Applying Vacuum Saturation to Study The Pore Structure of Tight Shales

Troy J. Barber and Q.H. Hu

2014 GSA Annual MeetingVancouver, British ColumbiaOctober 21, 2014

Outline

I. Tight gas production decline and fracture-

matrix interaction

II. Pore topology and macro scale fluid

migration

III. Vacuum saturation: how we use it

IV. Mapping edge-accessible pores

V. Preliminary Results

VI. Summary and Looking forward

Recovery factor of 5‒10% for tight oil (Hoffman, 2012; SPE 154329)

Problem: Steep production decline in tight shale gas

Chong et al., 2010, SPE-133874

Refrac rebound ???Slow matrix

diffusion

Pore structure: Geometry and Topology

LA ICP-MS

Multiple approaches to pore characterization

Nitrogen sorption

Imbibition Tests

Traced vacuum saturation

• Fluid (API brine; n-decane) and tracer imbibition tests

• Edge-accessible porosity after traced vacuum saturation

• Liquid and gas diffusion tests

• Mercury intrusion porosimetry and hysteresis

• N2 adsorption isotherm and hysteresis

• Ar ion milling, FE-SEM, and TEM

• 2-D/3-D tracer mapping using Laser Ablation-ICP-MS

• Small-Angle Neutron Scattering (LANL; ORNL; NIST)

Today’s talk will focus on these two methods

T

What is traced vacuum saturation?

FLUID (water, brine, n-decane)

AIR

AIRAIR

CO2

TO VACUUM

CO2P

T

T T

T T

T

T

T

T

T

T

T

T

T

T

T

CO2 highly soluble in water

Several cycles over 6-12hr removes air

After ~1 hr, vacuum in connected space =(0.01/743)Torr = 99.999%

Evacuation Duration:12-24 hr

API Brine - water wettingReO4- (nonsorbing)Cs+, Co2+, Ce+,Eu3+

n-decane – oil wettingRe (nonsorbing)I-

Different fluids to see effect of wettability. Sorbing and nonsorbing tracers.

After saturation, apply CO2 pressure to liquid surface12-24 hr. T

TT

T

T

T

• Large volumes• Prone to leaks,

99.91% vacuum• No flushing or

positive pressure capability

Our previous apparatus

Our current apparatus

Smaller chamber = less waste

99.99% vacuum

Mechanically sealed, allowing CO2 flushing/positive pressure

Sample holder for easy organization

Solid

Liquid

Granite

Laser Ablation-ICP-MS for micro scale profiling

100 µm hole diameter

Rb (intrinsic)

ReO4- (non-sorbing)

224 µm

2 µm

12 µm

54 µm

100 µm spot size

2 mm

Saturating Surface

-45

-35

-25

-15

-5

0 20 40 60 80 100 120 140 160 180 200

Verti

cal h

eight

(µm)

Horizontal distance (µm)

1 pulse5 pulses10 pulses25 pulses50 pulses

3D Tracer Distribution

2D Interior cross section

9.5 mm

Saturation Saturation

Epoxied Sides

100um spot size500um spacing

~ 2 order of magnitude drop within 500um from sample edge.

Summary and Looking Forward• Steep 1st year decline and low

overall hydrocarbon production observed in hydraulically-fractured shales.

• Investigating pore structure in natural rock requires several complimentary approaches.

• Traced vacuum saturation paired with LA-ICP-MS is effective at characterizing the edge-accessible pores.

• Results indicate low pore connectivity in shales, which reduces gas diffusion from matrix to stimulated fractured network – driving steep production decline

What’s next?

• Elevated pressure saturation• Comparing samples of different

mineralogy, maturation, TOC, bedding orientation, wettability, etc.

• SANS/USANSo Inaccessible poreso In-situ P-T conditionso Pore structure and flow

dynamics

AcknowledgementsQ.H. “Max” HuGSA On to the Future Program

ThankYou