Abstract:
The present invention generally relates to a method and an apparatus for drilling with casing. In one aspect, a method of drilling a wellbore with casing is provided, including placing a string of casing with a drill bit at the lower end thereof into a previously formed wellbore and urging the string of casing axially downward to form a new section of wellbore. The method further includes pumping fluid through the string of casing into an annulus formed between the casing string and the new section of wellbore. The method also includes diverting a portion of the fluid into an upper annulus in the previously formed wellbore. In another aspect, a method of drilling with casing to form a wellbore is provided. In yet another aspect, an apparatus for forming a wellbore is provided. In still another aspect, a method of casing a wellbore while drilling the wellbore is provided.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to wellbore completion. More particularly, the invention relates to effectively increasing the carrying capacity of the circulating fluid without damaging wellbore formations. More particularly still, the invention relates to removing cuttings in a wellbore during a drilling operation. 
   2. Description of the Related Art 
   In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed, and the wellbore is lined with a string of casing with a specific diameter. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore. 
   It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The well is then drilled to a second designated depth and thereafter lined with a string of casing with a smaller diameter than the first string of casing. This process is repeated until the desired well depth is obtained, each additional string of casing resulting in a smaller diameter than the one above it. The reduction in the diameter reduces the cross-sectional area in which circulating fluid may travel. 
   Typically, fluid is circulated throughout the wellbore during the drilling operation to cool a rotating bit and remove wellbore cuttings. The fluid is generally pumped from the surface of the wellbore through the drill string to the rotating bit. Thereafter, the fluid is circulated through an annulus formed between the drill string and the string of casing and subsequently returned to the surface to be disposed of or reused. As the fluid travels up the wellbore, the cross-sectional area of the fluid path increases as each larger diameter string of casing is encountered. For example, the fluid initially travels up an annulus formed between the drill string and the newly formed wellbore at a high annular velocity due to small annular clearance. However, as the fluid travels the portion of the wellbore that was previously lined with casing, the enlarged cross-sectional area defined by the larger diameter casing results in a larger annular clearance between the drill string and the cased wellbore, thereby reducing the annular velocity of the fluid. This reduction in annular velocity decreases the overall carrying capacity of the fluid, resulting in the drill cuttings dropping out of the fluid flow and settling somewhere in the wellbore. This settling of the drill cuttings and debris can cause a number of difficulties to subsequent downhole operations. For example, it is well known that the setting of tools against a casing wall is hampered by the presence of debris on the wall. 
   Several methods have been developed to prevent the settling of the drill cuttings and debris by overcoming the deficiency of the carrying capacity of the circulating fluid. One such method is used in a deepwater application where the increased diameter of the drilling riser results in a lower annular velocity in the riser system. Generally, fluid from the surface of the floating vessel is injected into a lower portion of the riser system through a flow line disposed on the outside of the riser pipe. This method is often referred to as “charging the riser”. This method effectively increases the annular velocity and carrying capacity of the circulating fluid to assist in wellbore cleaning. However, this method is not practical for downhole operations. 
   Another method to prevent the settling of the drill cuttings and debris is by simply increasing the flow rate of the circulating fluid over the entire wellbore interval to compensate for the lower annular velocity in the larger annular areas. This method increases the annular velocity in the larger annular areas, thereby minimizing the amount of settling of the drill cuttings and debris. However, the higher annular velocity also increases the potential of wellbore erosion and increases the equivalent circulating density, which deals with the friction forces brought about by the circulation of the fluid. Neither effect is desirable, but this method is often used by operators to compensate for the poor downhole cleaning due to lower annular velocity of the circulating fluid. 
   Potential problems associated with flow rate and the velocity of return fluid while drilling are increased when the wellbore is formed by a technique known as “drilling with casing”. Drilling with casing is a method where a drill bit is attached to the same string of tubulars that will line the wellbore. In other words, rather than run a drill bit on smaller diameter drill string, the bit is run at the end of larger diameter tubing or casing that will remain in the wellbore and be cemented therein. The bit is typically removed in sections or destroyed by drilling the next section of the wellbore. The advantages of drilling with casing are obvious. Because the same string of tubulars transports the bit as lines the wellbore, no separate trip into the wellbore is necessary between the forming of the wellbore and the lining of the wellbore. 
