devex-conference.orgdevex-conference.org/pdf/Presentations_2010/Day2B/DEVEX... · 2012-04-13devex-conference.org

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Ebrahim Heydari Senior Petrophysicist

New insight on the presence of residual oil below contact in the Middle East reservoirs

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u  Introduction and clarification of concepts u  Objective of the study u  Geology u  Available data u  FWL/OWC determination u  Why residual Oil? u  Residual oil area u  Conclusions u  Recommendations

Agenda

Residual oil

area

Why residual oil

Below contact

Applied data Introduction

Conclusion

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u  Although presence of residual oil below contact is very important, they are not commonly considered for reservoir modeling

u  There are a number of oil and gas accumulations worldwide that cannot be described using conventional drainage capillary pressure curves. In these situations and when there are residual hydrocarbons below contact, the hydrocarbon saturations are better modeled using imbibitions capillary pressure curves

u  Presence of residual oil below contact is common in the Middle East reservoirs

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Introduction

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u  To explain why there is residual oil below contact in some of the Middle East reservoirs

u  To inform both geoscientist and reservoir engineers of the important role of residual oil in reservoir characterization (i.e. HIP estimation and production scenarios particularly pressure maintenance, water injection and EOR projects)

Introduction Objective of the study

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FWL and OWC

u  Free water level (FWL): The depth at which the water saturation approaches 100% and capillary pressure is zero

u  Oil water contact (OWC): The depth at which the water saturation approaches 100% and capillary pressure isn't zero

PT = Pore throat P – Pore space

.

Clarification

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u  Drainage: Fluid flow process in which the saturation of the non wetting phase increases

u  Imbibition: Fluid flow process in which the saturation of the wetting phase increases

Drainage

Drainage and Imbibition

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Clarification

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u  Reservoir bitumens are hydrocarbons in reservoir rocks which are immobile under reservoir pressure and temperature (Rogers et all., 1974; Lomando, 1992)

Reservoir Bitumen

Note

Normally Reservoir Bitumens are located very

close to the OWC

Reservoir Bitumen

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Clarification

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Location and study area

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u  The subject was investigated by integration of geological, reservoir and petrophysical data of 3 fields located in the West, East and Mid of the Persian Gulf

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Petroleum system

u  Cap rock: Laffan Formation u  Reservoir: Mishrif Formation u  Source rock: Khatiyah Formation u  Trap mechanism: Structural u  Generation: 39Ma u  Migration: 26Ma

Mishrif

Laffan

Khatiyah

Geology

9/38

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Depositional setting

Field

u  The field is located in the eastern part of an intra-shelf basin developed during the Middle Cretaceous within the continental shelf setting

Geology

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u  6 vertical wells u  17 horizontal oil producer u  13 horizontal water injector

Vertical Well

Oil Producer

Water Injector

Wells

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• Tectonics • Structural Geology • Geochemistry • Stratigraphy • Depositional Environment • Sequence Stratigraphy

• DST • RFT/MDT • Water Flooding • PVT

• Core Description • CCAL • SCAL • Thin section

• C1 • C2 • C3

• Raw Data • CPI results

Data sources

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Available data in Eastern Field

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Data sources

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u  3 wells with mud logs, wireline logs, RFT, DST and cores

u  1 well with mud logs, wireline logs, DST and cores

u  2 wells with mud logs, wireline logs and RFT

u  3 wells with mud logs and wireline logs u  7 wells with mud logs and LWD recorded logs

u  All the remaining wells have MWD directional and GR logs, and mud logs

Core

DST

Wireline Log set including RFT

LWD data

Available data

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Data sources

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u  Free water level (FWL) was determined at 2900 mss

u  Residual oil saturation below FWL is around 50-60%

Well-5 D

ST

Fluid O

il and Formation w

ater

FWL@2900 mss

OWC determination

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u  OWC was picked at 2868 mss

Well-39 Fluid

OWC determination

u  Residual oil saturation below OWC is around 40%

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OWC@2868 mss

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u  ODT is shown at 2896 mss (base of Mishrif)

Well-4

SIE-5

Oil

Oil

DS

T

Fluid

OWC determination

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u  OWC was recorded at 2886 mss u  water saturation below OWC is about

100% (Residual oil saturation below WOC is negligible)

Well-39 Fluid

OWC determination

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OWC@2886 mss

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u  OWC has been encountered at 2930 mss u  Water saturation below OWC is about 100% (Residual oil saturation below OWC is negligible)

Well-37 Fluid

OWC determination

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OWC@2930 mss

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Well-38 Fluid

OWC determination

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u  OWC has been encountered at 2930 mss u  water saturation below OWC is about 100% (Residual oil saturation below WOC is negligible)

