Perforating Basics

Perforating BasicsBasic ConceptsFactors Affecting Gun PerformancePerforation x ProductionPerforation and Completion TypesPerforation TechniquesIntelligent Perforation

Basic ConceptsPerforation in Well CompletionExplosivesShaped ChargesPerforating Guns

Well CompletionPerforating is a critical part of well completionDeterminant for well productivity SafetyLower CostPressure Relation:Overbalance;Underbalance;Extreme Overbalance.

ExplosivesExplosives Types:Low Explosives:High ExplosivesPrimary Explosives: Electric and Percussion Detonators;Secondary Explosives: Detonating Cord, Boosters and Shaped Charges.Affected by temperature

Shaped ChargesComponents:Outer Case;Primer Explosive;Main Explosive;Liner

Shaped ChargesDetonation Mechanics:1. Detonation Cord - Primer Explosive - Main Explosive2. Liner collapses - perforating jet

Shaped ChargesDetonation Mechanics:1. Detonation Cord - Primer Explosive - Main Explosive2. Liner collapses - perforating jet3. Jet impacts casing surface (100 GPA )4. Jet passes through casing wall, cement and formation

Perforating GunsDivided in:Capsule Guns: Enerjet* and Pivot*Carrier Guns: High Density, HEGS*, ScallopeExpendable or RetrievableDebris are concernClearance x standoff

Perforating GunsDivided in:Capsule Guns: Enerjet and PivotCarrier Guns: High Density, HEGS, ScallopeExpendable or RetrievableDebris are concernClearance x standoff

Gun PerformanceDirect Influence well productionPerformance affected by:Gun DesignDownhole Parameters

Gun Design FactorsStandoffScallop ThicknessSmall effect in deep penetration than in Big Hole chargesCharge packed in the gunCharge to charge interference;

Downhole ParametersClearanceCasing StrengthEffective Formation StrengthWellbore FluidTemperature

Perforation x ProductivityProductivity LawsSkinPerforation ParametersSkin due to Perforation

Productivity LawsDarcys Law

Productivity Index

SkinDivided inMechanical Skin:Drilling, Completion fluidsPerforation damagePseudo Skin:Completion improper diameter;Shot densityTurbulencePerforation plugging

Perforation ParametersGeometry factorsImportance related to completion type and formation conditionsShot density more important than shot penetration

Skin due to PerforationSkin due to:Perforation LengthShot DensityCrushed Zone PermeabilityThickness Optimum Underbalance

Completion TypesNatural CompletionStimulated CompletionFracturing;Acidizing;STIMPAC*Extreme OverbalancePerforation affecting Sand Control and Fracturing

Perforation x Sand ControlPerforation Efficiency - PermeabilityFactors:GravitySand debries are difficult to removeStable ArcsMultiple PerforationsGravel mixed to Sand Debries tends to reduce permeability

Perforation x Sand PreventionThree main objectives:Acceptable sand productionMinimize formation stressOptimum flow rateRecommended Perforation Design:UnderbalanceDeep PenetrationOptimum gun phaseShot density by Critical FlowOil as wellbore fluid

Perforation x FractureObjectives:Minimize Wellbore Pressure DropsPerforation FrictionMicroannulusSide FracturesTortuosityLab Results:180 phase with 30 degrees offset PFPHole diameter: Saucier Method

Perforation TechniquesThrough-Tubing PerforatingHyperDome*, Enerjet* or Pivot* - Carrier GunsCompletion tested, Underbalance, RiglessLimited Interval - 30 ftCasing Gun/ High Shot Density GunsPort Plug Guns, HEGS* and HSD gunsGun size limited by casing sizeWireline conveyed in overbalance: larger interval (60 ft)Low Debries Guns

Perforation TechniquesWireline :Faster and EconomicLimited Intervals - Deviated WellsTubing Conveyed Perforation Long and multizone intervalsThrough tubing and casing/ HSD advantagesUnderbalance PerforationWide variety of firing systemsExpendable guns can be drop allowing through tubing operations

Perforation TechniquesGunsEnerjet*Pivot*HSD*Big ShotRelease MechanismsWAXR*SXAR*MAXR*

Perforation TechniquesCompletion without KillingFIV*CIRP*PERFPAC*GunStackPerfPAC*

Intelligent PerforatingCustom design each completion based on formation properties to optimize productionIntelligent Perforating.Completion Design

Intelligent PerforatingReservoirDescriptionCompletionDesignHardwareTreatmentGunDeploymentJobDesignExecution(Optimize)SeismicLogs...PerforatingParametersSimulatorsNodal, SPAN*..MonoboreMultiple..Performance Data BaseField Specific