   Drilling with casing is especially useful in certain situations where an operator wants to drill and line a wellbore as quickly as possible to minimize the time the wellbore remains unlined and subject to collapse or to the effects of pressure anomalies. For example, when forming a subsea wellbore, the initial length of wellbore extending from the ocean floor is much more subject to cave in or collapse due to soft formations as the subsequent sections of wellbore. Sections of a wellbore that intersect areas of high pressure can lead to damage of the wellbore between the time the wellbore is formed and when it is lined. An area of exceptionally low pressure will drain expensive circulating fluid from the wellbore between the time it is intersected and when the wellbore is lined. 
   In each of these instances, the problems can be eliminated or their effects reduced by drilling with casing. However, drilling with casing results in a smaller annular clearance between the outer diameter of the casing and the inner diameter of the newly formed wellbore. This small annular clearance causes the circulating fluid to travel through the annular area at a high annular velocity, resulting in a higher potential of wellbore erosion compared to a conventional drilling operation. 
   A need therefore exists for an apparatus and a method for preventing settling of drill cuttings and other debris in the wellbore during a drilling operation. There is a further need for an apparatus and a method that will effectively increase the carrying capacity of the circulating fluid without damaging wellbore formations. There is yet a further need for a cost-effective method for cleaning out a wellbore while drilling with casing. 
   SUMMARY OF THE INVENTION 
   The present invention generally relates to a method and an apparatus for drilling with casing. In one aspect, a method of drilling a wellbore with casing is provided, including placing a string of casing with a drill bit at the lower end thereof into a previously formed wellbore and urging the string of casing axially downward to form a new section of wellbore. The method further includes pumping fluid through the string of casing into an annulus formed between the casing string and the new section of wellbore. The method also includes diverting a portion of the fluid into an upper annulus in the previously formed wellbore. 
   In another aspect, a method of drilling with casing to form a wellbore is provided. The method includes placing a casing string with a drill bit at the lower end thereof into a previously formed wellbore and urging the casing string axially downward to form a new section of wellbore. The method further includes pumping fluid through the casing string into an annulus formed between the casing string and the new section of wellbore. Additionally, the method includes diverting a portion of the fluid into an upper annulus in the previously formed wellbore from a flow path in a run-in string of tubulars disposed above the casing string. 
   In yet another aspect, an apparatus for forming a wellbore is provided. The apparatus comprises a casing string with a drill bit disposed at an end thereof and a fluid bypass formed at least partially within the casing string for diverting a portion of fluid from a first to a second location within the casing string as the wellbore is formed. 
   In another aspect, a method of casing a wellbore while drilling the wellbore is provided, including flowing a fluid through a drilling apparatus. The method also includes operating the drilling apparatus to drill the wellbore, the drilling apparatus comprising a drill bit, a wellbore casing, and a fluid bypass. The method further includes diverting a portion of the flowing fluid with the fluid bypass and placing at least a portion of the wellbore casing in the drilled wellbore. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a cross-sectional view illustrating a flow apparatus disposed at the lower end of the run-in string. 
       FIG. 2A  is a cross-sectional view illustrating an auxiliary flow tube partially formed in a casing string. 
       FIG. 2B  is a cross-sectional view illustrating a main flow tube formed in the casing string. 
       FIG. 3  is a cross-sectional view illustrating the flow apparatus and auxiliary flow tube in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention relates to apparatus and methods for effectively increasing the carrying capacity of the circulating fluid without damaging wellbore formations. The invention will be described in relation to a number of embodiments and is not limited to any one embodiment shown or described. 