OWC@2930 mss

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Well ODT OWC

Well 1 2781.0Well 2 2857.0Well 3 2943.0Well 4 2893.0Well 5 2900Well 6 2865.7Well 7 2824.0Well 8 2843.0Well 9 2837.5Well 10 2855.0Well 11 2818.0Well 12 2853.0Well 13 2865.8Well 14 2914.9Well 15 2890.4Well 16 2860.2Well 17 2898 ?Well 18 2884Well 19 2868.7Well 20 2806.0Well 21 2868.2Well 22 2851.1Well 23Well 24 2838.9Well 25 2832.0Well 26Well 27Well 28 2877.0Well 29 2866Well 30 2863.9Well 31 2901.5Well 32 2870.7Well 33 2876.5Well 34 2886.5Well 35 2870.1Well 36 2871.9Well 37 2930Well 38 2930Well 39 2921

Well-37 (OWC 2930 mss)

Well-38 (OWC 2930 mss)

Well-39 (WOC 2921 mss)

Well-4 (ODT 2899 mss)

Well-5 (FWL 2900 mss)

Well-39 (OWC 2866 mss)

Depth of OWC

Depth of OWC increases toward the south of the

field

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OWC determination Important finding

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Fluid Fluid

Northern Flanks Presence of residual oil below OWC is the main

characteristic of the northern wells

Southern Flanks Absence of residual oil below OWC is the main

characteristic of the southern wells

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Important finding

OWC determination

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Correlation

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Residual oil below contact

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Western Field

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OWC@50% Sw

u  Obtained results for Eastern field were

confirmed by other fields

Residual oil below contact

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Middle Field

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Residual oil below contact

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Well-28 and Well-3 Well-4 and Well-5 Well-4 and Well-28

PVT data analysis

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Why residual oil below contact? Structural history and sedimentation

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DS

T Fluid

Oil and Form

ation water

u  Northwardly tilting……during last 11 my

u  Oil migration……26 Ma

u  Oil generation……39 Ma

u  Structure generation…… 90 Ma Oil Generation

Oil Migration

Str. Generation 28

5

2

Well-5

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Why residual oil below contact?

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N

Paleo Level

Pc Pc

Sw

Oil displaced by waters

FWL

S

Water displaced by oil

Sw

FWL

Free Water Level

Sw

Pc Capillary Pressure

Drainage

N S

Well-37

Well-39

u  Early accumulated oil was swept by water rise after northwardly tilting of the structure and residual oil has been trapped below OWC in the northern parts of the field

Structure Tilting

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Explanation

Why residual oil below contact

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u  WUT was recorded at 2912.5mss (Top of Mishrif) u  A paleo OWC is shown at 2943mss u  30 m of residual oil (Sor=40%) is observed between the paleo OWC and top of Mishrif

Location of paleo contact

Formation w

ater Form

ation Wate, Trace of oilr

DS

T

Fluid

Immovable oil

OWC determination

29/38

Paleo OWC

Paleo OWC

FWL

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u  This theory is also confirmed by the presence of reservoir bitumen 35 meters above the current contact in well-28 which is located in southern parts of the field.

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Why residual oil below contact Evidences: Bitumens in Well-28

N

Paleo Level Oil displaced by waters

FWL

S Reservoir undergoes imbibition

Reservoir undergoes

further drainage

Reservoir Bitumen

Note

Normally Reservoir Bitumens are located very

close to the OWC

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u  Considering Mishan and Aghajari formation thicknesses in different parts of the field, tilting axis was determined close to the well-2

Probably Axis of tilt towards the North-Northeast

Reservoir undergoes imbibition

Reservoir undergoes

further drainage

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Why residual oil below contact Axis of structural tilting

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Stress Direction

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Why residual oil below contact Regional tectonics

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u  Residual oil in the Mishrif locates in the northern parts of the field

Flushed area: Residual oil extension

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u  Presence of residual oil below contact in the northern part of the field implies that imbibition capillary pressure curves , rather than drainage, are representative of water saturation in the northern parts of the field

u  For southern parts of the field drainage capillary pressure curves are best for saturation modelling

u  Imbibition is due to the northwardly tilting of the structure essentially several million years after oil generation and migration to the reservoir

Conclusion

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u  The signs that imbibition may be important in reservoir include:

•  Presence of residual oil below the contact (sometimes acts as a barrier between aquifer and oil column)

•  Dry oil production very close to the contact (consequently higher oil in place and reservoir volumes)

•  Sharper log derived transition zone than the reservoir permeability suggests

Conclusion

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u  Imbibition capillary pressure curves should be used for saturation modelling in the northern parts of these fields

u  Presence of residual oil below contact impacts all disciplines not just Petrophysics and an integrated study is necessary to solve or prevent problems associated with the residual oil and bitumens in the reservoirs

u  Log derived porosity in residual oil sections is not reliable as sometimes they are completely solid but they are interpreted as porosity in log interpretation.

Recommendations

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u  Even CCAL and SCAL data from residual oil sections should be used with caution if they have gathered after sample preparing and washing (i.e. by Toluene)

u  RFT data from future drillings should be examined for permeability barriers through suspect residual oil zones

Recommendations

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Special thanks to AFES, Iranian Offshore Oil Company (IOOC) and Aberdeen University for their support in the development of this study

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