1. Field of the Invention
The present invention relates generally to well operations and, more particularly, pertains to a method of placing pipe, such as casing, within a highly deviated borehole.
The present state of the art related to well drilling has developed techniques to drill boreholes which angularly deviate from the vertical axis that extends downwardly from the drilling rig. It has, therefore, become necessary to devise a method for the placement of pipe longitudinally throughout the borehole regardless of the degree of deviation of the borehole from the vertical axis of the drilling rig.
2. Discussion of the Prior Art
The inventive arrangement provides a method for the placement of pipe longitudinally within the borehole by pushing the pipe through the vertical portion of the borehole down to the deviated portion of the borehole, subsequently filling the pipe with fluid having a lower density than the surrounding fluids, and then further forcing the pipe into and through the deviated zone of the borehole. Upon the final placement of the pipe within the borehole, the fluids are subsequently released into the mud slurry surrounding the pipe. The pipe is then in the desired position within the borehole and can be readily used.
As is well known in the prior art, numerous problems are encountered in pushing pipe down into a borehole which has a high angle of deviation from the vertical axis of the drilling rig. The major problem encountered in attempting to push a pipe through the deviated zone is the contacting of the pipe with the wall of the drilled borehole. In the non-vertical portion of the hole, or what is commonly referred to as the deviation zone, the borehole deviates from the vertical axis of the drill rig. Consequently, the pipe is forced against the wall of the borehole by the downward force provided at the drill rig. Additionally, the weight of the pipe itself forces the pipe to rest against the side of the borehole and, because of friction, causes a force opposite to the direction of the desired advance of the pipe. The greater the weight of the pipe, and the greater the borehole's angle from the vertical axis of the rig, the greater the drag force opposing the downward movement of the pipe. The downward applied force, as well as the weight of the pipe itself, produces a formidable frictional drag force acting against the downward movement of the pipe.
In addition to this drag force, as the pipe lays against the side of the borehole, the pipe displaces some of the filter cake, or wall of the borehole, thereby embedding itself into the cake forming an effective pressure seal within the embedded area. In the permeable sections of the formation, a loss of pressure between the fluid in the borehole and the fluid in the formation causes a force across this pressure seal to further push the pipe against the wall. This force may cause the pipe to become what is commonly called "differentially stuck". Consequently, the forces which hinder the downward movement of the pipe, notably frictional forces and the pressure differential force, vary directly with the force of the pipe which is directed against the borehole wall. As a result of the creation of these forces, it is considered highly desirable to minimize the contact of pipe to borehole wall. By minimizing such contact, the applied force required to push the pipe down the borehole will also be minimized.
The present invention effectively alleviates the aforementioned problems related to the differential sticking and drag forces associated with pipe being forced down into a deviated borehole. None of the prior art arrangements directed to solving these problems do so as effectively and inexpensively as the present invention. None of the prior art, of which Aulick U.S. Pat. No. 3,526,280 is most pertinent, show or even suggest the method of the present invention herein described.
As is illustrated in Aulick U.S. Pat. No. 3,526,280 a related well completion operation is outlined therein for highly deviated wells. As described therein, cement slurry is pumped down into the hole to partially displace and replace the mud slurry. The lower portion of the casing string is filled up with fluid of lower density than the cement slurry, thereby providing a buoyancy effect to the lower chamber of the casing string. Centralizers are further provided throughout the length of the casing string to minimize contact of pipe to borehole wall. The buoyancy chamber causes the pipe to float through the deviation zone of the well, thus avoiding extreme centralizer and casing deflections. This patent provides, therefore, a more complicated apparatus to overcome some of the same problems that the present invention is directed towards. However, this patent is inapplicable in very deep, highly deviated, wells. More particularly, the combination of slow-forming cement slurry, fluids and numerous centralizers, well known in the prior art, are combined to minimize the contact of the leading end of the pipe and the borehole wall. In the present invention, the placement of the pipe throughout the borehole is effected by the use of the fluid itself with no need for centralizers or slow-setting cement slurry. Additionally, Aulick is inapplicable in deep well applications. Where the wells are particularly deep, the flotation chamber described in Aulick will have insufficient bouyant capability to prevent the contact of the pipe with the borehole wall at a great distance from the chamber. As a result, great centralizer and casing deflections will occur. The pipe which is at a distance from the flotation chamber will come into contact with the borehole wall thereby creating the drag forces and the subsequent differential sticking problems, thereby rendering the teaching of this patent insufficient in regards to problems encountered when extremely long lengths of pipe are to be used.
The method of the present invention described herein overcomes these aforementioned problems. Contrary to the teachings of the Aulick patent, the present invention provides a method wherein the pipe being pushed down into the borehole can have differing lengths of pipe containing different fluids to aid in centering the pipe. This advantage is especially desirable in deep wells which have differing angles of declination at different points along the borehole's length. The operation of the rig can, in effect, control the bouyancy along the entire length of pipe being pushed down as required by the particular circumstances of the drilling operation, unlike in the prior art.