     FIG. 1  is a section view of a wellbore  100 . For clarity, the wellbore  100  is divided into an upper wellbore  100 A and a lower wellbore  100 B. The upper wellbore  100 A is lined with casing  110  and an annular area between the casing  110  and the upper wellbore  100 A is filled with cement  115  to strengthen and isolate the upper wellbore  100 A from the surrounding earth. At a lower end of the upper wellbore  100 A, the casing  110  terminates and the subsequent lower wellbore  100 B is formed. Coaxially disposed in the wellbore  100  is a work string  120  made up of tubulars with a running tool  130  disposed at a lower end thereof. Generally, the running tool  130  is used in the placement or setting of downhole equipment and may be retrieved after the operation or setting process. The running tool  130  in this invention is used to connect the work string  120  to a casing string  150  and subsequently release the casing string  150  after the lower wellbore  100 B is formed and the casing string  150  is secured. 
   As illustrated, a drill bit  125  is disposed at the lower end of the casing string  150 . Generally, the lower wellbore  100 B is formed as the drill bit  125  is rotated and urged axially downward. The drill bit  125  may be rotated by a mud motor (not shown) located in the casing string  150  proximate the drill bit  125  or by rotating the casing string  150 . In either case, the drill bit  125  is attached to the casing string  150  that will subsequently remain downhole to line the lower wellbore  100 B, therefore there is no opportunity to retrieve the drill bit  125  in the conventional manner. In this respect, drill bits made of drillable material, two-piece drill bits or bits integrally formed at the end of casing string are typically used. 
   Circulating fluid or “mud” is circulated down the work string  120 , as illustrated with arrow  145 , through the casing string  150  and exits the drill bit  125 . The fluid typically provides lubrication for the drill bit  125  as the lower wellbore  100 B is formed. Thereafter, the fluid combines with other wellbore fluid to transport cuttings and other wellbore debris out of the wellbore  100 . As illustrated with arrow  170 , the fluid initially travels upward through a smaller annular area  175  formed between the outer diameter of the casing string  150  and the lower wellbore  100 B. Generally, the velocity of the fluid is inversely proportional to the annular area defining the fluid path. In other words, if the fluid path has a large annular area then the velocity of the fluid is low. Conversely, if the fluid path has a small annular area then the velocity of the fluid is high. Therefore, the fluid traveling through the smaller annular area  175  has a high annular velocity. 
   Subsequently, the fluid travels up a larger annular area  140  formed between the work string  120  and the inside diameter of the casing  110  in the upper wellbore  100 A as illustrated by arrow  165 . As the fluid transitions from the smaller annular area  175  to the larger annular area  140  the annular velocity of the fluid decreases. Similarly, as the annular velocity decreases, so does the carrying capacity of the fluid resulting in the potential settling of drill cuttings and wellbore debris on or around the upper end of the casing string  150 . To increase the annular velocity, a flow apparatus  200  is used to inject fluid into the larger annular area  140 . 
   Disposed on the work string  120  and shown schematically in  FIG. 1  is the flow apparatus  200 . Although  FIG. 1  shows one flow apparatus  200  attached to the work string  120 , any number of flow apparatus may be attached to the work string  120  or the casing string  150  in accordance with the present invention. The purpose of the flow apparatus  200  is to divert a portion of the circulating fluid into the larger annular area  140  to increase the annular velocity of the fluid traveling up the wellbore  100 . It is to be understood, however, that the flow apparatus  200  may be disposed on the work string  120  at any location, such as adjacent the casing string  150  as shown on  FIG. 1  or further up the work string  120 . Furthermore, the flow apparatus  200  may be disposed in the casing string  150  or below the casing string  150  providing the lower wellbore  100 B would not be eroded or over pressurized by the circulating fluid. 
   One or more ports  215  in the flow apparatus  200  may be modified to control the percentage of flow that passes to drill bit  125  and the percentage of flow that is diverted to the larger annular area  140 . The ports  215  may also be oriented in an upward direction to direct the fluid flow up the larger annular area  140 , thereby encouraging the drill cuttings and debris out of the wellbore  100 . Furthermore, the ports  215  may be systematically opened and closed as required to modify the circulation system or to allow operation of a pressure controlled downhole device. 
