1. Field of the Invention
This invention relates to a method for conducting gas drilling operations, and more particularly concerns a method for conducting gas drilling operations to reduce the contaminant emissions during the drilling of a well through steam-bearing formations.
2. Description of the Prior Art
In conventional gas drilling operations, a gaseous drilling fluid, such as air, nitrogen, natural gas or other gaseous fluid, is passed from the earth surface downwardly through the drill pipe, outwardly through a rotary drill bit attached to the lower end of the drill pipe, and then upwardly through the annulus between the drill pipe and the walls of the borehole to the earth surface. Gas drilling using reverse circulation, wherein the gaseous drilling fluid is circulated downwardly through the well annulus and then upwardly through the drill pipe, is also known. The gaseous drilling fluid is circulated through the well at a volumetric flow rate sufficient to cool and drill bit and to lift the particulate drilling cuttings to the earth surface. In general the volumetric flow rate is selected such that the velocity of the solids-bearing gas rising through the well is sufficient to entrain the drilling cuttings and other solid particles found in the borehole. The prior art teaches that certain minimum velocities, and therefore minimum volumetric flow rates, are required and that velocities higher than this minimum velocity are preferred due to the very serious consequences of using an insufficient volumetric flow rate, e.g. a stuck drill pipe.
The solids-bearing gas rises through the well at velocities on the order of 10 to 100 feet per second, and higher. Solid particles transported at these velocities are highly abrasive and therefore erode the drill pipe and other metal surfaces to which they are exposed. Colisions between the particles and abrasion against the metal surfaces causes an overall diminution in size of the entrained particles. While the smaller particles are more easily entrained in the gas for removel from the borehole, they are more difficult to remove from the solids-bearing gas at the earth surface and therefore are usually vented to the atmosphere.
The problems of drill pipe erosion and contaminant emissions associated with gas drilling operations are further accentuated when drilling a well through steam-bearing formations, such as are encountered in the development of geothermal reservoirs. Since these formations have high temperatures, which can be in the order of 500.degree. F. or higher, and often contain corrosive brines, sulfurous compounds and carbon dioxide, corrosion and erosion of the drill pipe, casing and other metal surfaces in the well are often excessive. Also, as steam enters the borehole from the surrounding formation, the velocity of the solids-bearing gas rising through the well is substantially increased, even as high as sonic velocities, which further increases the erosion of metal surfaces exposed to this gas. The useful life of drill pipe employed under these conditions is relatively short, which fact alone may render impractical the use of gas drilling for a particular well.
The higher veolcities in the well also result in further diminution of the solids entrained in the gas rising through the well. A substantial fraction of the solids are reduced to particles with characteristic diameters of 2 microns and less. The removal of such fine particles from a large volume, high velocity gas stream is not practical with conventional separator devices, such as cyclones or filters. After passing the well effluent through the separator device, the gas, which still contains the finely divided solids, is conventionally vented to the atmosphere.
Emissions of particulate material and other contaminants, such as hydrogen sulfide, during the gas drilling of wells through steam-bearing strata have become a matter of environmental and regulatory concern. These emissions are often serious enough to render impractical the use of the otherwise desirable gas drilling techniques. Wells which are drilled during the development of a geothermal reservoir using other drilling techniques, such as drilling with oil or mud-based drilling fluids, often have much poorer steam production characteristics as compared to a well drilled by gas drilling techniques. Thus, there is a need for a method of conducting gas drilling operations in which contaminant emissions are reduced to a practical level.
Accordingly, it is a principal object of this invention to provide a method for reducing contaminant emissions during gas drilling operations.
Another object of this invention is to provide an improved method for drilling a well through steam-bearing formations in which the erosion of metal surfaces in the well and the emission of particulate material and other contaminants are reduced.
Yet another object of this invention is to provide an improved method for drilling a well through steam-bearing formations in which the emissions of contaminant-bearing steam is reduced and the high steam productivity of the well is maintained.
A further object of this invention is to provide an improved method for drilling a well through steam-bearing formations in which the emission of particulate material and other contaminants is reduced and the useful life of the drill pipe and other metal surfaces exposed to the solid-bearing gas rising through the well is increased.
Further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.