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Underbalanced Perforating. Underbalanced Perforating. Early 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
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Underbalanced Perforating • Early 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 • Published 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 • Plugs that formed when perforating in heavy mud were almost impossible to remove by reversing pressure. • "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." • "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 • Differential pressure required to initiate flow through each plugged perforation, varies. • 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. • “Crushed and compacted rock around the perforation has essentially zero permeability and further reduces the probability of perforation cleanout.”
Underbalanced Perforating • The 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. • High post-shot formation to wellbore flow generally provides optimum perforation cleanup and minimum skin.
Underbalanced Perforating • Underbalance 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 • Even when compared to surging and washing, underbalance perforating followed by flow can be superior
The Level of Underbalance • Must 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 • The pressure differentials necessary to achieve the flow rates required to remove perforation and/formation-skin damage are affected by: • Formation pressure • Reservoir permeability • Perhaps limited by formation integrity • Usually range from approximately 500 psi to over 5000 psi • Have been established by trial and error in many fields
Implementation • Downhole and surface mechanical equipment to achieve desired underbalance and maintain the integrity of the well • Control the well during deployment, perforating and retrieval, • Deploying the perforating device(s) to the proper downhole position, • Activating the perforating mechanisms, • Monitoring the downhole perforating process, • Retrieving the perforating system
Precautions! • It is more costly to employ than conventional perforating. • Safety and well control is a primary issue. • It requires the appropriate combination of reservoir pressure, reservoir fluid properties and formation permeability to achieve the required underbalance for effective application. • Prospects must be thoroughly screened, and there are those that may not be, or are not, appropriate candidates.
Jet PerforatingFormation Properties • Compressive strength, effective stress and specific rock characteristics can have significant impact • In unstressed rock, penetration decreases with increasing compressive strength • Pore fluid compressibility affects performance. • Increasing liquid saturation improves penetration • Stress reduces penetration (other factors being kept constant)
Penetration Multiplier Effective Stress (ksi) Jet PerforatingFormation Properties
Design • Perforator Type, Charge Strength and Gun Clearance • Conveyance Logistics, Surface and Downhole Equipment/Apparatus • Existing Wellbore Tubular/Cement-Sheath Limitations
Design • Formation Mechanical Rock Properties and Characteristics • Reservoir Pressure and Fluid Flow Characteristics • Perforating Downhole Environment Conditions/Limitations
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 • The major factors affecting the efficiency of a perforation include shot density (spf), penetration depth into the formation, angular phasing, and diameter • Injectivity increases as shot density increases? • Injectivity increases with increases in perforation penetration? • The effect is greater at shallow depths.
Efficiency • Angular phasing other than 0° increases injectivity by reducing the interference with flow resulting from the presence of the wellbore. • Perforation diameter plays a relatively minor role in determining injectivity? • The 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 • Deployment systems for multiple guns. • Guns are loaded at the surface, deployed downhole individually, and stacked on each other at the perforating zone, with the lower-most gun module being supported by the gun hanger.
Modular Gun System • An entire interval can be perforated over- or underbalanced • Gun sizes from 2 to 7-inch OD can be run for casing from 3 ½ to 8 5/8 inches, • Zone can be perforated and tested with no downhole restrictions below or above the packer.
