seismic pore pressure prediction - cgg · eage 2004 - seismic pore pressure prediction . 1. seismic...

EAGE 2004 - Seismic Pore Pressure Prediction 1

Seismic Pore Pressure PredictionEAGE, Paris 2004

Jean-Luc Formento

2EAGE 2004 - Seismic Pore Pressure Prediction

Seismic Pore Pressure Prediction

Seismic solution for predicting drilling hazards


Outline

Pore Pressure Concepts

Seismic Processing – HDPIC technology

Filtering and Calibration of Velocity Cube

Pore Pressure Estimation



- =

Total Vertical StressOver Burden Effective stressPore Pressure

Lithostatic Load+

Fluid ColumnMatrixPore Fluids

Stress equation σtotal – PP = σ’effective

Different methods can be used to relate σ’effective to velocity

Terzaghi


Over pressure Mechanisms

Under compaction+

Secondary Mechanism

Under- compaction

Compaction Overburden Gradient

Top Overpressure

PP

Effectivestress

Fluid expansion

8.3 -9 ppg

10-14 ppg

> 15 ppg

Bowers GL, 2001

Pore Pressure Gradient

Why Predict Pore Pressure?

OBG = overburden gradient

FG = fracture gradient

PPG = pore pressure gradient

MD = mud weight gradient real

MD = mud weight gradient ideal

Well stability risks,‘blow out’

Losing mud and fracture risk

DamagingReservoir risks

Key requirement for safe well planning in over-pressured formations

depth

pressure


PP = OBG - (OBG – GHyd ) * (Vintobs /Vintnor)x

PP = Pore Pressure gradient(ppg or psi/ft)OBG = Overburden Gradient (ppg or psi/ft)ρ(z) = Density from Gardner relationGhyd = Hydrostatic gradientVintnor = Normal Compaction Trend for interval velocity (m/s)Vintobs = Interval velocity observed (m/s)X = Exponent

Eaton’s Method

How to relate pore pressure to seismic velocity?

Vintnor

Vint

Dep

th

Vintobs

Effective stress gradient

( )bmudlineVVa

1

int1 −=σ

PPpressure = OB vertical stress - Effective stress σBower’s Method

1500170019002100230025002700290031003300

0.5 0.7 0.9 1.1 1.3

(g)

V = 2575* (Esfuerzo effect)1.4323Effective Stress

Bowers assumes an Effective stress σ - velocity relationship

∫z

dzzz 0

).(1 ρ

Where Vint = Interval velocity observed (m/s)Vmudline = cste (m/s)


The General Workflow

Calibration Pore Pressure Gradient with existing wells

Estimation and Calibration of 3D Velocity cube

Apply HDPIC high resolution automatic velocity picking

Perform Geostatistical Filtering (GSFIL)

Calibrate seismic interval velocitiesto well rock velocities

Convert Vint cube from time to depth

Analyze drilling history of wells

Analyze compaction regime and estimate trend

Calculate Overburden Gradient OBG, Pore Pressure Gradient PPG and

Fracture Gradient FG from Drilling data

Estimate Effective Stress and Correlation with Vint

Transform Vint cube into Effective Stress Cube

Compute OBG cube, then PPG cube and FG cube

Extract OBG, PPG and FG profiles at new drilling locations


Outline


Seismic processing – HDPIC technology





HDPIC = High-Density Anisotropic Velocity AnalysisHigh density accurate Geological Velocity (RMS) fieldsTwo parameter picking (V and η) Picking independent from muteConfidence (semblance) associated to each picked point

internal QC and editing of the picked values are performed

Vnmo = VRMS η


HDPIC overview

Near offset data(offset/depth≈0.5)

Full offset data

Standard Velocity Analysis

Bispectral Velocity Analysis

Hyperbolic Moveoutone parameter: V

Tim

e

Velocity

Non Hyperbolic Moveouttwo parameters picked

Tim

e

dtnτ0

Vrms < Vstack

(curvature at 0°)

Vrms = f(time, dtn τ0)η = f(time, τ0)

(offset/depth<2.5)

StackingVelocity

Stacking(dtn τ0) pair

Geological Velocity(RMS)

direct estimation

Vrms = 95%Vstack


Outline






Geostatistical Filtering of Velocity CubeRaw interval velocity section

Final interval velocity section

The 3D geostatistical filtering:

removes the residual noise from the automatic velocity picking

prevents anomalous interval velocity values

preserves main geological features

Filtered RMS velocity volume is converted into interval velocity volume, which is the key parameter in the pore pressure workflow


Calibration of the Velocity Cube

WorkflowMatch seismic velocity to sonic logs, check shots and geological tops when available.

• Calculation of seismic interval velocity (VIhz ) in each macro layer

• Calculation of well interval velocity from checkshots

• Estimation of VIhz correction coefficients for each interval to match the

wells

• Correction of the velocity cube by macro interval

• Calibrated velocity cube used for:

– Time-depth conversion

– Density estimation (Gardner)

– Vintobs (Eaton, Bowers,..)


Interval velocitybefore

calibration

Calibrated interval velocity

Calibration

Transformation into average velocity

Time/depth conversion

Average velocity

time

depth

Calibration of the Velocity Cube


Outline






Methodology• Used to derive the Overburden gradient ( key parameter of Terzaghi equation)

• Different techniques can be used according to well data available

Estimation of density using compaction law with depth

Estimation of density from Vint with modified Gardner equation at each well location and 3D kriging by macro intervals: Rho =Rho(0) + A*(Vint-V0)B => density trend cube

Density cubes obtained using different geostatistical algorithms (wells as hard data and density trend as soft data)

Estimation of the Density Cube

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

1 1.5 2 2.5 3

Density (g/cc)

Dep

th (m

)

Density from Gardner

Density from log

Density cube

OBG cube

∫z

dzzz 0

).(1 ρ


Pore Pressure Cube Generation

Pore Pressure analysis at each well location using PP indicator, geological and drilling reportsDefine overpressure mechanismDefine relationship between interval velocity observed and effective stress (Eaton, Bowers,…)

Pore Pressure profile from Vint

Overburden gradient OBG

Vintnor Definition:Low caseBest caseHigh Case

( )bmudlineVVa

1

int1 −=σBowers: PP = OBG - (OBG – GHyd ) * (Vintobs /Vintnor)xEaton:

Vintobs

Pore Pressure indicator

PP = OBG - Effective stress


Pore Pressure Cube Generation

OBG - Effective stress = Pore Pressure

OBG

Effective stress

Pore Pressure


Predictions at future well locations

From the 3D cube, OBG, PPG and FG can be extracted at any new location. Over-pressured zones are interpreted in their structural and seismic stratigraphy framework.

Top Over-pressured zone

PPG cube

Seismic Amplitude cube Extraction of Pore pressure cubes around

new well locations


PP low case (ppg)

Uncertainty assessment: sensitivity cubes

PP high case (ppg)

PP best case (ppg)


The CGG workflow

Careful well calibration of key geopressure parameters by geomechanical experts

Improvement in pore pressure prediction resolution using CGG’s High density anisotropic velocity

analysis

Advanced geostatistical techniques to derive rock properties and calibrate seismic and well

velocities

3D GOCAD geo-modelling technology for 3D estimations and predictions at new well location

Provides:

Detection of problematic drilling zones

Pre-drill pore pressure profiles along planned well trajectories

Multiple scenarios with Pore Pressure confidence margins

seismic pore pressure prediction - cgg · eage 2004 - seismic pore pressure prediction . 1. seismic...

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