Panteha Ghahri

Guillaume Berthereau

Estimated Fluid Contact Using Material Balance Technique &

Volumetric Calculation Improves Reservoir Management Plan

1

• Objective

• Gryphon Reservoir-Overview

• The proposed approach(Material Balance

Modelling & Volumetric Calculation)

• Comparison the calculated fluid contacts with

4D seismic and simulation model

2

Outline

• Estimate WOC & GOC for Gryphon area using

an analytical approach

• Direct measurement of the fluid contacts is costly and sometimes can be very challenging.

• It is also useful to validate the simulation model

3

Objective

•Production started from 1993

•Original oil column 190 ft

•Heavy Oil (21 deg API) Viscous oil (6 cP)

•Long tail end production at high watercut

•Long term plan to produce across a floating facility (first permanently moored FPSO)

•Developed exclusively with horizontal production wells

•Aquifer well to provide compatible injection water/large connected aquifer

•Water & gas much more mobile than the oil in the reservoir

4

Gryphon reservoir overview

Gas+ Oil Production Gas Injection

Aquifer Influx + Water Injection

Very Limited/Aquifer

Index

WOC

GOC

WOC

GOC

The Original Oil Rim Thickness

Gas+ Oil Production

(Gas Movement)

Moved GOC

Moved WOC Moved GOC

Moved WOC

5

Material balance modelling Gryphon and nearby field

6

Material balance & volumetric calculation

Gas, 1-Swi

Oil, 1-Swi

Water

Water, Sor

Oil, 1-Swi

Oil, 1-Swi-Sgr

Gas, 1-Swi

Initial Current

7

Material balance & volumetric calculation

•Material Balance Calculation • Oil Produced: 121 MMbbl

• Oil left inside the water zone:14% of STOIIP

• Oil moved up to Gas zone:10% of STOIIP

• Remaining Oil: 47% of STOIIP

8

WOC & GOC vs. Time

5200

5300

5400

5500

5600

5700

5000 5500 6000 6500 7000 7500 8000 8500 9000

Co

mp

leti

on

Inte

rva

l o

f w

ells

(T

VD

ss

)

WOC-1993

WOC-2011

Data from last test:GOR =402

WOR=89%PI=178

GOC-2011GOC-1993

9

Well position vs. WOC & GOC

W1

W2

10

Completion interval vs. WOC & GOC and WOR%

11

Completion interval vs. WOC & GOC and GOR

• The strategy is to keep the existing wells placed within the

oil rim by controlling the movements of the GOC & WOC.

• The WOC movements are controlled by re-injecting produced water

into the aquifer or the reservoir.

• The GOC movements are controlled by re-injecting the gas into the

gas cap and operating the wells at a GOR limit.

• Well performance data, 4D seismic, simulation modelling and

analytical calculation are used to monitor contact movements. This

understanding is applied to optimize new well placements.

12

Reservoir management strategies

4D modelling

Shale

Shale

Gas bearing Sand

Oil bearing Sand

Brine bearing Sand

(1990) Acoustic impedance (2011)

No signal in shale

No signal in shale

2011-1990

syn

theti

c

Co

lou

red

in

versio

n

• 4D observed theoretically only due to fluid movement (no significant pressure change)

• Fluid movements over Gryphon: • Gas into oil (due to production): softening • Water into oil (due to production): hardening • Oil into gas (water injection): hardening

Aco

usti

c im

ped

an

ce

Hard

enin

g

Soft

enin

g

4D (2011-1990): seismic Sum +ve amplitude

2011-1990 (CI Full stack) oGOC / oOWC

Hardening

Polygon used for initial STOIIP calculation

• Polygon used to calculate

STOIIP, input to material

balance

• 4D showing water movement

into the oil leg due to:

• production

• water injection

4D (2011-1990): seismic / simulation model Sum +ve amplitude

2011-1990 (CI Full stack) oGOC / oOWC

Column height (water in oil leg)

2011-1990

Hardening

Polygon used for initial STOIIP calculation

• Decent qualitative match

between simulation model and

4D seismic

4D and fluid sections

Hard

enin

g

Soft

enin

g

oGOC

oOWC

Moved OWC

HC

• Generally reasonable quantitative match between moved OWC from the simulation model

and 4D seismic

Moved contact comparison: material balance and simulation model

Original Simulation model* Material balance

GOC 5541 5514 (Sgas90%) , 5518 (Sgas70%) 5519

OWC 5731 5638 - 5545 5631

* Average of the moved OWC given by the simulation model

• Very similar moved GOC from simulation model and material balance (error within vertical resolution)

• The moved OWC between simulation model

and material balance are in alignment.

Moved OWC (Sim. model)

Multi-scale injectites

5619.00’ 5622.00’ 5625.00’ 5628.00’

5628.00’ 5631.00’ 5622.00’ 5625.00’

DYKE

DYKE INJECTION BRECCIA

DYKE

Feeder dyke system

DYKE

Stepped discordant

base

July 2, 2013

• Remobilisation of the turbidites sandstone in early Frigg times

• Creation of multi scale (from core to seismic scale) injection wings

• Below seismic resolution injectites may account for a significant volume, more than expected

T Sele

T Balder

Adapted from Hurst and Cartwright

• Water and gas oil contacts has been calculated using material balance and

volumetric calculation

• Contacts from material balance were compared with those observed from the

simulation model:

• Moved OWC from the simulation reasonably matches 4D

• Moved OWC from material balance and simulation model are in alignment

• The difference is possibly associated to missing sand volume unresolved by

seismic (below seismic resolution injectites)

• Material balance can be used, in addition to well performance evaluation, 4D

seismic and simulation modelling, to monitor the contact movements

effectively. Results can be used to control well operations and placement of

new wells.

19

Conclusion

• The authors would like to thank the partner

Sojitz for its approval to present this work.

•We also would like to thanks our colleges who

support us during this study, Steve Milner,

Hope Okhuoya, Miguel Orta, David Kirby,

Duncan Chedburn, Doug Smith, Tim Heijen

and Fabrizio Conti.

20

Acknowledgment