SPE-CSGM_Workshop_Mundle_May2015

© 2015 Chemistry Matters Inc.

Making chemistry data meaningful

Characterizing the source zones for surface casing vent leaks

Scott Mundle* and Court Sandau

Presented at: SPE/CSGM Gas Migration Challenges – Identification and Treatment Workshop

May 13-14, 2015

SPE Gas Migration Workshop May 13-14, 2015


o  Geochemical characterization of sources to identify ‘fugitive’ gas/fluid migration in the oil/gas sector

Geochemical ‘fingerprinting

2 SPE Gas Migration Workshop May 13-14, 2015


o  Surface casing used to protect shallow (potable) aquifers

o  Gas (and fluids) in SCVs migrate from deeper zones

o  Irregular and low flow rates (can be difficult to sample/monitor)

o  How can you determine the source(s) of the leaks?

Surface casing vent flow

SPE Gas Migration Workshop May 13-14, 2015 3


Geochemical characterizations & forensics

o  Gases/fluids for different zones can be ‘fingerprinted’

o  But why do you keep getting burned with this approach?



The geochemistry is never wrong, but the interpretation is often incorrect

o  Common problems: o Sample degraded prior to

analysis o Either the wrong ‘end-

members’, or no ‘end-members are used to identify sources

o Numbers are reported that are at or below the detection limits of the instrument



Don’t make assumptions based on regional geochemistry

o  Where do the ‘fingerprints’ originate?

o  Isotope “databases” often use regional values rather than local values that can mislead interpretations



Need high quality SCV samples

o  Can separate and collect gas and liquid o  Gas can be purged to remove “breathing”

gas from well o  Need low atmospheric content in gas

samples for compositional and isotopic characterization



Sampling can affect water chemistry

0.0001$

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Cobalt$

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Group 2 metals and f-block metals lower in bleed barrels - suspended solids settle out in bleed barrel (low energy environment)



How can you get it right every time?

Mudgas Geochemical Profile Characterized stratigraphy

Prod./Source Zones: 1.) Fluids 2.) Gases Characterized end-members

Shallow Fluids 1.) Surface water 2.) Groundwater Characterized inputs

SCVs: What are the Source(s)? 1.) Fluid 2.) Gas

GEOCHEMISTRY:



Complex case study: Fluid and gas flows

•  Collected four samples from each location: –  SCV gas sample –  SCV liquid sample –  Production casing gas sample –  Production casing liquid sample

•  Used local groundwater and deep aquifer wells in region for comparison

•  Used perforated wells with DST samples to acquire ‘end-members’ for other potential source zones



No insight from water isotopes

o  Water isotopes implicated groundwater sources that conflicted with other indicators (chlorides, TDS, etc.)



A clearer picture with advanced analyses

o  Different sources had characteristic ranges of concentrations and isotope values. This provided first line of evidence to indicate that different source zones can be identified



Identifying source zones

o  Fluids provide a line of evidence for potential sources, but they do not provide a refined interpretation



Combined fluid and gas characterization

*Exact values property of client, shift in isotope values indicated

δ13C Values GASES C1 C2 C3

SCV A - 1.9 B - 2.3 C Casing A* B* C*

o  Gases provide a second line of evidence for potential sources and help confirm a production zone leak



Complete geochemistry = complete picture


SCV A - 1.9 B - 2.3 C Casing A* B* C*

*Exact values property of client, shift in isotope values indicated


SCV A - 13.1 B - 8.1 bdl Casing A* B* C* o  Gases provide a second line of

evidence to confirm a cretaceous zone leak




SCV A - 13.1 B - 8.1 bdl Casing A* B* C*

Resolving the depth of source zone

o  Mudgas log can refine cretaceous leak to within 200-300m range



Simpler case study: High gas flows •  SCV had high gas flow •  No fluids •  Commercial isotope

interpretations were consistently picking the wrong depth/zone



Bulk gas compositions gave little insight

o  Data equally implicates different zones and/or mixtures



Carbon isotopes suggest different zones

o  Data could implicate two different zones



Advanced models implicate same zone

o  All leaks actually from similar zone!



Did it work?

•  Similar zone implicated for all SCV in area

•  Gas zone overlying production zone implicated

•  Cement squeezes successful at predicted depth for all interventions undertaken in 2014



Where can it all go wrong?

•  Similar altered isotope ‘fingerprints can be expected from different sources



Best practices for SCV ‘geoforensics’ •  Collect high quality samples •  Collect samples for potential source(s) and from the

production casing (if possible) •  Start with basic chemistry but may need more conservative

analytes and advanced interpretations •  Ask the right questions and get it right !

–  What source zone characterizations were used in the interpretation? –  Where were these source zones characterized (geographically)? –  What are the reliable detection limits for your instrumentation?

High quality samples, isotope analysis and accurate interpretation of microbial effects provide more

conclusive source determination.



Thank You

[email protected] www.chemistry-matters.com

403.669.8569 24

SPE-CSGM_Workshop_Mundle_May2015

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