For a slim hole casing design all the design loads, inclusive burst, collapse, and tensile load requirements, should be determined as for all other wells. For a fixed influx volume and assuming a single bubble of gas, the smaller annular cross section causes a greater influx height than in conventional holes. This might influence the casing setting depth and indicates the need for more sensitive kick detection techniques.
It is important, especially where non-standard casing sizes are introduced, that geometrical clearances, drilling hydraulics, and cementing hydraulics are closely examined:
Geometrical clearances
The following checks should be made for all proposed casing schemes:
- clearance between the bit and the drift diameter of the casing;
- clearance between the open hole and casing connections.
Drilling hydraulics
Selection of casing and drillpipe sizes should be made such that the low flowrates required for drilling small holes do not cause problems with respect to hole cleaning. Introduction of several drilling liners might be a solution.
Cementing hydraulics
Because of the high density and viscosity of cement slurries, pressure losses can be high in slim hole wells with small annular clearances. In some instances this may restrict pumping rates such that the slurry flow is not turbulent. Low pump rates may mean long pumping times and cement properties should be adjusted accordingly.
Attempts should also be made to include casing couplings in computer program string descriptions where annular clearances are small. The result of these simulations may indicate the requirement for flush couplings to reduce pressure losses.