 5 — Quaternary deposits; 6 — argillites.
greater than the bit diameter by a factor of 1.6–1.7, and below depths of 350 m—by a factor of 1.4–1.5. The greatest destruction in the walls of a borehole occurs in siltstones, argillites, and especially coals. The thicker the coal seam, the greater the caving from it (Fig. 1). Sandstones are subject to destruction to a lesser degree. A general regularity is that destruction and caving of rocks, and the corresponding increase in borehole diameter, occur at the contacts of different lithological varieties. Therefore, where sandy-clayey rocks and coals frequently alternate, the borehole walls have the most complex profile. In such sections, almost down to depths of 500 m, borehole diameters are nearly twice the bit diameter.
As was found, caliper logging in exploratory boreholes is performed very rarely, and abandonment plugging is carried out without information on the diameter of the exploratory borehole.
In view of the above, the plugging wooden plugs installed at the roof and floor of coal seams do not become wedged; at best they hang or are held on ledges, while between them and the borehole wall there remains free space into which the cementing mixture forced from the drill string can freely fall beneath the plug without cementing the gaps.
Moreover, there is absolutely no guarantee that the cement slurry in the borehole will set (harden) reliably and quickly, since poor-quality cement that has been stored longer than the prescribed period is often used in plugging operations. Special studies on the reliability of isolating aquifers in boreholes have not always yielded positive results because low-quality cements were used.
In the course of hydrogeological surveys, the floating up of plugging plugs has repeatedly been observed in exploratory boreholes that had penetrated confined groundwater with self-flow to the surface.
In order to avoid the shortcomings noted and not waste funds, before performing plugging operations it is extremely necessary to prepare the borehole walls for normal setting of the cement-concrete mixture with the rocks. In the intervals where plugging bridges are to be installed, one should know at least approximately the degree of increase in borehole diameter in order to determine the size of the gap between the plug surface and the borehole wall. This will make it possible to determine the dimensions of the plugs and to select the wedging material.
To increase the reliability of plugging, one should be guided by stable intervals of rock in the borehole walls nearest to the roof and floor of the coal seam. Of no small importance is the selection of cement, which must correspond to the grade “plugging” and must be suitable for use in terms of storage time. Otherwise there is no guarantee that the cement will set and that the separating bridge will be strong.
To refine the location of plugging barriers, the hydrogeological conditions in the study area should be analyzed. There is probably no need to plug every coal seam, since it may turn out that, in the course of mining the deposit, groundwater inflows along a number of seams will not threaten mine workings, and anti-filtration curtains along the borehole will simply be superfluous.
LITERATURE
A. A. Volokitenkov, V. N. Volkov. Installation of artificial bottoms and separating bridges in boreholes. Nedra, 1965.