Casing Setting Depth Determination

Factors influencing casing seat determination

1. Pore pressure and fracture pressure

2. Company policy and gov regulations

3. Lithology

4. Protection of freshwater aquifers

5. Hole problems

6. Control of formation P without fracturing shallow formations

7. Control of abnormal P zones

8. Kick tolerance

9. Well trajector/directional requirementsS

Sources of Pore P and Fracture P

1. Actual data from offset wells

2. Analysis from offset well logs

3. Seismic data analysis of velocity and transit times

4. Seismic lookahead technology

How to measure Fracture Pressure?

1. Leak-Off Test (LOT)

   • Keep pumping until deviation (crack initiation/formation of fracture)

2. Formation Integrity Test

   • to determine the ability of a subsurface zone to withstand a certain pressure

LOT Procedures

1. Drill 3m of new formation

2. Circulate and condition mud until uniform and pull bit inside shoe

3. Line up to high pressure low volume pump (cement unit) and close BOP

4. Pump in the annulus/drillpipe. Record volume vs pressure

5. Stop pumping once pressure curve changed is observed and identify the LOT from the graph

Possible issues during LOT/FIT test and mitigation method

Issues	Mitigation Method
Leaking	Perform pressure test for surface line-ups prior
Early leak through exposed sand layers	Allow filter cake build-up from circulations
Abnormal pressure trend signatures	Identify wellbore signature from casing pressure test performed at same rate

Overburden Pressure

Weight of the sediments (soil, rock). Used to estimate the fracture gradient if unknown

Formation/Pore Pressure

Pressure of native fluids contained within the rock matrix/pore spaces

Fracture Pressure

Fracture strength of the rock. Limit for fracturing and crack propagation of the formation.

Consequence of inaccurate pore pressure evaluation

1. Lost circulation

2. Blowout & Underground blow out

3. Stuck pipe

4. Hole/wellbore instability and enlargement

5. Increase of drilling cost

Kick Tolerance

The maximum influx volume that can be taken into the wellbore and subsequently circulated out without breaking down the formation. The volume that can be tolerated depends on the influx fluid type and intensity of the kick.

Assumptions in simple kick tolerance hand calculations method

1. Single gas bubble influx (no mixing of gas and mud)

2. Influx occurs at the bottom of the well and stays there at shut-in

3. The effects of gas migration, dispersion and solubility are ignored

4. Temperature effects and gas compressibility are ignored

5. Boyle’s law applies (P1V1 = P2V2)