Return of boreholes
RETURN OF BOREHOLES (EN: return of a hole, return of a well; DE: Bohrlochrucknahme; FR: restitution des trous; ES: de los agujeros para la extraccion retorno; RU: возврат скважин) is the conversion of the boreholes for the extraction of the useful minerals (petroleum, gas, and so on) from the certain objects (layers, horizons) to the other objects.
This return of the boreholes is performed during the development of the fields with several productive layers, which have been drilled using the unified network of the boreholes, when the boreholes, which have unsealed one of the objects, have been completely exhausted, have been flooded with the water, or there has changed their technical status (the crumpling of the drill strings, the accidents with the equipment). They distinguish the return of the boreholes to the overlying objects (relatively to the objects, which have been operated previously), and to the underlying objects. They perform the return of the boreholes to the overlying objects, using the disconnection of the mine workings of the layer (horizon), using the cementing (under the pressure), most often using the mortars of the plugging Portland cement, with the leaving of the hardened "goblet" within the casing string, or using the installation of the bridge plugs. In case of the absence of the quality zonal isolation, they perform simultaneously the restoration of the hermeticity for the external annulus of the borehole, sometimes with the rise of the cement mortar for the overlap of the object, which is brought into the operation. After the verification of the hermeticity for the operational drill string, and of the quality of the zonal isolation, they p erform the unsealing of the overlying object, and the bringing of this object into the operation.
In case of the return of the boreholes to the underlying objects, they disconnect the layer (horizon), which was operated previously, using the cementing, with the drilling of the hardened "goblet" of the plugging substance within the casing string. In case of the extraction of the hydrocarbon raw materials, the return of the boreholes to the underlying objects is performed in the rare cases. In case of the existence of the abnormally high layer pressure within the object, which is being disconnected, they install the secret drill string (the transient drill string), with the subsequent cementing of the annulus of the borehole between the pipes (see the Figure). The disadvantage of this technology is the decreasing of the diameter (of the flow cross-section) within the operational casing string. There is more progressive the disconnection of the overlying object with the help of the longitudinal corrugated thin-walled nozzle, which is installed with the help of the special flaring tool, namely, the mandrel, or using the method of the creation of the profiled gate within the metal for the excessive hydraulic pressure.
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