assignment 200

151PTE321/GEOL2101 Engineering Geology

Lecture 9

Dr. Seyed Mehdi Seyed Alizadeh

Heterogeneity

Definition

• Formation with two or more non-communicating sand members.

• Different specific- and relative-permeability characteristics.

• The reservoir heterogeneity is defined as a variation in reservoir properties as a function of a space.

• Oil/Gas reservoirs are complicated geological heterogeneous bodies.

• There is no homogeneous porous media.

• Well log and core analysis reports show that all reservoirs are heterogeneous.

• Permeability heterogeneities cause variations in the fluid movements compared to the equivalent homogeneous system.

• Efficiency management (RF).

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Reservoir Heterogeneity in Sandstone

Heterogeneity May

Result From:

Depositional Features

Diagenetic Features

(Whole Core Photograph, Misoa

Sandstone, Venezuela)

Heterogeneity

Segments Reservoirs

Increases Tortuosity of

Fluid Flow

Reservoir Heterogeneity in Sandstone

Heterogeneity Also May

Result From:

Faults

Fractures

Faults and Fractures may

be Open (Conduits) or

Closed (Barriers) to Fluid

Flow

(Whole Core Photograph, Misoa

Sandstone, Venezuela)

Scales of Geological Reservoir Heterogeneity

F ie

ld W

id e

In te

rw e ll

W e ll -B

o re

(modified from Weber, 1986)

Hand Lens or

Binocular Microscope

Unaided Eye

Petrographic or

Scanning Electron Microscope

Determined

From Well Logs,

Seismic Lines,

Statistical

Modeling,

etc.

10-100's mm

10-100's

mm

1-10's

m

100's

m

10's

m

1-10 km

100's m

Well Well Interwell

Area

Reservoir Sandstone

Scales of Investigation Used in Reservoir Characterization

Gigascopic

Megascopic

Macroscopic

Microscopic

Well Test

Reservoir Model

Grid Cell

Wireline Log

Interval

Core Plug

Geological

Thin Section

Relative Volume

1

10 14

2 x 10 12

3 x 10 7

5 x 10 2

300 m

50 m

300 m

5 m 150 m

2 m

1 m

cm

mm - mm

(modified from Hurst, 1993)

& Seismic

 Primary objective of geological characterization is concerned with predicting the spatial variation of geological variables.

Variable : • is any property of the geological subsurface that exhibits

spatial variability and can be measured in terms of real numerical values.

Spatial Variation: • Typically the subsurface is anisotropic, spatially complex

and sedimentary bodies are internally heterogeneous.

Geological Modeling

Reservoir Characterisation

• Modern reservoir characterisation started around 1980:

• Reason: deficiency of oil recovery techniques (inadequate reservoir description)

• Aim: predict inter-well distributions of relevant properties (φ, K)

• Subsurface (inter-well) heterogeneity cannot be measured:

• Seismic data (large support, low resolution)

• Well data (small support, high resolution)

• Complementary sources of information:

• Geological models

• Statistical models

• Combine data and models  ‘static’ reservoir model

Static reservoir models

• Reservoir geology is the science (art?) of building predictive reservoir models on the basis of geological knowledge (= data, interpretations, models)

• A reservoir model depicts spatial variation of lithology (porosity and permeability): “static” model

• Simulations of multi-phase flow (“dynamic” models) require high-quality “static” reservoir models

• Static reservoir models are improved through analysis of dynamic data: iterative process

Geological Modeling: different tracks

Static Reservoir Model

Reservoir Data

Seismic, borehole and wirelogs

Sedimentary Process Model

Stochastic ModelDeterministic Model

Data-driven modeling Process modeling

Flow Model

Upscaling

14

Geological model

•Elements of the geological model:

1. Bounding surfaces

2. Distributions of physical properties between surfaces

3. Faults

4. OWC, GWC, GOC

5. Conditioned to well data ?

15

Why is geological modeling difficult

• The output of many natural systems exhibits apparent randomness, which is usually caused by extreme sensitivity to initial conditions. Initial conditions and physical laws of such systems cannot be inferred from the output.

• Measurements are a finite sample of the output (all possible realisations of the system).

• Statistical models may be used to describe such measurements in the absence of a physical model.

• Geological modeling software (a worst-case scenario): • Designed by statisticians who know little about geology • Applied by geologists / engineers who know little about

statistics • Many things can and will go wrong !

16

Upscaling issues • In addition to the natural scales of heterogeneity in the

system and the scale of the measurements, there is also the scale of the discrete elements (grid blocks) in a reservoir model.

• Upscaling measurements to grid-block scale is a critical issue in geological modeling and the object of active research

• Common errors in numerical reservoir models: • Discretisation errors • Upscaling errors • Input errors

• Geological modeling aims at minimizing these errorsrrorsnput errors to

improve reservoir-model performance