natural fractures, “pore” pressure & variable formation...

Natural Fractures, “Pore” Pressure & Variable Formation Stresses: Linkages to Effective Gas

& Oil Recovery Improvements?

George E. King, P.E., Apache Corporation

ARMA Unconventionals Workshop

June 25, 2016

Westin Galleria, Houston

Importance of Geomechanics in Unconventionals

Rocks, Stresses & Production – Intimately Related?

•Discussion Starters• Where are the recoverable oil and gas reserves?• Stress effects on stability of flow paths in low perm rock.• Generating and maintaining access to flow paths.

Is shale porosity important?• Three Porosities:

• Interstitial (between the grains) • Natural fractures, fissures & micro-cracks• Solution/modification porosity

–organic material shrinkage during maturation.

• Interconnected porosity?• Size of pore throats limits the type of flow.• Saturation of liquids generates capillary blocking problems.

Learning – hydrocarbon generation is from some organic carbons –often deposited in TOC-rich layers in the shale.Low permeability may keep much of the hydrocarbon in this layer.

Crack (Fracture) Development in Shales Often Following the Natural Fracture System or Other Weak Points in the Rock

The development of network fracturing, i.e., opening small cracks in the very low permeability shales, enables gas to flow.

Propping these cracks with conventional methods, e.g. sand proppants, is virtually impossible, but the development of even the smallest crack delivers 100’s of times more flow capacity than the interstitial porosity offered by the matrix of the shale.

The enormous number of these cracks, plus the potential “propping” action, delivered by trapping of water (capillary blocking effects) as the hydraulically opened fractures close, delivers flow capacity where none existed before. The water in the wider parts of these cracks will be displaced by gas, leaving trapped water to “prop” open the narrower parts of the crack.

Shale Structure (URtec 2154081)

Where is Hydrocarbon Generated, Stored & How Does it Flow?

Relative Hydrocarbon Potential (RHP)

Transgressive – Regressive Shore lines and impacts in shale deposition.

• Organic carbon is in layers within low permeability rock.

• Access to the TOC layers is controlled by natural fissures.

• Some weak zones will preferentially fracture.

• Stable flow access is critical.

How Does Oil Move Through Shale?

R. Gonzales8

There are “Sweet Spots” in the Barnett shale (based on highest production rates).

Some reasons behind this are: • Presence of mature TOC,• Natural fractures,• Operator knowledge, • Remaining recoverable

resources.

Note that the greener areas (higher production) are somewhat co-located with stress modifying features

Effect of Regional Fractures?

Best month production over 4 years of development – two different fields – what is happening?

What is the Permeability Range for Conventional and Unconventional Rocks?

• Conventional 1 mD (10-3D) to Darcies (D).

• Unconventional: varies from <1 nanodarcy, nD (10-9 D) to 1 microdarcy, mD (10-6 D) for granites, shales & clays that form cap-rocks or compartmentalize a reservoir,

Typical Productive Shales

Do All Perf Clusters or Fracs Produce?

Net Stresses Depend on Frac Growth and Where it Grows.

300 ft heigth

100 ft heigth

Induced Stress vs. Distance from Center of Fracture

What happens to the stress as multiple fractures are added?

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Water molecule ~ 0.3 nmCH4 molecule ~ 0.4 nmC20 oil molecule ~ 1.7 nm

Locating a Wellbore Position

“An Integrated Workflow for Selecting Optimal Drilling Locations,” E&P Magazine, June 2010, by Gerald Kidd and Fred Jensen.

Is a “quiet” well path the best approach for well design?

Natural fractures are prevalent in productive shales.

Flow through these fractures may be 10x to 10,000x higher than through the matrix.

Fractures open at ~60% of pressure needed to fracture the rock.

Fracture Azimuth vs. Distance from Fault

Average Frac Spacing

Background Frac spacing = 24 ft.

Stress Relaxed (?) Zone frac spacing = 4.3 ft.

SPE 163846

EVALUATION OF HORIZONTAL WELLS IN THE EAGLE FORD USING OIL-BASEDCHEMICAL TRACER TECHNOLOGY TO OPTIMIZE STIMULATION DESIGN

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2 wells – first ten daysOverview: Tracking Frac Stage performance

with Oil-Based TracersObjective: Optimize Well and Frac DesignConclusion: Uncertain, but oil-based tracers

may help define frac performance and optimized stage #’s

Effects of Geologic Occurrences

Estimated gas in place(MMcfg/sq mi); modified from Marcellus Shale in New York, 2009.

Productivity Index Along the Lateral

Look for the Gas Shows

•Gas Show•Quantity•Ratio of gasses•Corresponding GR

•Other logs (CNL, Density) to help assess TOC•Density for Brittleness•Resistivity for water saturation and salinity•ROP (rate of penetration)•Is it a hot shale or a natural fracture?

The objective is to align the perf clusters with natural fractures.

The Curious Effect of ISIP.Why do pressures vary so much?

Is it possible that stress changes will create a repeating frac pattern?

Stress Changes Along the Wellbore3D seismic interpretation by Rich and Ammerman, illustrating significant differences in seismic attributes between toe and heel of the lateral.

In their analysis, the natural fractures are parallel to fracture propagation in the toe.In the heel, the natural fractures are oriented perpendicular to hydraulic fracture direction.

An alternate interpretation is that the differences between smin and smax are decreasing in the heel and are in the range that both fracture sets could grow and complexity is developed.

No Conclusions – that’s up to you.

Questions?

Mineralogy - Where do the UC Reservoirs fit?

Technology Made Shale Gas Production PossibleCan understanding of stress increase recovery?

Oil

Gas