Well Testing

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UTC

Well Testing Techniques Module PKR09 RAJNESH GOGOI

WHAT IS WELL TEST

•Well testing is the technique UTC and method for the evaluation of well conditions and reservoir characteristics.

•It involves producing a well at a constant rate or series of rates, some of which may be zero (well closed in), while simultaneously taking a continuous recording of the changing pressure in the well bore using some form of pressure recording device. Module PKR09

WHY TEST A WELL

To determine the following UTCparameters •Initial pressure (p) •Average pressure within the drainage boundary (p) •Permeability thickness product(kh) and permeability(k) •Mechanical skin factor (S) •Area drained (A) •Dietz shape factor (CA) Module PKR09

Types of Well Tests • Productivity Well Test

• Descriptive/Reservoir Test

Productivity Well Tests Pwf

• Well is produced at several flow rates • Stabilized BHP is measured • Plot of Pwf Vs Q = IPR • Slope of lines indicative of well productivity

Q

Applications of Productivity Well Tests • Identify produced fluids • Measure reservoir pressure and temperature • Obtain samples for PVT analysis • Evaluate completion efficiency

• Evaluate workover or stimulation treatments

Descriptive/Reservoir Testing • Involves introducing abrupt changes in production • Associated changes in BHP monitored • Disturbances penetrate more than near-wellbore • Other reservoir properties can be determined

– Shape of Pw Vs time curve is defined by reservoir characteristics

Applications of Descriptive Tests • Evaluate Reservoir parameters

– Skin,Flow Efficiency,Productivity index – Permeability – Reservoir pressure • Characterize reservoir heterogeneity • Reservoir extent/geometry • Hydraulic communication

Well Testing In Different Phases • Drilling phase – Open hole testing(Descriptive ) • DST • RFT

• Completion Phase – Productivity Tests

• Production phase – Cased hole testing • Descriptive Tests

Important datas required in well testing Core,log data(to check homogeneity,dual porosity,OWC,GWC etc.) RFT,pressure depth profiles(to check pressure equilibrium and thus homogeneity) Geological model(for structural interpretation,position of OWC,GOC) Drive mechanism PVT fluid properties(Pb,Bo,Co,

Well completion(partial completion and partial penetration) Tests in neighbouring wells completed in the same reservoir Equipment (position of pressure gauge from reservoir datum depth)

Types of Well Tests • • • • • •

Pressure Drawdown Pressure Build up Multirate Injection/fall-off Interference Pulse

Pressure Drawdown Tests • Shut in the well till Pwf pressure reaches static level & then flowing the well at a constant rate ,q & measuring Pwf. rate

Shut in

q time

t

Pressure Drawdown Tests •

BASIC EQUATION OF PRESSURE DRAWDOWN TEST:

Pwf=Pi – (162.6qµBo/kh)((log t + log (k/ФµCtrw2)–3.23+0.87s) This is the equation of a straight line with slope m = 162.6(q µ Bo /kh) From slope permeability and skin can be calculated as k = 162.6(q µ Bo /mh) and skin,S = 1.151(((P1hr - Pwf)/m) - log(k/ ФµCtrw2 )+3.23)

Pressure Drawdown Tests •

Advantages: – – –

•

Suitable in new wells No need to lose production K,S,reservoir size can be determined

Disadvantages: – –

Difficult to maintain constant production rate. Long shut in so that Pi is achieved is required.So suitable in new wells.

Mulitrate Tests • •

Accounts for variable rate history Applications

– Rate variations – kh product, Pr – Boundary configurations – Skin,FE,PI etc.

Pwf Pi

q1

0 t1

qn

qn-1

q2 t2

tn-1 Time,hrs

tn

Mulitrate Tests • Advantages: – No problem with variable flow rate – No loss of production – Reduced wellbore storage • Disadvantages: Q3 Q2 – Rate fluctuations are difficult Q1 to measure,especially on a continuous basis.

