METHOD AND APPARATUS FOR CEMENTING CASING IN A WELLBORE

Abstract: A method for cementing casing into a wellbore, which method comprises the steps of: (a) providing float equipment (4) into said casing (3) at or adjacent the downstream extremity thereof; (b) inserting a bottom plug (11) into said casing and pumping it down said casing with cement, and (c) inserting a top plug (15) into said casing above said cement and pumping it down said casing, characterised by (d) providing a plug seat (10) upstream of said float equipment, said bottom plug (11) being shaped and configured so that it will pass said plug seat (10) and land on said float equipment (4) and said top plug being shaped and configured so that it will land on said plug seat (10).

Description: Method and Apparatus for Cementing Casing in a ellbore This invention relates to a method and apparatus for cementing casing into a wellbore. During the construction of oil and gas wells a wellbore is drilled in the ground. A string of casing is then lowered down the wellbore and the annular space between the casing and the wellbore filled with cement. In order to facilitate cementation float equipment, for example a float shoe or a float collar is fitted at an adjacent the bottom of the casing. When the casing is in position conditioning fluid is pumped down the casing through the float equipment and flows upwardly through the annular space removing debris and preparing the annular space for reception of the cement. When conditioning is complete a bottom plug is inserted in the casing and pumped down with cement. When the desired volume of cement has been dispatched a top plug is inserted into the casing and the bottom plug, cement and top plug are pumped down the casing with a convenient liquid (typically drilling mud or salt water). The bottom plug typically includes a frangible diaphragm which, after the bottom plug lands on the float valve and increasing pressure is applied to the top plug, ruptures to allow the cement to pass through the bottom plug and the float shoe and up into the annular space. Cement flows until the top plug lands on the bottom plug. The float shoe prevents cement from flowing back into the casing when the pumps applying pressure to the top plug are turned off. After the cement has hardened the casing is pressure tested and, if satisfactory, the plug, bottom plug, float shoe and any residual cement in the casing are drilled out. Occasionally, the cement towards the bottom of the casing does not set properly. This condition, sometimes referred to as "wet shoe", requires very time consuming and expensive remedial action. The applicants have determined that one cause of "wet shoes" is directly attributable to mud and debris which accumulate on the inside of the casing. In particular, it is not unusual for a significant crust to build up on the inside of casing as it is being run into position, particularly if it becomes necessary to circulate the casing to facilitate its insertion in the wellbore. Although the bottom plug is intended to remove this crust it is not unusual for the bottom plug to be slightly skewed as it travels downhole with the result that a thin crust may be left on parts of the casing. Some of this crust will be washed off into the cement and tends to rise towards the top plug which will remove much of the remaining crust itself. The overall result is that the cement immediately below the top plug can contain a high proportion of varied solids which contaminate the cement. Such highly contaminated cement may well either not set or or have undesirable mechanical properties . The applicants believe that this problem can be reduced by ensuring that the final slug of cement remains in the casing rather than pass into the annular space. The applicants believe that this slug should be at least 1 joint (40ft (12m)) long and up to 5 joints (200ft (60m)) where the quality of the cement job is paramount . It is know to use both a float shoe and a float collar in a length of casing, the float shoe being placed at the bottom of the casing and the float collar a few joint thereabove. However, the bottom plug and the top plug seat on the float collar with the result that contaminates can enter the cement from the wall of the casing between the float shoe and the float collar. According to the present invention there is provi- ded a method for cementing casing into a wellbore, which method comprises the steps of: (a) providing float equipment into said casing at or adjacent the downstream extremity thereof; (b) inserting a bottom plug into said casing and pu p- ing it down said casing with cement, and (c) inserting a top plug into said casing above said cement and pumping it down said casing, characterised by (d) providing a plug seat upstream of said float equip- ment, said bottom plug being shaped and configured so that it will pass said plug seat and land on said float equipment, and said top plug being shaped and configured so that it will land on said plug seat. Preferably the plug seat is separated from the float equipment by at least 12m, preferably by at least 36m, and advantageously by at least 60m. The present invention also provides apparatus for cementing casing into a wellbore, said apparatus compri- sin: a) float equipment b) a bottom plug, and c) a top plug characterised by d) a plug seat which can be mounted in said casing upstream of said float equipment and which is shaped and configured to permit said bottom plug to pass therethrough but to prevent passage of said top plug therethrough. For a better understanding of the present invention reference with now be made, by way of example, to the accompanying drawing which is a schematic side view of an apparatus in accordance in the present invention in use. Referring to the drawing, there is shown a wellbore which is generally identified by the reference numeral 1. The bottom of the wellbore is temporarily sealed off by a packer 2. A casing string 3 extends downwardly through the wellbore 1 and terminates in a float shoe 4. The float shoe 4 comprises two non-return valves 5 and 6 which are set in cement 7 and are separated by a chamber 8. The float shoe 4 is covered by a baffle 9 intended to inhibit the ingress of voluminous debris into the non-return valves 5 and 6. A plug seat 10 is incorporated in the casing string 3, three joints (36m) above the float shoe 4. The plug seat 10 comprises an annular ring which projects radically inwardly from the inner wall of the casing string 3. In use, after the casing string 3 has been correctly positioned (as shown in the Figure) a bottom plug is released into the casing. A large variety of bottom plugs may be used. The bottom plug 11 shown in the Figure is particularly intended for release from a sub-sea wellhead by release of a dart 12 from the surface which blocks a central bore 13 in the bottom plug 11. When cement is introduce into the wellhead above the bottom plug 11, the bottom plug 11 is swept out of its retainer and moves down the casing string 3 ahead of the cement. The bottom plus 11 is provided with fins 14 which inter alia wipe crusts from the inside of the casing string 3. Whilst bottom plugs will normally remove most debris from the inside of the casing string 3 they sometimes travel down the casing string 3 in a skewed orientation with the result that thin areas of crust remain on the wall of the casing string 3. Some of this crust is removed by the cement as it flows down the casing string. However, because of its proximity to the wall of the casing string 3 and its composition it tends to migrate upwardly in the cement. When the required volume of cement has been introduced into the casing a top plug 15 is introduced by releasing a second dart 16 from the surface. The slug comprising the bottom plug 11 the cement and the top plug 15 is then pumped downhole by drilling mud which acts on the top of the top plug 15. The bottom plug 11 passes through the plug seat 10 and lands on the top of the baffle 9. Pressure is increased on the top plug 15 with the result that one or more of the diaphragms 17 in the bottom plug 11 rupture and permit the cement to flow through the float valve 4 and up into the annular space 18 between the outside of the casing string 3 and the wellbore 1. Cement continues to flow into the annular space 18 until the top plug 15 lands on the plug seat 11. The top plug 15 itself is provided with fins 19 which tend to remove crust from the inside of the casing string 3. It will be appreciated that if the casing string 3 was badly contaminated the volume immediately downstream of the top plug 15 will contain heavily contaminated ce- ment. After the top plug 15 has landed the casing string 3 is pressure tested, the pressure released and the cement allowed to set. The non-return valves 5 and 6 in the float shoe 4 inhibit cement flowing from the annular space 18 back into the casing. Once the cement has set the top plug 15, the bottom plug 11, the cement therebetween, the float shoe 4 and the packer 2 are drilled out. In this connection it is a relatively simple matter for a rock bit to drill through green cement before entering the formation below the casing string 3.