Natural CompletionsFormation DamageDeep Penetrating ChargesAnisotropyHigh Shot DensityPhasingPerforation SkinUnderbalanceTCPThrough-TubingFIV, X-ToolsCIRP, StackableLong IntervalsTCP, CTFIV, X-ToolsCIRP, StackableDeep/Hot WellsHPHT, HNS/HTXWirelineThrough-TubingHard FormationsHard-Rock Charges

Sand ControlGravel Placement & Skin ReductionArea-Open-to-Flow (AOF)Shot Density and Big-Hole ChargesPhasingReduction of Perforation Skin (Underbalance/Clean-up)

EfficiencyPERFPACIRDV, SXAR, HSD Guns, Quantum Packer

Sand PreventionRetain Structural IntegrityDeep Penetrating ChargesHigh Shot DensityOptimum Phasing (Maximize Perf-to-Perf Spacing)Limit DrawdownCustom Gun Designs7-in. 27 spf HSD Gun3 3/8-in. 4 spf 99

Hydraulic Stimulation - FracPressure DropPerforation Hole SizeInterval LengthNumber of ShotsTortuosity & LeakoffPhasing (180, 60 , 120)OrientationMicro Annulus or Multiple FracturesDeviation

Deviated/Horizontal WellsDebrisCleanSHOT*, CleanPACK* ChargesBigshot 21*SkinGun OrientationSpecial Guns (Top-side Shooting)EfficiencyCoiled Tubing PerforatingFIV*, LTIV*

ReferencesJan 99Schlumberger WL & Testing Perforating Services Manual

Perforating is a critical part of well completion. Well production depends on near wellbore pressure drop. Damages due to drilling or completion fluid invasion and the crushed zone formed by perforating procedure reduce sandface permeability, which affects not only production but other operations( Gravel Pack, Fracturing, etc..). Beside that, perforation size and density affect production flow.Safety is a main concern when working with guns. Surface detonations are catastrophic and may result in live loses. It is a Schlumberger commitment provide operation following rigid procedures, well prepared and safe equipment, handle by well-trained and high qualified personnel. Safety devices have been developed to ensure a properly operation (S.A.F.E.)Perforation procedures must be optimized in order to minimize the number of required trips, reducing costs. New procedures and technologies, like No Kill Perforating, and combined operations, like PERFPAC*, have been developed with this propose.Explosives were used by Chinese since 10th Century, passed to the Arabic in 13th century. They are divided in two types:Low Explosives with slow reaction rates ( 500 - 1500 m/sec) and low combustion pressures. They are used in the oilfield in bullet perforators, sample taker guns and stimulation ( high-energy gas fracturing, perf wash, etc...)High Explosives have very high rates, between 5000 to 9000 m/sec and generate enormous combustion pressures. They are classified by sensibility and ease detonation in Primary and Secondary High Explosives. Primary Explosives are very sensible and easily detonated by shock, heat or friction. Due to safety reasons, in Schlumberger they are used only in electrical or percussion detonators. Secondary Explosives are less sensitive, requesting high shock waves to detonate. They are present in the detonation cord, boosters and in shape charges.Temperature is a main factor that affect reaction, combustion and sensitivity of chemical explosives. It means that explosives have to be used in their safety range of temperature, avoiding auto-detonation or reduction in performance.Shaped Charges are the components of perforation guns. Their development came from research performed by US Navy in 1888, detonating different shaped charges close to steal metals and observing the shape of the cavities left. This experiment was used in the II Word War in development of antitank weapons. The main components are: Outer Case: It holds the detonation and allow the formation of liner jet. Besides that, it avoid interference between charges in the gun. Steel is normally used in high penetration guns, while Zinc and Aluminum in expendable guns ( less debris ). Primer Explosive: Secondary sensitive explosive that ensure correct detonation of the charge by the detonating cord. Main Explosive: Also known as secondary explosive, it detonates forming the liner jet. It is select by temperature rating of shaped charge. Liner: Metal cone that during the explosion forms the perforating jet. They are normally made of compressed powdered metal, except in big hole charges where solid metal is used. The liner shape and composition is fundamental for perforation size and penetration. Compressed powder metal, as cooper, zinc, tungsten, tin and lead, are used to obtain deep penetration perforations. These metals are mixed together in order to obtain uniform density and velocity gradients.Solid metal liners were normally used but, due to the fact that they leave debris into the perforation tunnel, nowadays they are only used in Big Hole charges. In this cases, the large hole sizes and high formation permeability where this kind of shot is normally used compensate the negative debris effects. The detonation starts in the detonation cord. The detonation front travels at 7000 m/sec, detonating the primer and, just after, the main explosive. By this moment, the detonation pressures are about 15 to 20 GPA, what melts the conical liner at the same time that is compressed formatting the perforating jet. At the same time, the case expands radically. The jet travels towards the casing wall, divided into two axial streams: a faster stream called tip ( velocities on 7000 m/sec) and the tail ( 500 m/sec). This differential velocity is responsible by the jet stretching. Perforating jet reach the casing wall at 7000 m/sec speed and almost 100 GPA pressure. This force is enough to deform plastically the casing material, that follows away from the jet. In the same process, the jet passes trough cement and formation, eroding the tip material which is completely expended at the end of perforation.A important consideration is that this process happen without heat, being the process a low temperature process. Perforating Guns are divided in two categories: Capsule Guns where the shaped charges are covered one by one in pressure type cases. They are normally used in through tubing perforating jobs, Wireline convey jobs; Carrier Guns have a group of shaped charges involved in a steel carrier. This kind of guns is used in TCP or Coiled Tubing convey perforating jobs.Perforating guns can also be expendable or retrievable. Expendable guns are destroyed when its charges are fired while Retrievable Guns casing are not affected by detonation, being retrieve after being fired.Debries production during perforating procedure are a concern. The main debries sources are: case, liner , charge jacket or carrying tube debries; spall debries from exit hole in the gun and entrance hole in the casing; formation material produced during perforation or when the guns are removed. In other to avoid debries formation, zinc alloy charge cases ( CleanPack*, CleanShot*) and powdered metal liners are normally used. Debries formed during detonation are easily dissolved by acid or circulated out off the well. Clearance is the distance between the gun and casing. Standoff, also called In-gun clearance, is the distance between charges and gun casing. This distance is designed to allow enough space for the collapse of liner and jet formation before it hits the gun internal wall.

