Part III - Well stimulation methods

Wormholes

Dissolution pattern obtained with a network model

The width of the bonds reflects the amount of material dissolved and removed.

Numbers correspond to the orders of magnitude in the Peclet number Pe

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Horizontal wells

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Horizontal wells

HW drilling provides powerful and attractive technology of hydrocarbon recovery due to the following features of horizontal wells:

Substantial length

Infinite conductivity

Control of the geometry

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Horizontal wells

Targets for horizontal wells:

Thin reservoirs

Reservoirs with no horizontal barriers

The greatest increase in productivity will be gained when a HW connects a very localized network of fractures existing in a low permeable reservoir.

Another advantage of HW is that it can be hydraulically fractured at several points or intercept natural vertical fractures. The letter will effectively increase an overall productivity of a HW.

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Advantages of horizontal wells

Formation damage

The additional pressure drop associated with skin effect in VW is proportional to

p S / h p

hp – perforated interval

The additional pressure drop associated with skin effect in HW is proportional to

	p	S
				kh / kv	L – length of HW
		L

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Advantages of horizontal wells

Sand control

Production from a HW of length L will reduces the mean flow velocity as compared with production from a VW in a ratio

h p / L

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Advantages of horizontal wells

Gas reservoir

In a vertical well, in vicinity of perforation holes, inertial forces play an important role. This cause additional pressure loss which is proportional to the square of the flowrate:

p 2 Aq Bq 2

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Advantages of horizontal wells

Critical rate

Often oil reservoirs must be operated at a flow rate lower than he socalled critical rate. Over and above this rate, the unwanted fluid, i.e. gas or/and water, appears in production. Examples where such phenomena might take place, are listed below:

–Gas reservoir with underlying aquifer

–Oil reservoir with a gas cap

–Oil reservoir with underlying aquifer

HW can efficiently reduce the “coning” effect:

–By placing the well at a maximum distance from the GOC/WOC

–Due to improved productivity of a HW the viscous forces can be lowered and the critical rate can be higher. In certain cases critical rate can be 5 times higher with HW 2000 ft long (Oliver de Montigny)

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Advantages of horizontal wells

Water drive or water injection

A HW can contribute to the enhancement of a water drive/water injection due to:

–Improved productivity

–A greater dispersion of the withdrawal. The pressure sink is no longer concentrated at a single point. This increases the volume swept by water

–In case of bottom drive, the distance from the WOC is greater which enables to delay the water breakthrough

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Advantages of horizontal wells

Better results can be achieved in cases of:

Heavy oil. The profits are maximum throughout water-free production period. This phase will be longer with HW

Low permeability reservoirs. In this case, it is the pressure drop in the reservoir which limits the oil + water production in vertical wells. The oil + water productivity increases in HW. This enable extension of oil + water production period from HW.

HW might increase the oil recovery (OR). Our evaluations show that OR can be enhanced by 2-4% (2-4% of STOOIP) that can be additionally produced by means of HWs.

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