Horizontal Well Cleanup
Horizontal and high angle wells are drilled by an increasing number of operators. These differ from conventional vertical and deviated wells in greatly increasing reservoir penetration - typically in the range 300 to 2000m (1000-6000ft). There are a number of advantages to horizontal wells in the cost effective exploitation of an asset. The two most important drivers for using horizontal wells are:
- improved productivity compared to conventional wells,
- multiple unconnected reservoir pockets can be assessed from one well, thus reducing the number of wells to be drilled.
These benefits can only be attained if horizontal wells are drilled and completed without significant near wellbore damage.
Most horizontal and high angle wells drilled are completed open hole - i.e. without a cemented and perforated liner. Open hole completions include barefoot completions, as well as those with sand control screens, slotted liners or pre-drilled liners.
In open hole completions the drilling mud can have an even greater impact upon well production performance than in conventional cased and cemented wells because:
- oil or gas must be produced through the mud filter cake and any mud filtrate induced formation damage, as there are no perforations to penetrate through these damage layers.
- sand control completions, such as pre-packed and wire wrap screens, can also be exposed to (and damaged by) the mud.
Thus avoiding mud damage or removing it during well clean-up is vital to the success of a horizontal well.
The approaches to cleaning up a horizontal well with an open hole completion can be conveniently divided into four categories. These are listed below:
- Design mud to flow through the sandface completion and bring well on production without clean-up.
- Displace mud to a low solids fluid and bring on production.
- Displace mud to a solids free, clear fluid and bring on production.
- Displace mud to a breaker system prior to bringing well on production.
It must be stressed that the best clean-up technique will vary from field to field.
Well Productivty Tests on Mud System
The mud system must not damage well productivity by either completely stopping flow from any reservoir section, or significantly increasing the near-wellbore pressure drop and thus reducing the well's productivity index. The following tests should be carried out to quantify the potential mud damage induced by a candidate mud system:
- Mud filtrate formation damage test
Reservoir condition core flood test to establish whether mud filtrate significantly impairs the permeability of the formation. For uncemented completions, hydrocarbon must be produced through the damaged zone. Most horizontal wells are fairly tolerant of filtrate induced formation damage, i.e. permeability loss < about 30%. If, however, damage levels around the well are high, i.e. > 50% permeability loss, this can significantly reduce well productivity. - Mud cake breakthrough test
Test to establish the pressure required for hydrocarbon to break through mud filter cake at the beginning of production. Permanent damage (permeability reduction) due to mud solids is also established.Breakthrough pressure should be compared to the expected well drawdown to establish if mud cake is likely to prevent oil production.
- Damage to sand control screens (where applicable)
Drilling mud can severely plug sand control screens. Laboratory tests can be carried out to quantify this damage and identify an optimum screen clean-up technique.
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