Standoff: The standoff size can be optimized to ensure complete formation of perforation jet or design to provide a fat jet, used in Big Hole charges;Scallop/ port plug: Scallop thickness do not affect so much perforating charge penetration, but are determinant in the casing entrance hole. It happens because gun scallop affects the penetration jet tip more than the tail, responsible to large penetrations. Clearance: It does not affect charge penetration in most conditions, but has considerable influence in entrance hole size. Basically, the large OD are obtained with centralized guns.Casing Strength: Affects entrance hole, without major effects in charge penetration.Effective Formation strength:Wellbore Fluid: Mud can affect operation of TCP firing systems. In high density mud, pressure firing heads are recommended. Beside that, wellbore fluid determine the differential pressure between wellbore and formation.Temperature: It is an explosive selection criteria. Excessive temperature for an explosive can cause loss of performance due to burn or autodetonation at the wrong depth. Its effect is cumulative. For bottom hole temperatures below 300F, HMX or RDX can be used. Darcys Law describes the well production, considering the production flow in steady-flow, homogeneous and flowing cross the entire zone. For a oil producer well, Darcys law has the following equation:

Where: q: oil rate (bbl/day) k: permeability (md) h: pay zone length ( ft) Pe: Formation Pressure (psi) Pwf: wellbore pressure (psi) b: formation volume factor (res bbl/day) m: viscosity ( cp ) re: Reservoir Radius (ft) rw: Wellbore radius (ft) S: Skin

Productivity Index is a inflow performance equation related with Darcys Law. Analyzing those two equations is possible describe some methods to increase production: Increase Permeability - fracturing/acidizing; Reduce viscosity - steam injection; Reducing or eliminating skin

The region from the wellbore to 3-5 ft into formation is called Critical Matrix, specially because most of the pressure loses happen in the this near-wellbore zone. The pressure drop occur due to formation permeability reduction, result of drilling and completion procedures. This damage is quantify by the adimensional number called Skin". The total skin is result of mechanical or proper skin add of other effects, called pseudo-skin. Mechanical skin is caused basically by drilling fluid or completion damage, crushed zone due to perforation, The main geometrical parameters that determine flow efficiency are shot density, perforation penetration, angular phase and perforation diameter. Those factors relative importance is determinate by completion type ( if it is in sand natural unconsolidated, consolidated, Stimulate consolidated or if it has remedial damage) and formation natural conditions ( type of permeability, if it has natural fractures, formation strength).Those factors also influence each other. The relation between production and some of those factor for different angular phase is present in the graphics below:

The principal factor in perforation damaged skin are: Perforation Length, Shot density, Crushed zone permeability and thickness. Crushed zone and debris effects can be minimized flowing the well after overbalance perforation or using Underbalance perforation.The optimum Underbalance can be obtained using the software SPAN. This Underbalance, that must be enough to obtain clean perforations and at the same time avoid formation failure, is function of permeability, porosity, reservoir strength and type and size of charge.Some considerations must be taken in order to avoid large transient sand production, casing collapse, sticking guns and tool movement.

