Underbalanced Perforating Underbalanced Perforating Early tests by Exxon

Underbalanced Perforating

Underbalanced Perforating èEarly tests by Exxon showed that flow patterns and perforation geometry prevent the cleaning out of an appreciable percentage of mud-or silt-plugged perforations by simple production from a well èPublished studies of the flow rate necessary to remove damage observed that serious perforation plugging occurred whenever the pressure was higher in the wellbore than in the formation

Underbalanced Perforating è Plugs that formed when perforating in heavy mud were almost impossible to remove by reversing pressure. è "Permanent plugging of a high percentage of perforations may result from killing a well with mud or dirty fluid during well completion, servicing, or workover. " è "When perforating in mud with a pressure differential into the formation, perforations are filled with mud solids, charge debris, and formation particles. " Not easily removed.

Underbalanced Perforating è Differential pressure required to initiate flow through each plugged perforation, varies. è When a few perforations requiring low differential pressures open up, flow into these perforations makes it difficult to create the higher pressure drawdown needed to open additional perforations. è “Crushed and compacted rock around the perforation has essentially zero permeability and further reduces the probability of perforation cleanout. ”

Underbalanced Perforating èThe post-shot flow into the wellbore (a function of the formation-wellbore pressure differential, the formation fluid viscosity and the formation permeability) helps remove the crushed formation from the perforation and provides improved flow channels. èHigh post-shot formation to wellbore flow generally provides optimum perforation cleanup and minimum skin.

Underbalanced Perforating èUnderbalance perforating followed by flow has been shown to be the best method for cleaning perforations and establishing high flow capacity from natural completions in moderate to high permeability core èEven when compared to surging and washing, underbalance perforating followed by flow can be superior

The Level of Underbalance èMust balance perforation cleanup and well performance potential enhancement against the downside aspects of mechanical problems such as perforators or wireline sticking in the wellbore, near-wellbore rock formation disintegration, downhole tubulars and equipment damage, etc.

The Level of Underbalance è The pressure differentials necessary to achieve the flow rates required to remove perforation and/formation-skin damage are affected by: è Formation pressure è Reservoir permeability è Perhaps limited by formation integrity è Usually range from approximately 500 psi to over 5000 psi è Have been established by trial and error in many fields

Implementation è Downhole and surface mechanical equipment to achieve desired underbalance and maintain the integrity of the well è Control the well during deployment, perforating and retrieval, è Deploying the perforating device(s) to the proper downhole position, è Activating the perforating mechanisms, è Monitoring the downhole perforating process, è Retrieving the perforating system

Precautions! è It is more costly to employ than conventional perforating. è Safety and well control is a primary issue. è It requires the appropriate combination of reservoir pressure, reservoir fluid properties and formation permeability to achieve the required underbalance for effective application. è Prospects must be thoroughly screened, and there are those that may not be, or are not, appropriate candidates.

Jet Perforating Formation Properties è Compressive strength, effective stress and specific rock characteristics can have significant impact è In unstressed rock, penetration decreases with increasing compressive strength è Pore fluid compressibility affects performance. è Increasing liquid saturation improves penetration è Stress reduces penetration (other factors being kept constant)

Penetration Multiplier Jet Perforating Formation Properties Effective Stress (ksi)

Design èPerforator Type, Charge Strength and Gun Clearance èConveyance Logistics, Surface and Downhole Equipment/Apparatus èExisting Wellbore Tubular/Cement. Sheath Limitations

Design èFormation Mechanical Rock Properties and Characteristics èReservoir Pressure and Fluid Flow Characteristics èPerforating Downhole Environment Conditions/Limitations

Design (King’s Method)

Hsia and Behrmann

Tariq

Cleanup Criterion Based on Reynolds Number

Design Of the factors influencing the determination of correct underbalance for cleanup the fluid properties, especially viscosity, are important but the key factor is the formation permeability

Efficiency èThe major factors affecting the efficiency of a perforation include shot density (spf), penetration depth into the formation, angular phasing, and diameter èInjectivity increases as shot density increases? èInjectivity increases with increases in perforation penetration? èThe effect is greater at shallow depths.

Efficiency èAngular phasing other than 0° increases injectivity by reducing the interference with flow resulting from the presence of the wellbore. èPerforation diameter plays a relatively minor role in determining injectivity? èThe strength, in-situ stress conditions and lithology can effect the penetration length, the extent and severity of the damage zone around the perforation, and the cleanup characteristics.

Modular Gun System èDeployment systems for multiple guns. èGuns are loaded at the surface, deployed downhole individually, and stacked on each other at the perforating zone, with the lowermost gun module being supported by the gun hanger.

Modular Gun System è An entire interval can be perforated over- or underbalanced èGun sizes from 2 to 7 inch OD can be run for casing from 3 ½ to 8 5/8 inches, èZone can be perforated and tested with no downhole restrictions below or above the packer.