Pressure Build-up Tests • •

•

Most preferred well testing technique The well is first produced at a constant rate till pressure is stabilized and then the well is shut in Pressure is recorded as a function of time

Pwf Pi

q flowing

tp

Shut-in t

Pressure Build-up Tests •

•

•

The basic equation of pressure build up test is Pws = Pi - (162.6qµBo/kh)log(tp+∆t/∆t) (Horner equation) This is the equation of a straight line when plotted as Pw Vs log (tp+∆t/∆t) (Horner plot) with slope m =162.6qµBo/kh and intercept Pi From this k & s can be determined as k= 162.6(qµBo/mh) and S= 1.1513(((P1hr-Pwf)/m)-log( k/ Ф µCtrw2 )+3.23) Fault distance=(0.0122k∆tx/ Ф µCt)1/2 ∆pskin= 0.87ms

Pressure Build-up Tests • Advantages: – Precise control of rate – P* can be determined • Disadvantages: – Loss of production due to shut in. Q(t)

Well Test Planning •

Before designing a test an engineer should ensure that there should not be

– more than one zone open to wellbore – interzone communication due to poor cement bond – partial zone completion •

It includes

– Test type – Test duration

Well Test Planning • Operational requirements – well completion data – location and pattern of wells completed in the same reservoir – rate data – fluid type – pressure data

Well Testing - Data Acquisition • Downhole Recorders – Record pressure and temperature – Components • Pressure Gauge • Power Source • Memory unit

– 1 to 2 recorders used – Surface pressure readout • Wireline link

Well Test Data Analysis • Diffusivity Equation

• Model Recognition

• Interpretation Software

DARCY’S LAW AND ITS APPLICATION • IN 1856, A FRENCH ENGINEER HENRY DARCY FORMULATED THE LAW • THIS IS THE MOTHER OF ALL RESRVOIR ENGINEERING CALCULATIONS • STUDY WAS ON UNCONSOLIDATED SAND FILTER BEDS

DARCY’S LAW AND ITS APPLICATION • DARCY’S LAW STATES THAT THE VELOCITY OF A HOMOGENEOUS FLUID IN A POROUS MEDIA IS PROPORTIONAL TO THE PRESSURE GRADIENT, AND INVERSELY PROPORTIONAL TO THE FLUID VISCOSITY • V= - (K/ µ) ( P/ L)=q/A

DARCY’S LAW AND ITS APPLICATION • APPLIES ONLY IN LAMINAR FLOW • DOESNOT APPLY IN TURBULENT FLOW • DOES NOT REPRESENT THE FLOW THROUGH INDIVIDUAL PORE SPACES BUT AVERAGES THE FLOW OF SEVERAL PORE CHANNELS.IT IS A STATISTICAL LAW

DARCY’S LAW AND ITS APPLICATION • THE UNIT OF PERMEABILITY IS DARCY. • A ROCK OF 1DARCY PERMEABILITY IS ONE IN WHICH A FLUID OF 1 CP VISCOSITY MOVE AT A VELOCITY OF 1CM/SEC UNDER A PRESSURE GRADIENT OF 1ATMOSPHERE/CM.

DARCY’S LAW AND ITS APPLICATION • THE UNIT OF PERMEABILITY IS DARCY. • A ROCK OF 1DARCY PERMEABILITY IS ONE IN WHICH A FLUID OF 1 CP VISCOSITY MOVE AT A VELOCITY OF 1CM/SEC UNDER A PRESSURE GRADIENT OF 1ATMOSPHERE/CM.

DARCY’S LAW AND ITS APPLICATION APPLICATIONS • TO CALCULATE THE FLOW RATE • TO CALCULATE THE P.I. OF A WELL • TO CALCULATE PRESSURE DRAWDOWN • IN RESERVOIR SIMULATION • TO INCREASE PRODUCTIVITY BY WELL STIMULATION

To sum up : Well Test A properly designed, executed, and analyzed well test can provide information: •about formation permeability •reservoir initial or average pressure •sand-face condition (well damage or stimulation)

•volume of drainage area •boundary and discontinuities •reservoir heterogeneity •distance or extension of the fracture induced

•validation of geological model.

To sum up : Well Test Further, it is important to determine the ability of a formation to produce reservoir fluids and underlying reason for a well's productivity.

Well Test data when combined with hydrocarbon production data, and laboratory data on fluid and rock properties: •afford the means to estimate the original hydrocarbon in-place •and the recovery that may be expected from the reservoir under various modes of exploitation •In addition, well test data and IPR well performance equations, combined with production data, help to design, analyze, and optimize total well production system or production optimization.

