DIFFERENT TYPES OF CASING / LINER SHOES. WHAT IS USED IN ERD? HOW LONG A RATHOLE SHOULD WE LEAVE?
It is easy to get lost in the variety of different shoes vendors want to convince us to buy. Below is the quick and simple overview of what are the main types of this piece of equipment.
Flat or slightly rounded guide shoes should not be used in ERD wells – lipstick shoe should be the first choice if no wellbore problems are expected, however, full or above swivel (if used) string rotation is required to change the toolface of the shoe. Self-orienting shoe provides better geometry-related obstructions passing ability. The only downside of this design (apart from higher price) is that it consists of movable parts that might come loose whilst tripping or turn into junk upon drill-out.
There is a number of different designs for orientating shoes, starting from simple lip-stick type shoe (full string rotation is required) and ending with reciprocation (i.e. Wildcat Smart Shoe) or hydraulically (i.e. Deep casing tools Turborunner) activated self-orienting shoes.
Reamer shoes are normally used for “drilling” past geometry related obstructions. Ideally, the wellbore should be conditioned with drilling BHA to the degree when such “drilling” with casing / liner is not required. Reaming through the obstruction with casing / liner has an elevated risk of pack-off / stuck-pipe and should be avoided if possible. If reaming is required it is preferred to utilize reciprocation / hydraulically activated shoes to minimize the volume of “mobilized” material.
Up-jets provide better displacement mechanics. Down-jets promote washing down through the loose material obstruction (cuttings, filter cake, etc.).
Another question commonly asked is: how long rathole should we leave on an ERD well?
The rat-hole length should be minimized as much as possible. Some small stand-off from TD is still required for casing space-out (mandrel type hangers in particular) and efficient spacer / cement displacement. Ideal rat-hole is 3-5 ft (1-1.5 m) long. Note that floated casing / liner would have an elongation effect when filled with fluid and this should be accounted for at the planning stage. Leaving open wellbore below casing / liner shoe could lead to wellbore instability, poor hole cleaning and cement break-down. Those risks severity will increase proportionally with the rat-hole length.
In some cases (normally, the last hole section), however, there is a need for some rathole to allow tools at the top of the logging string to reach and measure the deepest zone of interest. Depending on the completion design and wellbore geometry, extra rathole might be required to allow for junk, hole fill-in and other conditions that may reduce the effective depth of the well.