DRILLING BELOW THE SURFACE CASING

3.7.1 Drilling of Cement, Float Collar and Shoe Procedure

Make up used bit to drill cement with slick assembly.

1. Drill cement at 2-3 tones of weight on the bit and rotary rpm as 40-50 only. 2. Drill float collar, cement and float shoe carefully.

3. Continue drilling till 0.5 m below the casing shoe, formation should not be opened. 4. Circulate the cuttings out of the well.

3.7.2 Shoe Test

This test is done to determine the competency of cement job around the shoe.

Procedure

1. Pull the drill bit in side the casing. 2. Close pipe ram BOP and Kelly cock. 3. Hook up cementing unit with Kill line.

4. Flush the BOP stack, kill and choke lines with water. 5. Close gate valve on choke line.

6. Calculate the shoe test pressure. This is the sum of surface pressure and the hydrostatic pressure of the fluid being used during the test. It should be equal to the hydrostatic pressure at the shoe of the heaviest mud that will be used in the well before running the next string of casing.

7. Pump steadily at a rate of one litre per second till the time test pressure at shoe is reached, plot the increase in surface pressure against volume pumped. (For plotting the graph see leak off test).

8. If the shoe is hermetical, the plot will be linear.

9. Hold the required test pressure for 15 min. The shoe is considered OK if the pressure does not fall more than 10% of the test pressure during this time.

10. Release the pressure through choke line, and measure the volume of fluid recovered and compare this with the volume pumped. Both the volumes should be almost equal.

11. Open kelly cock.

12. In case the shoe does not hold the required pressure, squeeze cement and repeat all the above procedure for testing shoe.

3.7.3 Leak-off Test

This test is carried out to determine the competency of the formation.

Procedure

1. Drill down 1-2 meter fresh formation below the casing shoe. 2. Circulate to clean the hole.

3. Pull the drill bit inside the casing shoe. 4. Connect kelly and close lower Kelly cock. 5. Close the pipe ram BOP.

6. Close kill line and open both the outlet valves of casing head housing.

7. Connect the cementing unit through kill line. The pumping unit should have a graduated suction tank in litres or fitted with a pump stroke counter so that the volume of the mud pumped in can be accurately measured.

8. Start pumping mud in the well at a controlled rate of one litre/sec.

9. Record pressure readings after pumping each incremental volume of 50 litres. 10. Plot these values of pressures against volume of mud pumped on the graph.

11. It will be observed that pressure will proportionally increase after pumping each incremental volume of 50 litres of mud.

12. The first reading of pressure which does not change proportionally after pumping incremental volume of 50 litres is noted. Let it be ‘Ps’ kg/cm2.

13. Hydrostatic pressure due to mud column is calculated at the depth where leak-off is being performed as under:

Wm X D

Hs = —————— kg/cm2

10

Where,

Hs = Pressure due to hydrostatic head (kg/cm2_).

Wm = Specific gravity of mud. D = Depth in meters.

Ps = Surface pressure at which leak takes place (kg/cm2₎

Then

PLOT = Ps + Hs

10 x PLOT

LOT = ——————— gm/cc D

After the test is completed, release the pressure and measure the volume of return mud. 14. The volume of return mud for conducting the test should be almost equal to the volume of

Points to remember

a) Do not keep the pressure and volume data for plotting later on. This may lead to over pumping and formation breakdown.

b) The first straight line portion of the graph ‘OA’ indicates elastic deformation of the formation. When the curve starts getting visibly flat (at point A in the fig. 1.24) at the upper end of the plot, the testing is stopped, the point ‘A’ at which this change is noticed is marked. c) The surface pressure should not exceed 80% of the burst pressure of casing.

d) When such tests are conducted with heavy mud inside and lighter mud outside the casing, it is the lower part of the casing which is subjected to maximum burst loading.

Calculation of Maximum Specific Gravity of Mud: The maximum specific gravity of

mud is limited by the LOT pressure at the casing shoe. 10 x LOT Pressure at shoe (in kg/cm2₎

W max = ——————————————————— Depth of casing shoe (in metres)

3.7.4 Drilling after Leakoff

1. After conducting leak-off test, resume drilling with the drilling parameters as mentioned in GTO.

2. Drill down to shoe depth of intermediate casing .

3. Depending upon well condition wiper trip should be made as and when required. 4. Mud weight should not be allowed to increase beyond LOT value.

5. Totco reading or inclinometer survey should be recorded during each trip and also whenever required.

6. Maintain angle deviation with in 5° for straight hole.

7. Dog leg severity should not be allowed to go beyond 3° per 30 meter

8. Standard logs should be taken before lowering the casing and, as and when required. 9. If required, rise of cement may be changed as per the well condition.

10. Centralizers should be placed as per the plan.

11. After casing has been lowered and cemented, observe the WOC as per the plan or and casing on casing hanger (Sea Bed) immediately after cementation.

12. Install & test well head.

3.7.5 Intermediate Casing Test

After testing of BOP, choke & kill manifold etc, testing of intermediate casing is to be carried out as follows.

1. Run in drill string and bit, up to the top of the cement.

2. Circulate and condition the mud. Drill/ clear up to top of float collar. Circulate for hole cleaning. 3. Test the casing, test pressure should not exceed minimum of the following.

a) 80% of the internal yield pressure of the weakest section of the casing lowered. b) Maximum allowable casing head pressure.

Note : If casing shoe is drilled out, test pressure should not exceed the formation fracture