Wells (e.g., oil wells, gas wells, injection wells, water wells, etc.) are commonly formed by a process wherein a drilling apparatus is used to drill a well bore from the earth's surface to at least one subterranean formation. A typical drilling apparatus will include a tubular drill string having a drill bit positioned on the end thereof. Throughout the drilling process, a circulating fluid (e.g., a drilling mud) is typically pumped down the drill string and through the drill bit. After flowing through the drill bit, the circulating fluid flows up the well via an annulus defined between the wall of the well bore and the outer surface of the drill string. Among other things, this flow of circulating fluid assists the drilling process and carries drill cuttings out of the well.
Upon completion of the drilling process, the drilling apparatus is typically removed from the well bore and the well is cased. A well is cased by running a casing (i.e., a sturdy tubular conduit) into the well bore. Once in place, the casing prevents the well bore from collapsing and facilitates the performance of various downhole processes.
While inserting a casing into a well bore, it is sometimes necessary to pump a circulating fluid (e.g., a drilling mud or a completion fluid) out of the lower-most end of the casing. Before the casing is inserted into the well bore, portions of the well bore may collapse and/or shrink. However, by pumping a circulating fluid out of the end of the casing, and by rotating the casing as necessary, obstructions, such as those created by well bore shrinkage and/or partial well bore collapse, can oftentimes be cleared from the path of the casing so that the casing is enabled to descend into the well bore. As these obstructions are cleared, the circulating fluid also carries the resulting debris out of the well via an annulus defined between the wall of the well bore and the outer surface of the casing.
As is well known in the art, a high deviation well, e.g., a horizontally completed well, can be drilled when it is desirable to obtain a well bore which is not strictly vertical. As used herein and in the claims, the term "high deviation well" refers to any well having a well bore which is intentionally drilled such that one or more portions of the well bore are nonvertical. A high deviation well bore can be drilled, for example, when it is desirable to direct the well bore around, to, or through a given formation. The term "horizontally completed well", as used herein, refers to a well wherein the well bore has been drilled to include one or more substantially horizontal sections.
Subterranean formations, although typically very thin, can extend great distances horizontally. Thus, although the well bore of a strictly vertical well can extend only a few feet through a typical thin formation, a horizontally completed well can include one or more horizontal well bore sections which extend several hundred or several thousand feet through the formation. By providing much greater contact between the well bore and the formation, the horizontally completed well will typically provide a higher production rate than is provided by the strictly vertical well.
The installation of a well casing is particularly desirable when the well in question is a high deviation well. The installation of a well casing is especially desirable when the well in question is a horizontally completed well. Unless a casing is installed in the well bore of a high deviation well, the nonvertical portions of the well bore, and particularly any substantially horizontal portions of the well bore, are highly susceptible to cave-ins.
Methods of drilling high deviation wells are well known in the art. Such methods typically involve the use of a double acting drill bit assembly which: (a) drills along a substantially linear path when the drill stem is rotated and a circulating fluid is pumped through the drill bit assembly and (2) drills along an arcuate path when a circulating fluid is pumped through the drill bit assembly but the drill stem is not rotated.
In order to place a cased well in fluid communication with a subterranean formation through which the well casing runs, the Casing must be perforated. Typically, a casing is perforated after it is installed in the well bore. However, the process of perforating a casing after the casing is installed in the well bore can be time consuming and extremely expensive, particularly when the well in question is a horizontally completed well. In order to obtain the above-mentioned benefits of horizontal completion, at least one long horizontal section of casing must typically be placed in fluid communication with a subterranean formation through which said horizontal section of casing extends. Consequently, perforations must be formed along substantially the entire length of the horizontal section.
A need presently exists for (1) a useful, preperforated well casing and for (2) an effective and efficient method of installing a preperforated casing in a well. These are not adequately provided by merely perforating a casing aboveground and then inserting the perforated casing into the well bore. As indicated above, it is sometimes necessary to pump a circulating fluid out of the end of the casing as the casing is inserted into the well. Further, in order to remove drilling mud filter cake from the walls of the well bore and to remove unwanted fluids from the well bore, it is also desirable to pump a circulating fluid out of the end of the casing after the casing has been positioned in the well bore. If, however, even a few unsealed perforations are present in the wall of the casing during these circulating operations, a large quantity of circulating fluid will flow out of the perforations rather than through the end of the casing. Consequently, the amount of circulating fluid flowing out of the end of the casing typically will not be sufficient to accomplish the above-mentioned objectives.
