Cementing Process
- Casing Running
- Casing comes in various sizes and grades. Surface casing typically uses 20~13 3/8 inch casing, mostly “J” grade casing which has lower strength.
- Intermediate casing usually employs 13 3/8~7 inch casing with higher grades.
- Production casing cementing typically uses 7~5 inch casing with the same grade strength as intermediate casing.
- The strength of the casing is designed based on its purpose, predicted formation pressure, and depth of insertion to determine the wall thickness, grade, and thread type.
- Unlike drill pipe, casing is a one-time insertion without thickened sections, and its length is not strictly defined. To ensure cementing quality and smooth insertion, the structure of the casing string must be designed.
- Cement Slurry Injection
- This is a key step after the casing is run into the hole. Its purpose is to seal the annular space between the casing and the wellbore to isolate oil, gas, and water zones, making the casing a conduit for oil and gas to the surface.
- Well cement is a crucial material for sealing the annular space between the casing and the wellbore. Due to varying depths, pressures, temperatures, and chemical compositions of formations, well cements need wide adaptability. Currently, China uses nine grades and three types of well cements, each suitable for different downhole conditions.
- Selecting the appropriate cement grade based on well depth and temperature is the primary task before cementing. Limited grades may not meet all performance requirements; therefore, G and H grade cements often use additives to adjust their properties, which are the most commonly used types. Additives can be categorized into seven types: density modifiers, setting time modifiers, loss circulation control agents, fluid loss control agents, viscosity control agents, and special additives for exceptional situations. A single grade of cement can use one or more types of additives.
- After running the casing, connect the circulating lines and cementing lines immediately to prepare for cement injection. First, circulate drilling fluid to clear the mud cake from the wellbore walls due to the small annular space between the casing and the wellbore. Adjust the drilling fluid properties until the pump pressure stabilizes.
- Before injecting the cement slurry, pump a volume of spacer fluid to separate the drilling fluid from the cement slurry to prevent mixing. When the spacer returns to the surface, it also cleans the annular space.
- The displacement speed of the cement slurry depends on downhole conditions. Typically, high-speed turbulent displacement should be used unless the formation cannot withstand high pressure, then low-speed displacement is necessary.
- Wellhead Installation and Casing Pressure Testing
- After running the casing and cementing, install the wellhead during the cement curing period. For surface casing, the top of the casing connects to the casing head shell. Each layer of casing hangs within the casing head, which primarily supports the weight of the intermediate and production casings, especially important when the cement does not return to the surface. The casing head also seals the annular space between casings to prevent pressure communication and serves as a transition connection for blowout preventers and tubing heads. On land, casing heads have two side ports for remedial cementing, monitoring well conditions, and pumping balance fluids.
- Casing pressure testing is an essential part of verifying cementing quality. After installing the casing head and connecting the blowout preventer and related lines, perform a pressure test on the casing head seal and the seal connecting to the blowout preventer. After verifying the cement plug in the casing, perform a pressure test on the casing string. After drilling through the shoe track by 2~3 meters (for intermediate casing), conduct a formation fracturing test. For production wells, inspect the quality of the cement sheath using acoustic logging to evaluate the bonding between the cement sheath, casing, and wellbore. Only after all cementing quality indicators are met can the next operation proceed.
Cementing Operations
- Install float shoes and collars following the sequence, and run the tailpipe sequentially.
- Hold a safety meeting before lifting the tailpipe hanger onto the rig floor. Ensure that the joint above the tailpipe hanger is not torque down and avoid any collisions or scraping during lifting. Secure the safety line.
- Connect the tailpipe plug to the adapter and apply grease.
- Connect the tailpipe hanger to the 7-inch tailpipe and torque it up. Do not remove the slips during the lift.
- Slowly lower the assembly, stripping off the protective covering.
- Torque the short drill pipe to the tailpipe hanger at 22,000 ft-lbs.
- During casing running, cement each joint. Every five joints, fill the annulus with mud.
- Make up a stand of drill pipe.
- Establish circulation to check the float collars and shoes. Ensure the pump pressure does not exceed the formation’s capacity to prevent fracturing.
- While tripping in, cement each stand and every five stands once. The rate of descent should not exceed 3 minutes per stand. Notify the rig engineer if encountering resistance exceeding 5 tons.
- Measure the weight every 1,000 meters. Fill the annulus with mud before entering the open hole section, circulate, and measure weight.
- If circulating and rotating while tripping in, ensure the maximum torque does not exceed 4,000 ft-lbs (5,400 Nm).
- After reaching bottom, circulate and measure weight while pulling and lowering the string. Leave sufficient clearance.
- Connect cementing equipment and pressure test the lines to 70 MPa.
- Pump the spacer fluid according to design. Control the density and flow rate.
- Pump the cement slurry according to design. Control the density and flow rate.
- Pump another spacer fluid according to design. Control the density and flow rate.
- Release the drill pipe plug at the surface.
- Displace the drilling fluid with cement slurry. Control and maintain the flow rate. Record the displacement volumes during different stages: the volume when the drill pipe plug meets the tailpipe plug, the volume when the cement reaches the float shoe, and the total displacement volume. Check for backflow after displacement.
- Gradually increase the pressure to break the frangible disk and circulate to seat the ball. The pressure to break the disk is approximately 5,000 psi (35 MPa). Note: If the disk does not break, drop a ball and wait for it to fall into place.
- Overpull the string by 10,000 lbs (5 tons) to ensure it is in tension.
- Apply pressure to the expandable tailpipe hanger at no more than 0.5 bbl/min (0.08 m³/min) up to about 4,500 psi (31 MPa). Stop pumping and release pressure after observing five pressure fluctuations. The entire process should be continuous without stopping the pump.
- Overpull the string by 100,000 lbs (50 tons) to confirm the expandable tailpipe hanger is set.
- Lower the string to its original weight.
- Further lower the string by 5~7 tons (10,000~15,000 lbs) to ensure the setting tool releases from the tailpipe body.
- Pull the string to retrieve the setting tool from the tailpipe hanger. Observe and record changes in the weight during the pull.
- After pulling five stands of drill pipe, circulate to clean the bell nipple. While circulating, move the string up and down. Do not exceed a flow rate of 1 m³/min.
- Pull out of the hole. Do not rotate the string while pulling out.