This is an integral part of the completion of a well and helps to ensure its integrity. The choosing of the right type of casing will help in determining the type of completion that can be run in the well and as such it can determine the production characteristics of the well.
On a technical level the casing helps to prevent the borehole family, isolates the borehole fluids from the formation fluids and at the same time prevent formation fluids from mingling. It also helps to provide an environment to control the sub-surface environment from the surface.
A good example would be that the placement of the casing during drilling, which gives the rig a point to mount the BOPs and as such help to control the pressure of sub-surface fluids.
Similarly during the production the casing allows for the mounting of surface control equipment and valves to control fluid flow, also, the casing slows for the mounting of sub-surface equipment that can monitor and control the fluid flow and as such well production rates.
One of the major points to consider in casing design is the depth to which the casing is set. To do this it is necessary to know the pore pressure gradient and the fracture gradient.
- Plot pore pressure gradient, mud pressure gradient, and fracture gradient on a graph of depth vs. pressure.
- Start at the highest mud weight.
- From this point draw a line vertically from mud point to fracture gradient line.
- This depth point is the depth for a casing point.
- From this point draw a line horizontally to intersect the mud curve.
- Draw a line vertically until it intersects the fracture curve.
- This depth is the depth for the next casing point.
- This is repeated until the surface is reached.
- On looking at the graph the horizontal lines would represent where to set the casing shoe.