   The flow apparatus  200  is arranged to divert a predetermined amount of circulating fluid from the flow path down the work string  120 . The diverted flow, as illustrated by arrow  160 , is subsequently combined with the fluid traveling upward through the larger annular area  140 . In this manner, the annular velocity of fluid in the larger annular area  140  is increased which directly increases the carrying capacity of the fluid, thereby allowing the cuttings and debris to be effectively removed from the wellbore  100 . At the same time, the annular velocity of the fluid traveling up the smaller annular area  175  is lowered as the amount of fluid exiting the drill bit  125  is reduced. In this respect, the annular velocity of the fluid traveling down the work string  120  is used to effectively transport drill cutting and other debris up the larger annular area  140  while minimizing erosion in the lower wellbore  100 B by the fluid traveling up the annular area  175 . 
     FIG. 2A  is a cross-sectional view illustrating an auxiliary flow tube  205  partially formed in the casing string  150 . As illustrated with arrow  145 , circulating fluid is circulated down the work string  120  through the casing string  150  and exits the drill bit  125  to provide lubrication for the drill bit  125  as the lower wellbore  100 B is formed. Thereafter, the fluid combines with other wellbore fluid to transport cuttings and other wellbore debris out of the wellbore  100 . As illustrated with arrow  170 , the fluid initially travels at a high annular velocity upward through a portion of the smaller annular area  175  formed between the outer diameter of the casing string  150  and the lower wellbore  100 B. However, at a predetermined distance, a portion of the fluid, as illustrated by arrow  210 , is redirected to the auxiliary flow tube  205  disposed in the casing string  150 . Furthermore, the auxiliary flow tube  205  may be systematically opened and closed as required to modify the circulation system or to allow operation of a pressure controlled downhole device. 
   The auxiliary flow tube  205  is constructed and arranged to remove and redirect a predetermined amount of high annular velocity fluid traveling up the smaller annular area  175 . In other words, the auxiliary flow tube  205  increases the annular velocity of the fluid traveling up the larger annular area  140  by diverting a portion of high annular velocity fluid in the smaller annular area  175  to the larger annular area  140 . Although  FIG. 2A  shows one auxiliary flow tube  205  attached to the casing string  150 , any number of auxiliary flow tubes may be attached to the casing string  150  in accordance with the present invention. Additionally, the auxiliary flow tube  205  may be disposed on the casing string  150  at any location, such as adjacent the drill bit  125  as shown on  FIG. 2A  or further up the casing string  150 , so long as the high annular velocity fluid in the smaller annular area  175  is transported to the larger annular area  140 . In this respect, the annular velocity of fluid in the larger annular area  140  is increased which directly increases the carrying capacity of the fluid allowing the cuttings and debris to be effectively removed from the wellbore  100 . At the same time, the annular velocity of the fluid traveling up the smaller annular area  175  is reduced, thereby minimizing erosion or pressure damage in the lower wellbore  100 B by the fluid traveling up the annular area  175 . 
     FIG. 2B  is a cross-sectional view illustrating a main flow tube  220  formed in the casing string  150 . As illustrated with arrow  145 , circulating fluid is circulated down the work string  120  through the casing string  150  and exits the drill bit  125  to provide lubrication as the lower wellbore  100 B is formed. Thereafter, the fluid combines with other wellbore fluid to transport cuttings and other wellbore debris out of the wellbore  100 . Subsequently, as illustrated with arrow  170 , a first portion of the fluid at a high annular velocity travels upward through a portion of the smaller annular area  175  formed between the outer diameter of the casing string  150  and the lower wellbore  100 B. A second portion of fluid, as illustrated by arrow  210 , travels through the main flow tube  220  to the larger annular area  140 . In the same manner as discussed in a previous paragraph, the annular velocity of fluid in the larger annular area  140  is increased and the annular velocity of the fluid in the smaller annular area  175  is reduced, thereby minimizing erosion or pressure damage in the lower wellbore  100 B by the fluid traveling up the annular area  175 . 
