## Review of Stimulation Fluid Retention

Mechanisms and Likelihood of Fluid Communication with Shallow Aquifers

–What’s New and Different Since the 2004 EPA Study?

By Scott Cline, petroleum engineer

Stanley, NY*

*I do not represent my primary employer in this matter; information and opinions do not represent the views of my employer and are presented on my own time with prior approval.

# Why is it Relevant to Revisit the 2004 EPA Fluid Transport and Retention Discussion? There are differences between shale gas and shallow coal bed development that should be distinguished and addressed. Continuing Public Mistrust on the subject: Public thinks of this fluid as “pools” of subsurface toxic waste that will eventually “float” to the surface. Public looks to EPA for assurance on this topic. New Information and monitoring techniques have shed some light on mechanisms and fracture propagation.

### • Where Does the Frac Fluid go?
• Why Does so Little Return?
• Is Retained Fluid a Risk?
• How do we Know?

# Natural Fracture Generation Because pore space did not expand during burial of the Marcellus Shale, the generation of natural gas in this organic - rich unit resulted in an additional increment of pore pressure to such a magnitude that the rock cracked in a massive network of fractures! 1 

## Resulted over- pressured characteristics indicative of a closed system.

1 Engelder, Terry: “Unconventional Natural Gas Reservoir Could Boost U.S. Supply,” Penn State Live (January 17, 2008).Picture from November 2009 OilandGasInvestor.com Peter M. Duncan founding president of MicroSeismic Inc. Horizontal well image courtesy of Bill Kappel USGS, Ithaca, NY J 2 Fracture’s Strike NW Joints Systematic and Widespread Faults Unsystematic and Isolated

# Shale Gas Production Mechanism Shale - both gas source and “sealed” reservoir Mechanisms by which gas is produced from a shale reservoir :

– depletion of the free gas stored in the fracture network (natural and induced)

– depletion of the free gas stored in the matrix porosity, and

– Desorption

– A dual porosity
– triple mechanism system

## Why We Fracture Shale Gas Wells Horizontal Wells are not Enough Even Over- pressured Sealed Naturally Fractured Shale System Alone Will Not Produce Economic Quantities of Gas Goal is to give every gas molecule a high - speed path to the well - bore The hydraulic fracturing process creates not only tensile fractures, but also shears existing fractures
Diagram from: Packer’s Plus: Good Fracturing Practices in Horizontal Wells Multi - Stage Fracturing System Solutions

# How Much Fluid Retained? Inject Average 4+ million gallons of fresh water plus 4500 gallons of chemicals and 1400 gallons HCL (Range Resources and others)

– Approximately 1000 gallons of combined scale reducer , friction reducer and biocide per million gallons of water

– Plus another 300 gallons of HCL per million that is expected to immediately react and decompose into inorganic salts and CO2 

## Produced Fluid?:

– Less than a million gallons of water is produced to the surface over the life of the well

– Rates decline and become more saline over time

# Why Does so Little Water Return to the Surface and Does it



# Matter?Mini

-frac (cased hole) or leak -off test (open hole) pressure test versus time, showing definitions of pressure terminology

http://spec2000.net/10
-closurestress.htm

# Mechanisms for Fluid Retention Fluid “leak- off” into the fracture face. In formations with low permeability and low water saturation this fluid is permanently phase trapped by capillary forces. Hydrophilic clay adsorption and swelling in pore throat. Narrow fracture branches (shear fractures as opposed to tensile fractures) trap fluid by capillary forces and stranding
. Fluid in proppant packs may be unable to move as fluid prefers going around the packs. Fluid may move by gravity to the bottom of the fractures and unable to move as reservoir flow dynamics slow with depletion. ( Daneshy , 2010)Adapted and Modified from Penny, Pursley, Holcomb, SPE paper 94274 April, 2005 Areas of low porosity/perm high Pc – No flowIc.ucsc.edu/~ casey /ear150/Lectures/Joints/CrackModes.jpgProppant Pack

Modified from Daneshy, 2010 E&P Magazine

### Other Considerations Vertical Separation:

– The developable formations separated from potential aquifers by at least 3,000 feet (and probably more) of sandstones and shales (of low permeability). 

### Pressure Applied Limited Time

– Fracturing pressures applied for short duration , while the required travel time for fluid to flow from the shale to the aquifer under those pressures is measured in years 

### Pore Volume

– The volume of fluid used could only fill a small percentage of the void space between the shale and the aquifer and the already diluted chemicals would be further diluted by the formation water. (100 million gal/acre with 3000 ft of separation from aquifer - 16 billion gal/160 acres)Other Considerations cont. 

### Adsorption

– Some of the additives would be adsorbed by and bound to the organic - rich shales
. 

### Flow Direction
– Pressure Sink

– Any flow of frac fluid toward an aquifer through open fractures or an unplugged wellbore (remote) would be reversed during flowback , with any residual fluid further flushed by flow toward the production zone and well bore as pressures decline in the reservoir during production. (Pressure Sink) 

### Anomalous Stimulation Pressures

– The fracturing contractor would notice an anomaly if conditions led to the inability to maintain the predicted fracturing pressure. 

### History

– The historical experience of hydraulic fracturing in tens of thousands of wells is consistent with the analytical conclusion.Picture Source API Hydraulic Fracturing Operations
—Well Construction and Integrity Guidelines API GUIDANCE DOCUMENT HF1 FIRST EDITION, OCTOBER 2009
- In NY, any proposed drilling less than 2000 feet deep or 1000 feet below USDW requires site specific SEQRA (7.1.5 dSGEIS ) 

## Likely Development Depths for Large Volume

Stimulations? 1. Least Principle Stress Vertical at < 2000 ft. so Fractures Propagate Horizontal API 2. We Need Depth for the Desired Over
- pressured ConditionsPacker’s Plus: Good Fracturing Practices in Horizontal Wells Multi - Stage Fracturing System Solution http://www.packersplus.com/pdfs/Good%20Fracturing%20Practices%20SPE%20Dallas%20July%202010.pdf 

## How Do We Know Fractures Height is Limited?American Oil and Gas Reporter, July 2010 Kevin Fisher, Pinnacle ResourcesAmerican Oil and Gas Reporter July 2010 Kevin Fisher, Pinnacle Resources

# Recommendations Examine the New Literature on Fracture Propagation and Water Retention Mechanisms Since 2004 EPA Study. Distinguish between the various types, sizes and depths of fracture stimulation. Seek help from industry on simulation and extensive micro - seismic data already collected. Convey conclusions decisively to public .