Well Control Problems and Well Control Procedures in Well Drilling

Well Control Problems

If we start from spud then the first well control problem encountered in well drilling would be Shallow Gas. About 20 years ago we used to drill and set the 30" casing and then run a Pin Connector, this was a hydraulic latch and flex joint assembly which you ran on the bottom of Riser. The idea behind it was that you could use mud to give you a hydrostatic column or hydrostatic pressure and control the shallow gas. In reality if Shallow Gas was encountered the riser became a funnel or channel for the gas to the rig and over the years after several incidents it was decided not to use a Pin Connector. If you drill into a Shallow Gas zone and the kill mud has been pumped then the rig can be moved off location by either lowering the drill pipe stand as the rig is moved or dropping the drill string and moving the rig up wind or current. The stability of the rig is greatly reduced due to the sea being aerated by the gas, further problems with fire or explosions compound the incident.
If an over pressured zone is drilled into and the casing is set and cemented correctly then 99% of well control problems can be controlled and corrected.

Well Control Procedures

The most common causes of kicks are;
1. Improper hole fill whilst tripping.
The most common is human error or not filling the hole with the correct volume of mud for the steel displacement. Basically every tubular has an equivalent volume of mud to either replace it when pulled out of the hole or when the drill string is run in the hole. If it is not monitored or measured correctly then the height of the mud will reduce and the hydrostatic pressure from the column of mud will reduce when the drilling string is pulled out of the hole. The Driller will use the Trip Tank to monitor the well whilst tripping or flow checking. On the Eirik Raude it is located in the deep well next to the ROV umbilical winch. Maximum volume for the trip tank is 14m3. When the Driller lines up to trip out of the hole, he will close off the Flowline to the mud pits and open the trip tank line, the Trip Tank has it's own dedicated centrifugal pump which circulates mud from the trip tank into the Diverter Housing, when the hole / riser is full it flows back down the dedicated return line back to the Trip Tank. The volume is calibrated /monitored carefully to the nearest 10 Litre of volume change.

2. Swabbing whilst POOH.
Swabbing is caused by pulling the drill string to fast, possible small clearance between the drill string and the hole, balled up bit or BHA, mud properties wrong. The most critical part when POOH is the first 5 or 10 stands out of the hole, if a slug has been pumped then it has to be allowed to equalise before tripping commences. The volume of mud required to fill the hole is measured every 5 stands and must be accurately measured and logged. The Driller will maintain a log of displacement for the whole trip out.

3. Shallow flows.
Shallow flows is as dangerous as Shallow Gas. The geometry of the different formations could allow formation fluids which are trapped at higher pressures to transfer through Faults or Fissures into the well bore creating a well control problem. Quite often a problem when drilling on land and near mountains where the water table is charged due to communication with rivers / glaciers / on the mountain. The extra height increases the hydrostatic pressure and when the formation is drilled into instead of being a Normal Pressured formation it is an Abnormal Pressured formation.

4. Insufficient mud weight.
Insufficient mud weight is where the hydrostatic of the mud column is less than the formation pressure. This could be human error where lighter mud has been pumped down hole or a higher pressured zone drilled into.

5. Drill into abnormal formation pressure.
Over pressured zone is where the overburden or the weight of the formations above are pressing down on a plastic / pliable formation or where there is communication due to a fault or from volcanic movement of the earth.

6. Lost circulation.
Lost circulation is where the mud weight is to high for the formation pressure and the weak formation breaks down.

7. Incorrect well planning.
Incorrect well driling planning could be used for wrong casing depths / wrong mud weights / wrong well geometry with regards to BHA design with soft-clay type formations.