understanding shale fractures l

Understanding Shale Fractures Leads To Better Production,Lower Cost

Bill Dershowitz and Mark Cottrell, Golder AssociatAugust 1, 2011

DFN simulation is shedding new light on natural and hydraulic fracture systems.

Much of the public and political concern about shale gas extraction is centered on water impacts.

This includes complaints from residents in states such as Pennsylvania and New York that their waterwells have become contaminated with methane, speculation by some environmental groups that thefluids used in the hydraulic fracturing process will migrate upward to levels where it can contaminategroundwater, and concern that spills will impact surface water.

Such concerns, whether legitimate or not, are threatening development of an extremely valuableenergy source, with estimated recoverable reserves currently estimated at more than 60 Tcf in the US(worth approximately US $250 billion at a wellhead price of US $4/mcf).

Shales hold enormous potential. The Barnett shale underlying the Dallas-Fort Worth area of Texashas produced natural gas commercially since 1981, when Mitchell Energy drilled the first gas well.Today, improved technologies are moving the shale gas industry toward ever safer, cleaner, and moreefficient production.

This DFN computer-based model shows natural fractures in a rock mass with eachindicated by color. (Images courtesy of Golder Associates)

Natural fractures are key

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Natural fractures exist in almost all shale gas reservoirs. In many, the fractures are healed or sealedand are therefore assumed to play no role in production. There is increasing evidence that naturalfractures influence the hydraulic fracturing process, the delivery of gas to the wellbore, and the totaloil content (TOC) or richness of the gas shale itself and thus the "grade" of the resource. Naturalfractures also can influence the vertical extent of fracs, the vertical gas flow, and the potential forenvironmental impacts.

Discrete fracture network (DFN) analysis of natural fracture systems provides a new tool to betterunderstand the hydraulic fracturing process and the production of gas resources from such tight rocks.Although the DFN approach was introduced to the oil industry in the 1980s, the ability of DFNsimulators to incorporate both natural and hydraulic fractures is relatively recent. This ability dependson improved understanding of the underlying mechanics of hydraulic fracturing.

This computer-based model shows the tributarydrainage volume for fractures in a rock mass,illustrating the amount of interference between gasproduction wells.

Until recently, all frac fluid that did not directly support fracture propagation was treated as "leak off"and accompanied by a reduction in the hydraulic efficiency of the process. It is now recognized thatfracing also can reactivate natural fractures, providing an increased tributary drainage volume for gasproduction. Natural fractures thus have a dual role – in some reservoirs, higher natural fracturedensities imply reduced production, while in other reservoirs, higher natural fracture intensities areindicative of higher production possibilities.

Similarly, when considering the potential environmental impacts of hydraulic fracturing, fullyunderstanding the direction and extent of any hydraulic fracture treatment prior to implementing thefrac is beneficial.

The vertical height of the hydraulic fracture is controlled by the presence of lower modulus rock unitsabove the reservoir layer – hydraulic fractures consistently terminate when they encounter these



ductile quenching units. The existence of high fracture intensities in these overlying units means thoseunits are softer and therefore better able to terminate vertical frac propagation. It is through suchgeological understanding that the risks to the environment from hydraulic fracturing can beeffectively controlled.

Improved observations of natural fracture systems through careful log interpretation can provideessential insights to understanding fully the in situ stress state – essential for improving fracing designand gas production and maintaining environmental safety.

DFN modeling couples the geomechanics and fluid flow in the fracture network and the rock matrixand supports the design of a well-constrained, reliable, and productive drilling and fracing program.

Golder Associates has developed and used FracMan DFN models for both natural and hydraulicfracture assessment. FracMan provides a key tool for improving production from shale gas andunderstanding and reducing environmental risks. The predevelopment analysis of hydraulic fracturingand fluid flow demonstrates the care being taken to manage gas and fluid migration and can go a longway toward assuring regulators and other stakeholders, such as local residents and political leaders,that their concerns are being satisfied.

This DFN model shows the natural fractures with simulations of the microseismicresponse that can be expected during hydraulic fracturing.

Microseismic monitoring comes of age

Recent advances in microseismic monitoring, including the development of low-energy surface-basedmonitoring techniques, provide a key insight to the hydraulic fracturing process. This can be used tovalidate DFN geomechanical analyses and to help demonstrate control of fracing operations toenvironmental regulators.



Microseismic monitoring detects the hundreds of small seismic events that occur when rocks slip pasteach other during network fluid flowing. Using arrays of geophones to record and triangulatemicroseismic events makes it possible to build a 3-D image of the fracturing rock mass. This imagecan be combined with the simulated microseismic picture generated inside the Frac-Man model,providing understanding and validation to both the DFN modeling and the fracing process.

Geomechanical modeling also can support a company's decision-making process. It can contribute tothe planning of drilling, completion, and stimulation and production strategies required for horizontalmultifrac wells designed to complement natural fracture systems. Geomechanics can help optimizethe best trajectory of a well; it can help determine the in situ stress conditions for safe and efficientfracing around geohazards. And it also can help determine the optimal pressure conditions duringdrilling to avoid unwanted processes like hole collapse and formation damage.

No two shale gas reservoirs are the same (even within the same basin). It is of paramount importancethat integrated approaches be used to simultaneously address safe and efficient production from shalereservoirs.

Through combining geomechanics, petrophysics, and seismology, the use of horizontal wells andhydraulic fracture stimulation can be used to safely develop shale based resources.

This image is a simulation of the microseismic response during hydraulic fracturingin a rock mass. Small earthquakes generated as parts of the rock mass slide past eachother.



understanding shale fractures l

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