Claims: Claims : - 1. A method for cementing casing into a wellbore, which method comprises the steps of: (a) providing float equipment (4) into said casing (3) at or adjacent the downstream extremity thereof; (b) inserting a bottom plug (11) into said casing and pumping it down said casing with cement, and (c) inserting a top plug (15) into said casing above said cement and pumping it down said casing, characterised by (d) providing a plug seat (10) upstream of said float equipment, said bottom plug (11) being shaped and configured so that it will pass said plug seat (10) and land on said float equipment ( 4 ) , and said top plug being shaped and configured so that it will land on said plug seat (10). 2. A method according to Claim 1, wherein the plug seat (10) is separated from the float equipment (4) by at least 12m. 3. A method according to Claim 1, wherein the plug seat (10) is separated from the float equipment (4) by at least 36m. 4. A method according to Claim 1, wherein the plug seat (10) is separated from the float equipment (4) by at least 60m. 5. A method according to any preceding Claim, wherein said float equipment comprises a float shoe ( 4 ) . 6. A method according to any preceding Claim, wherein said float equipment comprises a float collar. 7. An apparatus for cementing casing into a wellbore, said apparatus comprising: a ) float equipment ( 4 ) , b) a bottom plug (110, and c) a top plug (15), characterised by d) a plug seat (10) which can be mounted in said casing upstream of said float equipment ( 4 ) and which is shaped and configured to permit said bottom plug (11) to pass thereto but to prevent passage of said top plug (15) therethrough.

Inventors: GIROUX RICHARD LEE, GIROUX, RICHARD, LEE
Applicants: LUCAS BRIAN RONALD, WEATHERFORD LAMB, LUCAS, BRIAN, RONALD, WEATHERFORD/LAMB, INC.
Application year: 1998
Main classification: E21B33
IPC codes: E21B33