Once an oil/gas well is being drilled and commercially viable quantities of oil/gas are present for extraction, the well must be 'completed' to allow for the flow of petroleum out of the formation and up to the surface. This process includes strengthening the well hole with casing, evaluating the pressure and temperature of the formation and installing proper equipment to ensure an efficient flow of oil/gas out of the well.
Condensate wells are wells that contain natural gas, as well as a liquid condensate. This condensate is a liquid hydrocarbon mixture that is often separated from the natural gas either at the wellhead, or during the processing of the natural gas. It is important to remember that natural gas, being lighter than air, will naturally rise to the surface of a well. Because of this, in many natural gas and condensate wells, lifting equipment and well treatment are not necessary.
Completing a well consists of a number of steps: installing the well casing, completing the well, installing the wellhead, and installing lifting equipment or treating the formation should that be required.
Installing well casing is an important part of the drilling and completion process. Well casing consists of a series of metal tubes installed in drilled hole. Casing strengthens the sides of the well hole, ensures that no oil or natural gas seeps out of the well hole as it is brought to the surface. A good deal of planning is necessary to ensure that the proper casing for each well is installed. The type of casing used depends on the subsurface characteristics of the well, including the diameter of the well and the pressures and temperatures experienced throughout the well. The diameter of the well hole depends on the size of the drill bit used. In most wells, the diameter of the well hole decreases the deeper it is drilled, leading to a type of conical shape that must be taken into account when installing casing.
There are five different types of well casing. They include:
Conductor casing is installed first, usually prior to the arrival of the drilling rig. The hole for conductor casing is often drilled with a small auger drill, mounted on the back of a truck. Conductor casing is usually no more than 20 to 50 feet long. It is installed to prevent the top of the well from caving in and to help in the process of circulating the drilling fluid up from the bottom of the well. Onshore, this casing is usually 16 to 20 inches in diameter, while offshore casing usually measures 30 to 42 inches. The conductor casing is cemented into place before drilling begins.
Surface casing is the next type of casing to be installed. It can be anywhere from a few hundred to 2,000 feet long, and is smaller in diameter than the conductor casing. When installed, the surface casing fits inside the top of the conductor casing. The primary purpose of surface casing is to protect fresh water deposits near the surface of the well from being contaminated by leaking hydrocarbons or salt water from deeper underground. It also serves as a conduit for drilling mud returning to the surface, and helps protect the drill hole from being damaged during drilling. Surface casing, like conductor casing, is cemented into place.
Intermediate casing is usually the longest section of casing found in a well. The primary purpose of intermediate casing is to minimize the hazards that come along with subsurface formations that may affect the well. These include abnormal underground pressure zones, underground shale, and formations that might otherwise contaminate the well, such as underground salt-water deposits. In many instances, even though there may be no evidence of an unusual underground formation, intermediate casing is run as insurance against the possibility of such a formation affecting the well.
Liner strings are sometimes used instead of intermediate casing. Liner strings are commonly run from the bottom of another type of casing to the open well area. However, liner strings are usually attached to the previous casing with 'hangers', instead of being cemented into place.
Production casing, alternatively called the 'oil string' or 'long string,’ is installed last and is the deepest section of casing in a well. This is the casing that provides a conduit from the surface of the well to the petroleum-producing formation. The size of the production casing depends on a number of considerations, including the lifting equipment to be used, the number of completions required, and the possibility of deepening the well at a later time.
Installing Well Casing
Well casing is a very important part of the completed well. In addition to strengthening the well hole, it provides a conduit to allow hydrocarbons to be extracted without intermingling with other fluids and formations found underground. It is also instrumental in preventing blowouts, allowing the formation to be 'sealed' from the top should dangerous pressure levels be reached.
Well completion commonly refers to the process of finishing a well so that it is ready to produce oil or natural gas. In essence, completion consists of deciding on the characteristics of the intake portion of the well in the targeted hydrocarbon formation. There are a number of types of completions, including:
Open Hole Completion
Conventional Perforated Completion
Sand Exclusion Completion
Multiple Zone Completion
It is the philosophy that during the drilling, - testing, completion water injection and workover of an oil or gas well, any work undertaken should be executed in such a manner that:
1. Loss of human life and injury to crewmembers shall be avoided
2. Pollution of the surrounding environment shall be avoided
3. Loss of rig and damage to equipment shall be avoided.
If all of the aforementioned conditions are fulfilled then the economic and ecological result shall be successful. It is also the philosophy :
1. That detection and controlling a kick takes a team effort from all members of the rig crew. Each member must be completely familiar with his duties so that any well control operation can proceed smoothly and efficiently
2. To maintain all well control equipment in first class condition and ready for use whenever required
3. To ensure that all personnel directly involved in well control situation shall be educated to a standard that ensures complete understanding of any situation that may arise.
However, for any other Well Control procedures and guidelines to take precedence over the ones shown in the ”Well Control Manual”, the following is a must and can not be deviated:
1. At least two tested Safety Barriers shall be present in drilling and producing wells. If for some reason only one Safety Barrier is present, all activities shall cease until two Safety Barriers are reestablished
2. All Safety Barriers must be tested in accordance with approved procedures, or as specified in the programme prepared for the specific operation
3. Activity Typical Independent Barriers:
• Drilling: A sufficient amount of mud or fluid of adequate density to control the well bore pressure; certified and tested BOP stack dressed with suitable rams; unperforated, cemented and pressure tested casing or liner; tested downhole plug; back pressure valve (BPV)
• Casing: A sufficient amount of mud or fluid of adequate density to control the well bore pressure; certified and tested BOP stack dressed with suitable Casing rams
• Snubbing: BOP stack; back pressure valve in the work string; independent safety shear seal immediately on top of Christmas tree; wireline set plug
• Production: Christmas tree; surface controlled subsurface safety valve (SCSSV); downhole plug
• Wireline: Christmas tree valves; wireline BOP; wireline lubricator.
For Jack-Ups Operating under the API Standard
Frequency : - All blow out prevention components that may be exposed to well pressure shall be function and pressure tested as follows:
1. When installed
2. After casing has been run, cemented and BOP nippled-up; but prior to drilling out of the shoe
3. Prior to production testing or completion
4. At any time the integrity of the BOP and casing becomes suspect
5. Function test BOP with low manifold pressure, flush kill - and choke lines each week, unless well operations prevent testing
6. Or at least every 21 days (API RP 53 Third Edition section 17.3.3), unless well operations prevent testing. Well operations that may prevent testing are stuck pipe and kick control. If a period greater than 21 days has elapsed since the previous test the reason for the test postponement must be entered on the drilling report.