The present disclosure is directed to both a method and apparatus for drilling wells, and in particular for drilling wells which are drilled into geothermal formations. Geothermal wells are normally drilled to obtain steam at relatively elevated pressures and temperatures. Particular emphasis must be placed on the elevated temperature. To be sure, when an oil or gas well is drilled, there is a temperature gradient experienced as the well passes through various formations of the earth. Assume for vis h vis illustration that the surface temperature is about 75.degree. F. As a well is drilled to great depths, 10,000 feet, even 15,000 feet or deeper, there is a well known temperature gradient. Absent any geothermal activities in the near region, the temperature experienced by the well drilling equipment increases with depth at a specified rate. That rate, however, assumes the absence of a geothermal formation. A geothermal formation is characterized by the fact that heat radiates outwardly into the various geological strata between the well and the geothermal formation. The geothermal formation normally is porous having water which is captured in near proximity of the relatively hot underground formation. This fills with water which is so hot that, absent the high pressure containment, it would otherwise flash into steam. Thus, when a well is drilled into the formation, the hot water is tapped and released into the well and flashes into steam when exposed to pressure reduction sufficient to permit formation of steam. It is not uncommon for underground formations to achieve a temperature level of 500.degree. F. or 600.degree. F. or even hotter. When tapped, a typically released flow of steam, which is quite hot, travels rapidly at high velocity up the producing well to the surface. It is not uncommon to recover steam at the surface which is delivered at a temperature of perhaps about 500.degree. F. and in many instances even hotter.
This should be contrasted with oil production from equal depths. Ordinarily, the oil is delivered at the ambient temperature of the oil in situ in the oil bearing formation. In other words, the temperature of the oil is fairly well predictable and is normally dependent on the depths which the oil producing formation or tapped. As will be understood, the oil can sometimes be slightly hotter or colder, but it fairly well does not stray excessively from the temperature levels common for that formation.
The present disclosure sets forth a method and apparatus for drilling into geothermal formations which deliver extremely hot water, and more commonly which deliver saturated steam. When steam is delivered, it flows at such high velocities that the production equipment, even to the surface, is heated. Indeed, heat loss is held to a minimum because of the high velocity of the steam flow. Thus, the wellhead equipment can be installed at a high mountainous region where it is typically cold all winter and yet the wellhead equipment can quickly assume the temperature of the produced steam and maintain that temperature as long as production continues.
One of the uses of a geothermal well is that it provides a tremendous amount of energy in the form of heated steam for operation of steam generators and the like. The present disclosure is a method for drilling and an apparatus assisting in the production of steam from such wells. More particularly, a significant and substantial problem is encountered in the operation of such wells. Assume for purposes of illustration that the geothermal well is 6,000 ft. in depth. They can vary widely. In any event, the well typically must be cased so that the produced steam does not erode the borehole, destroy the definite shape or profile of the borehole, thereby destroying the well. The casing is installed in the well at the time of drilling. The casing string is exposed to a tremendous swing in temperature. With the swing in temperature, the casing is heated. It is heated so hot that the casing will elongate and thereby shift the wellhead equipment. Consider in a well of 6,000 ft. that the temperature differential for the casing on the average is 400.degree.. In one example, the casing is installed without steam and without heating from the producing geothermal formation at a temperature of 90.degree. along the length of the casing while after production has been initiated, the casing will be heated to 490.degree. A change of 400.degree. causes elongation of the casing sufficiently great that the casing will move the wellhead equipment unless otherwise constrained. Casing is cemented along the full length of the casing. When the change occurs, the cemented casing will be placed under a strain by virtue of the steam flow. This runs the risk of breaking the bond either between the casing and the cement or between the cement and the open hole. In either instance, it is undesirable. In both instances, it is undesirable because it may risk a migration channel along the casing on the exterior defeating the basic purpose for which the casing is normally cemented in place. As a matter of fact, cementing of the casing is normally required as a matter of good sense in well completion techniques and also as a matter of obedience to various and sundry regulatory agencies. It is highly undesirable that external migration along the cased well occur because such migration may poison clean water or artesiat formations. Artesian formations in adjacent aquafiers can be severely damaged by such migration.
The present disclosure enables the drilling and installation of a casing string in a geothermal well with a view of avoiding the stress which is so commonly encountered once the casing has been heated by the steam flow and a stable temperature has been established. The casing is normally heated so that the casing will move, thereby creating damage either by tearing free at one end or the other of the well, or by breaking the bond in the casing and cement structure which is formed. In the well borehole. The borehole is thus protected by the implementation of the present procedure. This procedure will be described by setting forth a number of sequential operational steps.
In one aspect, the present invention enables one of average skill in the art to drill and complete a geothermal well. The well is drilled by drilling through various formations of the earth toward the geothermal formation of interest. Preferably, when the well is drilled, the well is stopped short of the geothermal formation of interest. As a generalization, the depth of the geothermal formation is normally well known in advance. The depth is normally identified in a wild cat well. In a production well drilled thereafter, it is usually known where the geothermal formation will be. Alternately, with the use of temperature detectors supported in the drill string, and especially at the very bottom portion of the drill stem next to the drill bit, temperature measurements can be obtained and the temperature measurements can be used to make a prediction of when the geothermal formation will be penetrated. A geothermal formation normally is thermally isolated by the formations there above. To the extent that the geothermal formation is pervious, one or more adjacent formations tend to be impervious. This typically involves a formation which provides effective isolation to the geothermal formation so that the hot water does not penetrate the adjacent formations. While the hot water does not penetrate, heat is transferred into adjacent formations and the temperature will increase even at distances of 50 or 100 ft. from the geothermal formation targeted by the drilling process. Also, the temperature profile at depth might be well known. That adds an incremental increase in temperature as the well is drilled which can be readily known. Examples will be given below in which the, geothermal formation is located at a depth of 6000 ft. Proximity can be detected at least 200 ft. there above.
The procedure of the present disclosure includes the step of drilling to that specified depth. That depth will be distinguished from other depths. The well is cased at the upper portions of the well. The casing is extended to the bottom of the well drilled to that stage, and a cement shoe is then installed so that cement can be pumped down into the annular space. Separate cement jobs are implemented. The first cement job is to anchor a surface conductor string at the surface. The conductor casing string is specifically anchored by the first cement job. The surface cement job has a depth of a few hundred feet. The second cement job is pumped through the casing and cement shoe and flows into the annular space on the exterior of the casing. The second cement job will be defined below in more detail but it is primarily a cement job in which the lower end of the casing is anchored. The casing string is pulled upwardly, and the casing at the top end is cemented to anchor within the surface conductor string. The surface conductor pipe is thus cemented on both the exterior and the interior. Furthermore, the casing is left under tension, being anchored at its lower end, and is cemented in placed by the third cement job. After the casing is placed under tension, it is also equipped with a spool which surrounds the casing which thereafter grips and locks the casing and enables any surplus casing to be cut off. Wellhead equipment is completed and a Christmas tree is installed on the wellhead equipment. The latter step occurs after drilling has extended the well through the cementing shoe and into the producing geothermal formation there below.
While the foregoing mentions a few of the details with regard to the procedure contemplated in practice of the present invention, the structural apparatus of the invention is likewise disclosed in some detail and is incorporated in this brief summary. The wellhead completion equipment particularly emphasizes a sleeve system which holds the casing so that it can be placed under tension.