9.5: Summary and Final Tasks

Summary

We began this lesson by discussing the five major systems on a modern rotary drilling rig. These are:

• the Power System
• the Hoisting System
• the Circulation System
• the Rotary System
• the Well Control System (Blowout Prevention System)

We discussed each of these systems along with the more important sub-systems that comprise these systems.

The power system on a drilling rig provides the for the other main systems on the rig and other ancillary systems, such as electrical systems, pumps, etc. The system typically consists of a prime mover (the component of the power system that generates the raw power) and a means to transmit the raw power to the end-use components on the rig. The sub-systems of the power system that were discussed in detail or listed and shown in Figure 9.02 include:

• the Fuel Storage
• the Engines and Generators
• means of power transmission
  • mechanical
  • direct current
  • alternating current

The hoisting system on a drilling rig does the heavy lifting on the rig. It is used to raise, lower, and suspend the drill string and lift casing and tubing for installation into the well. The sub-systems of the hoisting system that were discussed in detail or listed and shown in Figure 9.02 include:

• the Crown Block
• the Mast/Derrick
• the Monkey Board
• the Traveling Block
• the Hook
• the Swivel
• the Drawworks
• the Weight Indicator
• the Drilling Line

The circulation system on a drilling rig allows for circulation of the Drilling Fluid or Mud down through the hollow drill string and up through the annular space between the drill string and wellbore. It is a continuous system of pumps, distribution lines, storage tanks, storage pits, and cleansing units that allows the drilling fluid to fulfill its primary objectives. The sub-systems of the circulation system that were discussed in detail or listed and shown in Figure 9.02 include:

• the Swivel
• the Rotary Hose
• the Mud Return Line
• the Shale Shaker
• the Choke Manifold
• the Mud Gas Separator
• the Degasser
• the Reserve Pit
• the Mud Pits
• the Desander
• the Desilter
• the Mud Pumps
• the Mud Discharge Line
• the Bulk Mud Components Storage
• the Mud House
• the Water Tank

The drilling fluid (mud) is a critical part of the drilling process. Muds can be water-based fluids, oil-based fluids, foam, or air. The objectives/functions of the mud are:

• lift drill cuttings from the bottom of the wellbore to the surface;
• suspend cuttings to prevent them from falling downhole if circulation is temporarily ceased;
• release the cuttings when they are brought to the surface;
• stabilize the borehole during drilling operations (exert hydrostatic or hydrodynamic pressure on the borehole to prevent rock caving into the wellbore);
• control formation pore pressures to assure desired well control (apply hydrostatic and hydrodynamic pressures in excess of the formation pore pressures to prevent fluids from entering the wellbore);
• deposit an impermeable filter cake onto the wellbore walls to further prevent fluids from permeable formations from entering the wellbore;
• minimize reservoir damage (assure low skin values) when drilling through the reservoir section of the well;
• cool the drill bit during drilling operations;
• lubricate the drill bit during drilling operations;
• allow for pressure signals from Logging While Drilling (LWD) or Measurement While Drilling (MWD) tools to be transmitted to the surface (LWD and MWD data are transmitted to the surface using pressure pulses in the drilling fluid);
• allow for pressure signals to be sent to the bottom of the well to pressure actuate certain downhole equipment;
• minimize environmental impact on subsurface natural aquifers.

The rotary system on a drilling rig is the system that causes the drill bit rotate at the bottom of wellbore. We have discussed some components of the rotary system when we discussed rotary table and top-drive rigs, but we have not yet discussed the entire system. The sub-systems of the rotary system that were discussed in detail or listed and shown in Figure 9.02 include:

• the Swivel
• the Kelly or (Top-Drive Unit on a top-drive rig)
• the Kelly Bushing or (Top-Drive Unit on a top-drive rig)
• the Master Bushing or (Top-Drive Unit on a top-drive rig)
• the Rotary Table or (Top-Drive Unit on a top-drive rig)
• the Mousehole
• the Rat Hole
• the Rotary Hose
• the Drill String
• the Bottom-Hole Assembly
• the Drill Bit

Drill bits come in different shapes and sizes. The choice of the appropriate bit depends on the formations to be drilled. The drill bits that we discussed in this lesson include:

• milled-tooth tri-cone bit (for drilling through soft rock formations)
• insert tri-cone bit (for drilling through medium to hard rock formations)
• fixed-cutter bit (for drilling through hard rock formations)

The well control system or the blowout prevention system on a drilling rig prevents the uncontrolled, catastrophic release of high-pressure fluids (oil, gas, or salt water) from subsurface formations. The sub-systems of the well control system that were discussed in detail or listed and shown in Figure 9.02 include:

• the Accumulator
• the Blowout Preventer

A kick is an unwanted but controllable entry of subsurface fluids into the wellbore; while a blowout is a catastrophic (usually) and uncontrollable entry of subsurface fluids into the wellbore. Blowouts can be catastrophic because of the volatile, combustible nature of hydrocarbons.

The causes of a kick include:

• insufficient mud weight (density): the hydrostatic or hydrodynamic pressure exerted by the mud column is less than the formation pore pressure (the pressure is underbalanced);
• improper mud replacement during tripping: while tripping out of the hole mud volumes must be pumped into the wellbore at high enough rates to replace drill pipe being removed from the wellbore;
• swabbing: removing drill pipe from the wellbore can create a negative pressure (suction) if the drill pipe is removed too quickly;
• cut mud: if gas is entering the wellbore, then it may effectively reduce the wellbore pressure gradient;
• lost circulation: as discussed earlier, if large volumes of drilling fluid enter the subsurface in (1) high permeability formations, natural fractures, or drilling-induced fractures, then the effect is a shortened height and weight of the mud column.

The indicators/warning signs of a kick include:

• increase in the rate of flow of the drilling fluid returns at constant pump rates (primary indicator of a kick);
• volume of mud in the mud pit increases when no additional drilling fluids are added to the mud system (primary indicator of a kick);
• drilling fluids returns continue to flow when the mud pumps are turned off (primary indicator of a kick);
• improper wellbore fill-up/volume-balance on trips (primary indicator of a kick);
• pump pressure decrease and pump stroke increase (secondary indicator of a kick);
• change in the apparent weight-on-bit (secondary indicator of a kick);
• occurrence of a Drill Break or Bit Drop (secondary indicator of a kick);
• reduction in the mud weight (secondary indicator of a kick).

Safety is of the utmost importance in the oil and gas industry, and detailed well procedures are developed for each well. In addition, daily safety meetings discussing the status of the well, the day’s operations, and the safety concerns for that day’s operations are typically performed by all responsible drilling companies and operating companies.

Finally, we discussed the steps in the drilling procedure (Making Hole). An oil or gas well is drilled in a very ordered sequence. The steps in this sequence are almost universally applied to the drilling of all wells. These include:

1. plan the Well
2. perform Shallow Gas Survey
3. prepare the Wellsite
4. Set the Conductor Casing
5. Move-In and Rig Up
6. Spud the Well
7. Drill Down to the Surface Casing Depth
8. Run and Cement the Surface Casing
9. Continue this Process to Drill to the Next Casing Point
10. Continue this Process to Drill to each Casing Point
11. Continue this Process to Drill to Total Depth (TD)
12. Log the Well with Open-Hole Logs
13. Run and Cement the Production Casing String or Liner (if part of the completion design)
14. Compete the Well – install the well completion:
    1. Tubing
    2. Gravel packs
    3. Packers
    4. Sliding sleeves
    5. Stimulation
       1. Acidize the well
       2. Hydraulically fracture the well
    6. Artificial lift
15. Rig Down and Move Out

Final Tasks