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