North Sea Chalk - A Challenging Reservoir

Mike Millar

LPS Seminar - Challenging Reservoirs

Thursday 21st September 2017

North Sea Chalk - A Challenging Reservoir

Talk Outline

• The reservoir and the issues - why is Chalk a challenging reservoir ?

• Upper Cretaceous to Lower Palaeocene Carbonate Micrite

• high porosity but low permeability

• fractures and re-sedimentation can be important influence on reservoir quality

• potential for tilted Free water level (FWL)

• potential for compaction and subsidence

• What was done

• Collected enough data to understand as much of reservoir as possible

• in-place volumes and potential reserves, we should be able to make money from this field

• platform off-set from structure, avoid subsidence issues

• well placement and completion design, built-in flexibility

• Carry on collecting data during field life to aid reservoir management.

• Results and lessons

• Cased-hole logs show sweep efficiency and

• identified by-passed oil and opportunity for work-over

• Cumulative production exceeds sanction case estimates

• Summary

3

Why is North Sea Chalk a Challenging Reservoir?

• Upper Cretaceous to Lower Paleocene Carbonate

• Micrite - made from coccolithophores

• Very different grain size to sandstone reservoirs, which has a big impact on Chalk reservoir quality.

Chalk

Sandstone

Chalk

4

Why is Chalk a Challenging Reservoir?

• Generally good to high porosity, but low matrix permeability

• Generally very ‘clean’ limestone, but can have marls, flints and 'hardgrounds'

0.5

m2

m

Marl

Flint layers

Why is Chalk a Challenging Reservoir?

• Chalk Saturation Profile

• Chalk has very fine capillaries

• leads to long entry-pressure height,

• so FWL does not equal OWC

• and a long transition zone, generally much longer than Sandstones with similar porosity

• can lead to uncertainty in populating saturations in Geological and Dynamic models

Saturation vs. height above Free water level

6

Why is Chalk a Challenging Reservoir?

• Resedimentation, may enhance porosity

• Hardgrounds, reduce porosity

• Flints can make it difficult to drill

• Structure and fractures may enhance permeabilityFlint layers

7

Why is Chalk a Challenging Reservoir?

• Possibility of tilted Free water levels (FWL)

• Mapping Overpressure indicates pressure gradient in the aquifer which can cause tilted

FWL in Chalk fields

• Uncertainty for in-place volumes and planning well locations

• Dennis et al 2005; Tilted Oil-water Contacts, in Dore and Vining, Proc of 6th Petroleum Geology

Conference.

• Water Saturation and analysis of a tilted free water level in the Judy Joanne chalk field. Goldsmith and

Harris, LPS December 2001 Seminar

Joanne Chalk

8

Why is Chalk a Challenging Reservoir?

• Seismically Obscured Areas

• Shallow gas causes attenuation of seismic response at the Chalk horizons

• Common in North Sea Chalk Oil Fields

• Uncertainty for building the Geological model and thus in-place volumes and planning

well locations

• Compaction and subsidence in Chalk once production starts

• high porosity and relatively weak rock

• water injection might maintain pressure, but it makes Chalk weaker

• Platforms had to be jacked-up at the Ekofisk complex

• Compaction of North-Sea Chalk by Pore-Failure and Pressure Solution in a Producing

Reservoir. Daniel Keszthelyi et al. Front. Phys., 16 February 2016

9

What was done ?

• Collect enough data to understand the resource;

• Plan data acquisition in a team with all the sub-surface disciplines and facilities

engineers

• Mud log and gas log, especially mud losses

• Electric logs, including image logs and FPT

• Core (although recovery might not be very good)

• Well tests

• top reservoir pick to reduce mapping uncertainty

• full petrophysical evaluation integrating all log and core data to reduce

uncertainty on in-place volumes

• well test to reduce uncertainty on production potential and influence of fractures

• Understanding the Fractures, key to productivity

• Fractures on Image Log, Core (UV light) and Thin-section

• Varying scale of the fractures

10

What was done ?

• Property Modelling Porosity

• Porosity modeled by reservoir unit

• For each reservoir unit data analysis performed on petrophysical logs to determine

porosity min, max, mean and standard deviation.

• Using the data analysis as input, porosity was modeled stochastically within the

Geological model using truncated sequential Gaussian simulation on a unit by unit

basis.

• Compare the log data, the upscaled logs and the final stochastic model as a quality

control check on the final 3D volume

• Property Modelling Water Saturation

• Log derived Sw was used to build a saturation height function which is then used in

the Geological model.

• Core has limited influence as we only get recovery from generally the poorer quality

rock, but the core suggests high entry pressure height

11

What was done ?

• Property Modelling Permeability

• Permeability modeled by reservoir unit

• For each reservoir unit, maximum, minimum and mean permeabilities were estimated

from all the available data (core, well test, NMR, analog)

• Permeability was then stochastically modeled in Geological model using truncated

sequential Gaussian simulation conditioned on porosity as a trend on a unit by unit

basis.

• Field development

• Platform offset from main structure

• Well inventory - 2 development pilot wells, 2 horizontal production wells, and 2 high

angle water injectors

• LWD and Wireline (Pipe conveyed) including Acoustic image logs

• J meter used to monitor losses and help to identify open fractures

12

What was done ?

• Field development

• Completions

• perforated 7" liner, with isolation packers between perforations

• acid wash across perforations to enhanced flow

• sliding screens to give flexibility in production/injection

• downhole gauges installed

• gas lift plumbing installed

• saturation monitoring logging

• cased-hole pulsed neutron tools

• production logging tools

13

Results and lessons

• Cased-hole Pulsed Neutron Saturation Monitoring logs

• Horizontal Oil Producer, fairly even sweep

• Production Logging

• Horizontal Oil Producer, fairly even sweep

• Cased-hole Pulsed Neutron Saturation Monitoring logs

• Originally a water-injector

• PNC highlighted possibility of by-passed oil

• Well successfully turned around into oil producer

• Cumulative production exceeds initial estimates