TOP HOLE DRILLING

In upper part of the hole, drilling rate is normally too fast, hole sizes are large & porosity is very high. Cutting carrying capacity of the drilling fluid is poor since hole is about 30% to 40% larger than nominal size & high annular velocities can not be obtained. This leads to concentration of cuttings in the annulus thereby increasing the equivalent mud density in the annulus. Formation strength in the top hole section is normally low, more so in a marine well since part of the overburden there consists of seawater rather than formation. As such while drilling a top-hole section the chances of bulk mud losses are high, which if happens can lead to a shallow formation flows. The various problems faced while drilling top-hole section are as below:

a) The formation being weak is vulnerable to bulk mud losses.

b) Soft, fast drilling formation generates large volumes of cuttings that tend to accumulate in the bore hole.

c) Annular velocity is very low (due to large hole) and top-hole mud has a poor cutting carrying capacity.

d) Kicks occur quickly & since shallow reservoir can have high permeability, time for action is very limited.

The shallow gas kicks are difficult to detect at an early stage because most of the standard flow detection techniques fail. Flow checks on drilling breaks become impractical as drilling rates generally are very fast & penetration rates vary tremendously. Mud volume is continuously being added to active mud system therefore pit level indicator at times can not be made use of. The only reliable indicator is differential flow sensor. On the other hand reaction time is minimal since gas expands almost immediately upon entering the well bore, which further reduces the hydrostatic head and allows more and more influx to enter the well bore. As such well can not be closed because flow might broach to the outside of the shallow casing.

The major hazards of shallow gas influx

i) It can lead to blow-out.

iii) It can broach through outside of shallow casing leading to instability of bottom supported rigs. iv) Risk of crater, fire & loss of rig is very high.

A typical approach to a shallow gas kick is to allow the well to flow through a diverter. The diverter system is designed to pack off around Kelly, casing or drill string. It does not shut in the well, but allows the flow to be diverted through a vent line to a safe distance away from the rig. Well is allowed to flow and simultaneously mud or water is pumped through the drill string at maximum rate to keep as much fluid in the well as possible. Either the well shall flow till the formation depletes (or annulus bridges) or the well is brought under control by increasing the mud weight.

12.11.1 Shallow Gas Control Procedure

Diverter system should be used to control shallow gas kicks as discussed below.

During Drilling

a) At first sign of flow, immediately stop rotary, raise the kelly until tool joint is above rotary. b) Open diverter overboard line valves depending upon wind direction.

c) Close diverter packer.

d) Circulate out with available drilling fluid at maximum possible pump rate. e) Remove the non essential personnel from the rig.

While Tripping

a) Set pipe on slips.

b) Install FOSV and close it.

c) Open diverter line valves depending upon wind direction. d) Close diverter packer.

e) Connect Kelly or circulating head. f) Open FOSV.

g) Circulate out with available drilling fluid at maximum possible pump rate. (Note : Step b and f are not required if string contains a float valve) h) Remove the non essential personnel from the rig.

Control of some critical drilling parameters

Since shallow gas kicks occur quickly and time for action is limited, it is useful to control some critical drilling parameters to prevent shallow gas kicks.

a) Penetration Rate:- The rate of drilling is normally very fast in top holes, it adds tones of

drilled cuttings in the hole to create mud density much higher than what is required. It can lead to fracture of formations & bulk mud losses & therefore result in shallow gas influx. Thus there is need for limiting the actual penetration rates to a value less than that can be achieved.

b) Mud Density:- Avoid mud density increase down hole by -

(i) Drilling large diameter holes in two stages (i.e. drilling a pilot hole) (ii) Circulating out the cuttings with viscous mud sweeps.

c) Tripping:- Higher tripping speeds should be avoided, upper formations areusually sticky

(more so in offshore) & has more tendency to ball the bit thus enhancing the probability of swabbing. If necessary the drill string should be pumped out of the hole to limit swabbing. Whenever the pilot hole has been drilled, hole enlargement with under reamer should be preferred as it can be collapsed before starting pulling out in order to reduce the swabbing effect.

In addition to above, following measures are suggested.

i) Heavy Mud

A minimum of one reserve mud tank weighing about 2-3 ppg more than the drilling mud should be kept reserve.

ii) Mud Losses

When ever losses are encountered, they should be sealed before drilling ahead. Bits should have large nozzles to allow pumping of LCM material.

iii) Active Mud System & Flow Checks

Mud pit volumes should be continuously monitored so as to detect any change in active mud volumes. Periodic flow checks should be made while drilling in potential gas zones.

iv) Float Valve

CHAPTER - 13