Typical layout of well test setup Surge Tank

Flare

Pumps

Lab cum data center I/Heater

Choke manifold

3 Phase seperator

48” THREE PHASE SEPARATOR

CHOKE MANIFOLD

3 HEAD SEA SNAKE BURNER

GROUND FLARE

Diffusivity Equation Assumptions: Horizontal flow of a single phase fluid inward to a wellbore located at the center of a radial volumetric element Formation is both homogeneous and isotropic

The central well is perforated across the entire formation thickness Negligible gravity effect Applicability of Darcy’s law Ф,µ,Ct ,k are independent of pressure

Single phase fluid is present in the reservoir

Diffusivity Equation Partial differential equation Describes physical processes occurring in the reservoir Solutions depend on boundary conditions Reservoir Extent Initial Pressure

2p/

r2 + 1/r ( p/ r) = (Ct

k) p/ t

Pressure Build-up Tests •

MDH plots ETR

MTR

LTR Constant pressure boundary

Semi log straight line

Pws

Closed boundary Small wbs Deep fracture,small wbs

Pressure increase due to fault

+s,small wbs No s,no wbs

- s,large wbs +s,large wbs No s medium wbs

log (∆t)

Q(t)

Pressure decline from offset production

LOG-LOG PLOTS •

Log-log plots are used in welltest interpretation

• •

It is a plot of pressure change Vs log( tp+∆t/∆t) in BU & Vs log∆t in DD In wellbore storage affected region it gives unit slope line Derivatives are differentiation of pressure change wrt time Pressure change & Boundary effects are evident by late time derivative Q(t) variation of the derivative value

• •

Elapsed time

LOG-LOG PLOTS Pressure change & derivative

1=Infinite homogeneous reservoir 2=No flow barrier-faulted reservoir 3=Parallel faults-linear reservoir 1x104 4=Perpendicular faults 5=Constant pressure boundary 6=Parallel no flow &constant pressure boundary 1x103 7=Net pay variation-linear

4 3 2 7

1x102

1

1x101

5 6

1.5 log cycles 1x100 0.010

0.100

1.000

10.000

Elapsed time

100.000

Special Cases In Well Testing • Afterflow – occurs because of surface shut in – end of this region can be approximately 1 1/2log cycle away from unit slope

Pressure change & derivative

LOG-LOG PLOTS 1x104

1x103

1x102

1x101

1.5 log cycles 1x100 0.010

0.100

1.000

10.000

Elapsed time

100.000

Special Cases In Well Testing • Partial penetration or completion – results in additional pressure drop – S=(ht/hp)sd+sp where s=total skin ht=total formation thickness hp=perforated interval sd=damage skin sp=partial penetration skin – sp=(ht/hp-1)(ln(ht/hp(kh/kv)0.5 )-2)

Some Special Formation Types • Composite System – the fluid & rock properties varies in a steplike fashion in radial direction away from the well

Zone 1: k1,ct1, Ф1, , µ1 .

well Zone 2:k2,ct2, Ф2 , µ2

Some Special Formation Types • Layered Reservoir:Two types – layered reservoir with crossflow in which the layers are hydrodynamically communicating at the contact planes – layered reservoir without crossflow in which the layers communicate only through wellbore(commingled production)

Some Special Formation Types • Double porosity system – two distinct porosity types-matrix & fracture porosity – the condition applies to fractured reservoir or layered reservoir with a large contrast in rock properties

HORNER PLOTS(Double porosity system

Dimensionless pressure

1x104

1x103

Pw 1x102

1x101

1.5 log cycles 1x100 0.010

0.100

1.000

Elapsed time

10.000

Elapstyed time

100.000

)

Pressure Transient Analysis  Methodology Raw Data Model Identification log-log analysis Specialized Analysis Well and Reservoir Parameters Checking Procedures

Pressure Transient Analysis - Steps • • • • •

1. Draw diagnostic (log-log) plot 2. Identify flow regimes 3. Draw specialized plots(if any) 4. Calculate well and reservoir parameters 5. Choose basic well and reservoir model and re-compute parameters using Type Curves • 6. Check for consistency by comparing results

Model Recognition

Step 5: Re-compute parameters using Type Curves

• Type Curves – – – –

Generalized solutions to diffusivity equation Graphical form Depends on reservoir models and flow regimes Most common • Homogeneous reservoir with wellbore storage

– Type Curve Matching • Diagnostic Plot is matched with a Type Curve • Calculate parameters from given equations

Type Curve for a Homogeneous Reservoir • Wellbore Storage Effect