If the evaluation of the well logs indicates a potential zone to be completed, ie potential hydrocarbon bearing zone present, the well must be completed. This will enable the well to produce hydrocarbons to the surface, until the reservoir is depleted. The first and critical step is to run and cement casing in the hole.
The main reasons fore running and cementing casing in the open hole is to:
1. Prevent caving of the hole.
2. Confine production to the wellbore
3. Prevent contamination of fresh water sands, particularly in the surface hole.
4. Facilitate installation of surface equipment
5. Facilitate installation of downhole equipment
6. Provide means of controlling pressure
7. Exclude water from producing formation
Casing String and Design Factors
The casing is the steel pipe which is run to different depths in the well. This depth at which the casing is set is the casing point. A casing string is casing that is run from it’s casing point back to the surface or to the seafloor in offshore wells. A casing liner is casing that is run from it’s casing depth back to a casing liner hanger downhole inside a previously run and cemented casing string or liner.
The following are considered when selecting a casing to be run:
i) Axial load in tension
ii) Axial load in compression
iii) Burst as a thin walled cylinder (due to internal pressure)
iv) Collapse as a thin walled cylinder (due to external pressure)
v) Corrosion
vi) Abrasion
There are four typical types of casing that may be run in a well:
i) Conductor pipe
ii) Surface string
iii) Intermediate string or liner
iv) Production string or liner
Conductor Pipe
The conductor pipe may also be called drive pipe for offshore wells since it may be driven in to the seafloor with a pile driver. One function of the conductor pipe is to support the wellbore through the unconsolidated materials present in the surface ho,e such as dirt, gravel, clay, sand, rock boulders, silt and sediment. It is therefore desirable to set the conductor pipe either on solid rock or into solid rock. A second function of the conductor is to protect the wellbore near the surface from washout, which may result from circulation of the drilling mud from the lower section of the wellbore, and therefore to restrict the well diameter at the surface to the ID of the conductor.
The Surface String
The surface string serves a primary function of protecting the surface environment from contamination from downhole fluids such as hydrocarbons and drilling mud. This environmental protection requirement makes the cementing of the surface casing to the surface necessary. Once the surface string is run, a bolt flange connection is welded to the top of the casing to which the BOP stack will be attached. The combination of the casing head and BOP stack will protect against blowout during further drilling operations. It is important that the surface string be set at sufficient depth within solid rock to provide protection against downhole pressures.
Intermediate String
A primary function of intermediate strings or liners is to seal off zones of high fluid pressures. The determining factor for this casing point will be to drill through an impermeable rock formation below the high pressure reservoir, thereby permitting further drilling with a less dense drilling mud. A second function of intermediate strings or liners is to seal off zones of lost circulation, which can occur when drilling a low pressure reservoir. Another function of intermediate strings or liners is to seal off zones of wellbore washout in unconsolidated sandstones or mobile shales.
The Production String
The production string or liner is that casing through which the reservoir fluid will be produced. This casing is run all the way through the reservoir and set some depth below it. The casing depth is therefore dependent on the depth of the bottom of the reservoir and the amount of rathole required for that particular well. It is usually desirable to produce hydrocarbons through production tubing rather than through the production casing in order to minimize exposure to possible corrosion from the reservoir fluids. The production casing may also serve the functions of sealing off high pressure zones, zones of lost circulation, and zones of potential wellbore washout.
Production Choke
The production choke can be used to control the production flow rate of the well and hence the drawdown. The drawdown is the difference between the reservoir pressure and the flowing bottomhole pressure. This is very important in preventing sand entry into the wellbore. If the drawdown is too high, the cohesive forces due to the cementing material between the sand grains within the rock can be exceeded. This will result in sand grains becoming loose and flowing into the wellbore. This can result in plugged tubing, washed out choke and plugged flowline. The choke could be either adjustable or fixed choke (with a bean inserted). Adjustment of the choke and hence the flow rate can maintain a bottomhole pressure above the bubble point, preventing the breakout of gas out of solution at the bottom of the well. This guarantees only liquids flowing into the well.
Factors affecting Production:
i) Reservoir Fluid Pressure
ii) Reservoir Fluid Temperature
iii) Formation Volume Factor
iv) Bubble Point Pressure
v) Reservoir Fluid Saturations
vi) Solution Gas Oil ratio
vii) Reservoir Fluid Viscosity
viii) Reservoir Fluid Compressibility
ix) Porosity
x) Permeability
xi) Well Depth
xii) Flow area variations
xiii) Perforation size, penetration and density
xiv) Choke size
xv) Flowing Bottomhole Pressure
xvi) Production history of the reservoir
xvii) Skin Factor