     FIG. 3  is a cross-sectional view illustrating the flow apparatus  200  and auxiliary flow tube  205  in accordance with the present invention. In the embodiment shown, the flow apparatus  200  is disposed on the work string  120  and the auxiliary flow tube  205  is disposed on the casing string  150 . It is to be understood, however, that the flow apparatus  200  may be disposed on the work string  120  at any location, such as adjacent the casing string  150  as shown on  FIG. 3  or further up the work string  120 . Furthermore, the flow apparatus  200  may be disposed in the casing string  150  or below the casing string  150  providing the lower wellbore  100 B would not be eroded or over pressurized by the fluid exiting the flow control apparatus  200 . In the same manner, the auxiliary flow tube  205  may be positioned at any location on the casing string  150 , so long as the high annular velocity fluid in the smaller annular area  175  is transported to the larger annular area  140 . Additionally, it is within the scope of this invention to employ a number of flow apparatus or auxiliary flow tubes. 
   Similar to the other embodiments, fluid is circulated down the work string  120  through the casing string  150  to lubricate and cool the drill bit  125  as the lower wellbore  100 B is formed. Thereafter, the fluid combines with other wellbore fluid to transport cuttings and other wellbore debris out of the wellbore  100 . However, in the embodiment illustrated in  FIG. 3 , a portion of fluid pumped through the work string  120  may be diverted through the flow apparatus  200  into the larger annular area  140  at a predetermined point above the casing string  150 . At the same time, a portion of high velocity fluid traveling up the smaller annular area  175  may be communicated through the auxiliary flow tube  205  into the larger annular area  140  at a predetermined point below the upper end of the casing string  150 . 
   The operator may selectively open and close the flow apparatus  200  or the auxiliary flow tube  205  individually or collectively to modify the circulation system. For example, an operator may completely open the flow apparatus  200  and partially close the auxiliary flow tube  205 , thereby injecting circulating fluid in an upper portion of the larger annular area  140  while maintaining a high annular velocity fluid traveling up the smaller annular area  175 . In the same fashion, the operator may partially close the flow apparatus  200  and completely open the auxiliary flow tube  205 , thereby injecting high velocity fluid to a lower portion of the larger annular area  140  while allowing minimal circulating fluid into the upper portion of the larger annular area  140 . There are numerous combinations of selectively opening and closing the flow apparatus  200  or the auxiliary flow tube  205  to achieve the desired modification to the circulation system. Additionally, the flow apparatus  200  and the auxiliary flow tube  205  may be hydraulically opened or closed by control lines (not shown) or by other methods well known in the art. 
   In operation, a work string, a running tool and a casing string with a drill bit disposed at a lower end thereof are inserted into a wellhead and coaxially disposed in an upper wellbore. Subsequently, the casing string and the drill bit are rotated and urged axially downward to form the lower wellbore. At the same time, circulating fluid or “mud” is circulated down the work string through the casing string and exits the drill bit. The fluid typically provides lubrication for the rotating drill bit as the lower wellbore is formed. Thereafter, the fluid combines with other wellbore fluid to transport cuttings and other wellbore debris out of the wellbore. The fluid initially travels upward through a smaller annular area formed between the outer diameter of the casing string and the lower wellbore. Subsequently, the fluid travels up a larger annular area formed between the work string and the inside diameter of the casing lining the upper wellbore. As the fluid transitions from the smaller annular area to the larger annular area the annular velocity of the fluid decreases. Similarly, as the annular velocity decreases, so does the carrying capacity of the fluid resulting in the potential settling of drill cuttings and wellbore debris on or around the upper end of the casing string  150 . 
   A flow apparatus and an auxiliary flow tube are used to increase the annular velocity of the fluid traveling up the larger annular area by injecting high velocity fluid directly into the larger annular area. Generally, the flow apparatus is disposed on the work string to redirect circulating fluid flowing through the work string into an upper portion of the larger annular area. At the same time, the auxiliary flow tube is disposed on the casing string to redirect high velocity fluid traveling up the smaller annular area in a lower portion of the larger annular area. Both the flow apparatus and the auxiliary flow tube may be may selectively opened and closed individually or collectively to modify the circulation system. In this respect, if fluid is primarily required in the upper portion of the larger annular area then the flow apparatus may be completely opened and the auxiliary flow tube is closed. On the other hand, if fluid is primarily required in the lower portion of the larger annular area then the flow apparatus is closed and the auxiliary flow tube is opened. In this manner, the circulation system may be modified to increase the carrying capacity of the circulating fluid without damaging the wellbore formations. 
   While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.