Natural Completion are recommended for sandstone or carbonate formations with good permeability (>10 md), small damaged zones and skin values and stable formation. High shot density and depth penetration are required in this cases. Underbalance perforation is recommended.Stimulated Completion can be divided in three categories: Fracturing, Acidizing or StimFRAC/ Acid Frac. Extreme overbalance perforation technique can be also used. Each one of these scenarios request different types of perforation in order to ensure efficiency.Perforation jobs can be also design in order to avoid sand production or complete Sand Control interventions.Sand Control: The efficiency of perforation is very important for minimize sand production, restricting flow rates, with minimal damage for formation permeability ( Gravel Pack ). The two laboratory procedures are Large Block test and Single Shot Test. Some of the conclusions are:Gravity affect downside perforations cleanup;To remove sand debris from perforation is very difficult;Production of stable arcs reduce Sand production;Multiple perforations have different behave in flow rate, sand production and tubing cleaning aspects.Beside that, in test performed to Shell Company, prove that interaction between rock debris and gravel pumped into perforation channel tends to reduce gravel pack permeability.Some recommendations for Gravel Pack are:Perforate close to the optimum Underbalance - SPAN;Flow the well after perforation - problems with PerfPac;Flow to surface minimize crushed zone damage;Perforating using brine or a mutual solvent as completion fluid;Optimize gun phasingInjection of produce oil after packingUse big hole with high shoot density. Deep penetration charges can be used ( less formation damage)

Sand Prevention: Three main objectives of perforating driving to sand prevention;Ensure production with acceptable sand production risk;Minimize formation stress;Keep production flow below critical value.The laboratory procedures are Large Block and Single Shot , checking increasing of formation stress due to perforations, effects of two-phase flow and determination of rate that induces sand production. The results showed that formation erosion is not significant, being sand production due to perforation failure or water production dominant.The main recommendation for sand prevention completion without Gravel Pack are:Perforate close to optimum Underbalance, keeping it below the critical value;Use deep penetration charges;Optimize gun phase, minimizing influence between perforations;Shot density determinate by critical flow;Use oil as wellbore fluid;

Fracture Stimulation: Perforating for fracturing should minimize pressure drop effects, Perforation Friction, Microannulus formation, tortuosity and side fractures. Beside that, the integrity of cement/ sandface must be kept and production of fracture sand must be avoided.Lab test prove that 180 deg phased guns with 30 deg offset to preferential fracture plane had good results for vertical wells. Deviated and horizontal wells have the same recommendation if wellbore is in the PFP. If not, perforation interval have decrease - avoiding multiple fractures - shot density is maximized and multiple phase angles.Penetration depth does not have to exceed 4 to 6 inch and hole size follow Saucier Method ( six times proppant diameter ).Recommendations for Frac & Pac jobs:With Screens:60 or 45 deg phasing angle;12,16 or 21 shots/ft;Without Screens:Eliminate intervals that could produce sand;0,180 deg phasing

There are two perforation techniques available: through tubing and Casing/ HSD guns. Through Tubing Perforating guns can be conveyed by Wireline, tubing or Coiled Tubing. Schlumberger through tubing guns are HyperDome, Enerjet or Pivot; all of then are carrier type guns. The use of these guns allow wellhead and completion pressure test before perforation, Underbalance perforation and perforations during well life time - Rigless. Its normally a low cost operation, but for ;limited perforation intervals (30 ft).Casing guns and High Shot Density guns are larger diameter guns lowered into cased well before production tubing being run. Schlumberger options for this technique are: Port Plug gun, HEGS* High-Efficiency Gun System, HSD guns. Those guns size is limited just by casing size, allowing deep penetration and big hole charges to be used in their optimal shot density and perforating pattern. Overbalance allow run larger guns with Wireline than through tubing limit of 30 ft. Normally, 60 ft can be active using this system. Other advantage is the low amount of debries left into the well by these guns.Perforating guns can be conveyed into the well by Wireline or Tubing conveyed methods. The chose between those two methods depends on completions objectives and operational conditions.Wireline is a faster and more economic method compared with TCP. The speed is an important consideration for high temperature wells. It avoids explosives prolongate high temperature exposure. Wireline can be used for limited intervals and moderate deviated wells.In Tubing Conveyed Perforation method, production tubing, drillpipe and Coiled Tubing is used to run the guns. This method is more convenient for long interval and multizone perforations. Beside that, it allows perforate long intervals in Underbalance conditions and the large variety of firing head and accessories permit to perform job in a wide rage of well conditions.TCP guns can be dropped after firing, allowing future through-tubing operations.