A need also exists for (1) a useful, preperforated casing which can be installed in a well bore as the well bore is being drilled and (2) an efficient and effective method for installing a preperforated casing in a well bore as the well bore is being drilled. Until a casing is installed therein, a well bore is vulnerable to those forces which can cause the wall of the well bore to collapse. As indicated above, the danger of collapse is particularly great in the case of high deviation wells and is especially great in the case of horizontally completed wells. However, if a preperforated casing could be successfully installed at the same time that the well is being drilled, the danger of well bore collapse would be substantially eliminated. Additionally, the successful installation of this preperforated casing would eliminate the need to perform subsequent downhole perforation operations.
U.S. Pat. No. 2,224,630 issued to Dean et al. discloses a liner which is simply lowered into an oil or water well which has already been cased. The liner includes a preperforated screen. A portion of the preperforated screen remains open as the liner is lowered into the well casing. The remainder of the preperforated screen is covered with a thin layer of cement. Dean et al. further disclose a process including the steps of: (a) lowering the liner into the well casing until it reaches a predetermined depth; (b) depositing a gravel pack around the bottom exterior portion of the screen by pumping a gravel filled drilling mud solution down the annulus defined between the interior surface of the casing and the exterior surface of the liner; and (c) after the gravel pack is deposited, drilling through the thin cement layer in order to open the remainder of the screen.
Due to the high material density of cement, the cement debris produced by drilling a cement coating of the type described by Dean et al. is not easily circulated out of the well. Any cement debris remaining in the well can cause the drilling tool, or any other tool which is later inserted into the well, to become stuck in the well. Additionally, cement coatings of the type described by Dean et al. are difficult to install, particularly when long screen lengths are involved. Further, due to their low strength, cement coatings of the type described by Dean et al. are typically highly susceptible to cracking and breakage during shipping, handling, and installation.
U.S. Pat. No. 4,498,543 issued to Pye et al. discloses the insertion of a preperforated liner into a cased well. The lower end of the liner is closed, thus preventing the performance of circulation operations while the liner is being inserted and/or after the liner is inserted. The holes of the Pye et al. liner are temporarily sealed with hollow plugs. Each plug has an open flanged outside portion, which is urged against the outside surface of the liner, and a closed inner end which projects into the interior of the Pye et al. liner. When the liner is positioned in the well, a cutting tool is run down the inside of the liner in order to sever the closed ends of the plugs and thus open the perforations for fluid flow.
Plugged perforation systems of the type disclosed by Pye et al. produce various undesirable plug parts which, due to their size, shape, and material density, are very difficult to circulate out of a well. Thus, these plug parts deposit in the well and present obstructions which can cause the cutting tool, or a later inserted tool, to become stuck in the well. As is also apparent, the amount of trash (i.e , plug parts) deposited in the well will increase as the number of plugged perforations increases.
U.S. Pat. No. 3,386,510 issued to Schnabel, Jr. discloses a well point pipe having a strainer portion and a continuous, uniform, noncircular transverse cross section. The well point pipe is drilled into the ground by turning the pipe and applying downward pressure thereto while simultaneously jetting water through the end of the well point pipe. The strainer portion of the well point pipe is temporarily sealed with a sleeve. The sleeve is composed of a material which is insoluble in cold water (i.e., less than 100.degree. F.) but is highly soluble in warm water (i.e., at least 150.degree. F.). Thus, when the well point pipe is in place, the screen is removed by pumping warm water down the pipe. Several of the Schnabel, Jr. well point pipes are drilled down around the circumference of an excavation and are used to prevent water from flowing into the excavation. However, due to its noncircular (preferably square) cross section, the Schnabel, Jr. well point pipe is not suited for use as a casing for a well bore of any substantial depth. Further, due to the high temperature conditions existing downhole in deeper wells (e.g., in common oil or gas wells), the soluble sleeve portion of the Schnabel, Jr. device would dissolve before the Schnabel, Jr. pipe could be fully lowered into the well bore.