Chapter 2: Casing Design con't lec (10 )

Cementing

Cementing a liner in place requires very closely controlled application of existing technology4s46 and a fair amount of risk. Three cementing methods are generally accepted for liners.47 Calculating the volume of cement to be used in a liner cementing job is extremely difficult and requires more information than available from a simple caliper run. For maximum caliper information, a four arm device capable of determining elliptical holes should be utilized for hole volume. Cement excesses of between 20% and 100% have been used on a number of liner jobs with larger excesses being responsible for better bonding and less channels. There is also a direct correlation with absence of channels and pipe movement. In liners of 500 ft or less, Bowman and Sherer4s46 recommend 100% excess
over the calculated annular volume and on liners of 3000 ft or more at least a 30% excess is recommended. A single-stage cementing job in which cement is circulated to the top of a liner; much like a

primary cement job and may include pipe movement during cementing. A planned squeeze program in which the lower part of the liner is cemented and the top part of the liner is squeezed later. This technique does not have good middle support and should not be used to isolate high pressure zones. The procedure is more widely followed in worldwide operations because of perceived problems of disengaging the liner running assembly from the liner and of flash setting of cement. Disengaging from the liner before cementing eliminates the ability to move the liner and almost universally results in poor cement jobs.
A third procedure commonly reserved for short liners is to fill part of the hole with cement and then slowly run the casing string into the cement, forcing the cement to flow up around the pipe. While this method can be accomplished with the minimum amount of pumping, the lack of circulation can result in poor removal of drilling mud. The technique is called a puddle job. Most liner jobs do not include plans to move the liner during the primary ~ e m e n t i n g .T~h~e ?re~as~o ns for this include:
1. Detaching the drill pipe from the liner before cementing minimizes the risk of being unable to detach from the liner once the cement is in place.
2. It may be necessary to change to a higher strength drillstring to allow pipe movement.
3. Movement may cause the liner hanger to become tangled with the centralizers near the top of the string.
4. Swab or surge pressures may be created during liner movement, especially in close tolerance wellbores.
5. Movement of the liner during cementing may knock off debris from the borehole wall. The debris may cause bridges and reduce the possibility of circulating cement. Despite the quoted disadvantages of staying attached during the cementing operation, Bowman and
Sherer4346 site several serious disadvantages with releasing the liner before cementing.

1. If the liner is hung off, the small bypass area around the liner offers a greater restriction to flow and causes more lost circulation because of the backpressure on the flowing cement.
2. If a downhole rotating liner hanger is used (rotation only), additional torque is required to initiate rotation to overcome bearing friction. Pipe often rotates easier when it is being raised or lowered. The difference in torque required is often substantial.
3. The potential for sloughing shale and annulus bridging is lessened when the operator can alternate between rotation and reciprocation.
4. Premature shearing of the pins in the liner-wiper plug is less likely because there is no relative movement between the liner and the setting tool (these two pieces of equipment move together). 49
5. If cement channels and there is a large hydrostatic pressure difference between inside and outside of the running tools, the cups or seals can give way before cementing of the liner is complete.
6. The displacement efficiency of cement around the tubulars when pipe is not moved is lessened. When liners are close clearance, then the density differences between mud and cement should be as close as possible. This negates the advantages of hole cleaning by higher density cement. Reciprocation4’ of the string is helpful because it produces lateral pipe movement that causes the pipe to change sides in the wellbore while it is alternately compressed and stretched (slacked off and picked up).43 R o t a t i ~ hnelp~s b~y ~mix~in~g th~e ~cem ent into wellbore irregularities and displacing mud due to drag forces produced by the flowing cement.43 Although liner movement should be a goal in any liner operation, well conditions may prevent any type of movement. In many cases, however, liner movement can be achieved in a well conditioned hole. Two clear cases where liner should not be moved are:43
1. When a short or small liner (3-1/2 in. or smaller) is run in a deep well, the liner should be hung off first since it may be impossible to tell from the weight indicator whether the liner had been released from the drill pipe.
2. In cases of hole deviation over 35O, reciprocation may be difficult due to high drag forces.
Many of the problems in liner running can be lessened by drilling a usable hole. Problems with keyseats, ledges, washouts, and other nongauge problems intensify when close tolerance liners are to be run. For additional information on problems involved in drilling a usable hole, refer to the chapter on Drilling The Pay.
When cement is circulated from the liner bottom to over the liner top, the cement must remain fluid long enough to detach from the liner and to circulate the cement from the well or to pull up above the top of the cement with the drillstring. If the cement flash sets, then the drillstring will become cemented in place and the hole most likely will be lost. Cement may prematurely set, thicken, or cement circulation may be lost for a number of reasons
1. Improper thickening or pump times caused by a poor design, ineffective field operations, or bad test results.
2. Poor density control on the cement or poor mixing of the cement at the surface. ,
3. Bridging in the annulus caused by a buildup of cuttings. This is caused typically by the increased number of particles picked up by higher annular velocities with a liner in the hole (due to its larger ID) than around the drillstring.
4. Plugging from dehydration of cement caused by excessive water loss in openhole sections below the overlap.
5. Increased hole cleaning of the cement as compared to4he drilling mud.
One of the most troublesome problems in cementing design is inadequate hole cleaning prior to cementing. This is especially true when light weight, low viscosity muds are used and little attention is paid to cuttings removal. Heaving shales are also a problem in hole fill and may cause washouts. Under no circumstances should circulation be halted with the liner in the hole before all of the cement has been displaced. Due to the small clearances and the yield point of cement, it may be very difficult to start circulation again.