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Drilling a well
Drilling is a subset of Petroleum Engineering to provide a pathway to the produced fluids from the reservoir. Typically, a hole is drilled of a diameter according to the diameter of drill bit if diameter of the hole is kept smaller the bit can get stuck in hole and will cause a adverse problem for the crew to POOH therefore a significant hole is made in order to ensure near gauge of the hole. As drilling proceeds, bit is changed according to different diameters of the hole ensuring the safe operation.
When the well is spudded, reference is taken at wellhead usually Rotary Table Kelly Bushing (RTKB) or Below Kelly Bushing (BKB) to ensure what depth drill string is reached one should guess if reference is not taken how it will be known that where is the desired depth to stop the drilling further or where is the depth of perforation, the operator will stop drilling before the total depth or the perforations will be made few meters above the payzone therefore with the help of reference the distance of wellhead equipment is also counted with the depth of drillstring and this way engineer can meet the target.
REFERENCE: RTKB, BKB.
We usually measure the height of KB from MSL and subtract it from TVD to know what depth actually we are reached.
- Spudding: Drill a cellar up to the depth of 15 ft to provide a pathway for wellhead equipment and casing strings to be RIH.
- Drill another larger hole (Conductor hole) AND RIH conductor pipe through cellar by hammering.
- Use a pilot bit to drill smaller diameter of the hole so that the larger bit cannot get slippage.
- 1st Stage: Continue the drilling further to the depth of surface casing (20’’ O.D) and RIH surface casing with casing head housing (CHH).
- Cement in place the surface casing.
- 2nd Stage: Drilling continues to the depth of intermediate casing (13 3/8 “) and RIH intermediate casing.
- Set Casing Head Spool on Casing head housing and cement in place. Run casing hanger on CHS.
- 3rd Stage: Continue the drilling process up to the depth of production casing (9 5/8’’) and run casing hanger. Cement the casing and RIH THS on CHS and perforate (optional: if required).
- 4th Stage: Run tubing components and space out (POOH the assembly to add the length of pipes i.e. adding spacers) to meet the required length of string.
- Run tubing with tubing hanger on THS.
- Set Packer.
- Set BPV (Back Pressure Valve) on tubing hanger.
- Remove the BOP and set X-mas tree.
- Conduct TCP (Tubing Conveyed Perforation) if you haven’t done perforation.
Drill bit is changed at every stage, as we go deeper diameter of the hole is decreasing and so the bit. To prevent the overguage and underguage a suitable drill bit is required. For Surface Casing (20”): 24’’ hole is drilled. For Intermediate Casing (13 3/8 “): 17 ½ ‘’ hole is preferred. For Production Casing (9 5/8) : 12 ¼ ‘’ hole is required to be drilled. For tubing or liner (7”): 8 1/ 2” hole is recommended.
References: Drilling Engineering, Heriot Watt University Petroleum Engineering. Carl Gatlin Internship Tenure at Pakistan Petroleum Limited, KGF Pakistan Applied Drilling Engineering by Adam T. Bourgoyne Oil Well Drilling Engineering by H. Rabia, University of New Castle upon Tyne