Pressure Transient Analysis in Drawdown and Buildup lesson (1)

Pressure Transient Analysis
in Drawdown and Buildup

Dual flow Dual shat shat in test


Exploration Well Test Objectives
1. Determine the nature of the formation fluids
2. Measure the well productivity
3. Measure temperature and pressure
4. Obtain samples for lab analysis
Exploration Well Test Objectives
5. Obtain information for reservoir description
 (permeability , heterogeneity)
6. Estimate completion efficiency

Dual Flow - Dual Shut in Test

Initial flow and shutin designed to establish
communication with the reservoir
Initial flow as short as possible
Major flow period long enough to give
sufficient depth of investigation
Dual Flow - Dual Shutin Test
Often 6 - 12 hours is adequate
At least six hours of stable operation to
ensure reasonable estimate of productivity
and good samples
Multirate necessary in gas wells
Major shutin 1 - 2 times the duration of
the flow period
Methods of Gaining Information on Reservoir Characteristics
A. Seismic and associated geological studies
B. Information obtained during the well drilling program
C. Wireline formation testing
 1. Virgin Reservoir (Exploration and Appraisal Wells)
Methods of Gaining Information on Reservoir Characteristics
 2. Produced Reservoir (New development wells)
D. Pressure - Flow testing of wells
 1. Exploration and appraisal wells (DST)
 2. Production or injection wells
E. Analysis of reservoir performance
 - simulator history matching

Principal Objectives of Well Testing

Determine the average permeability of the reservoir
Determine the near wellbore alteration i.e. the skin factor
Measure the reservoir pressure
Attempt to locate the position of boundaries / discontinuities
Types of Pressure Transient Test
Principal Objectives of Well Testing
Pressure Drawdown (Reservoir Limit) Test
Pressure Bu ildup or Fa lloff Test
 - Drill Stem Te st (Downhole valve)
 - Production or Injectio n Well Test
In terference Test
Pulse Test - horizontal or vertical


Transient Well Testing
Buildup Analysis - Horner (Theis) Plot

From Steady-State Radial Flow TheoryNear Wellbore Altered Zone


Hawkins Equation - Open-Hole

From Steady-State Radial Flow Theory

Well Productivity



The well P.I. depends mainly on:
Permeability - Thickness Product
Oil Viscosity
Overall Skin Factor
 Drainage radius
 Wellbore radius
 Formation volume factor
are of secondary importance

Well Productivity Index, Jsss



Determination of Average Pressure


Flow Regimes


Detection of Depletion



Some Well Test Models
Homogeneous Finite
Composite Infinite
Reservoir
No Flow
Boundary
Composite Infinite
Reservoir
No flow boundary
Single Linear Fault






Model Reservoir


Assumptions

Well completed over entire thickness of formation
Homogeneous and isotropic porous medium
Uniform formation thickness
Bounded above and below by impermeable barrier
s
Porosity and permeability constant
Assumptions
Lead to Radial 1-D Flow
Formation contains a single phase liquid with constant viscosity
and small and constant compressibility
Leads to Diffusivity Equation

Nomenclature

k Permeability of porous medium
 Porosity of porous medium
 Fluid viscosity Fluid density
c Fluid compressibility h Formation thickness
φ
µ
ρ
p Pressure t Time
Nomenclature
r Wellbore radius
q Oil flow-rate (stock tank conditions)
B Formation volume factor r External radius
r Radial coordinate p Initial pressure
 Hydraulic diffusivity

To be continued 


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