American Oil and Gas Reporter - October 2017 - 23

Geomechanics
Key To Understanding
Fracture Design
As the quest to optimize hydraulic fracture treatments continues, I have noticed an increased interest in one of hydraulic
fracturing's fundamentals: the science of geomechanics.
Geomechanics is a geologic specialty dealing with how rocks
respond to changes in stress, pressures and temperatures. A solid
understanding of these interactions can help an engineer more
effectively solve problems such as optimizing hydraulic fracturing, especially in low-permeability reservoirs.
In NSI Technologies' Frac Tips, Michael B. Smith writes that
geomechanics-sometimes called rock mechanics-helps us understand key parameters essential for designing and applying effective hydraulic fracture treatments. These come in two categories.
The first includes fracture height, fluid loss coefficient, tip
effects and Young's modulus, representing things over which
frac engineers have little control, but that are important to fracturing success. The second category includes pump rate and fluid
viscosity, and represent the things that can be controlled, but
often have less impact on the fracturing process.
In the early 1980s, one of the founding fathers of the American
Rock Mechanics Association (ARMA), Jean Claude Roegiers,
discussed how geomechanic principles could impact stimulation.
In his 1983 SPE paper, A Coupled Fracturing Model and its Application to Hydraulic Fracturing (SPE-12311-MS), Roegiers asserted that, while hydraulic fracturing worked, a thorough
understanding of exactly what went on down hole was lacking.
Roegiers and co-author Y. Ishijima presented a coupled model describing a two-dimensional numerical approach to studying the
fluid/rock interaction (geomechanics) of hydraulic fracturing.
In the intervening decades, geomechanical experts have continued to study how rocks respond during fracturing. A more modern example of how understanding these key parameters can affect
a fracture treatment comes from a presentation delivered by Neal
Nagel from OilField Geomechanics in Houston.
In Fully-Coupled Numerical Evaluations of Multiwell Completion Schemes: the Critical Role of In-Situ Pressure Changes
and Well Configuration (SPE-168581), presented at the 2014
Society of Petroleum Engineers Hydraulic Fracturing Technology Conference, Nagel outlined geomechanical challenges encountered during the development of many shale plays, which
commonly experience low production rates and rapid declines.
Nagel illustrated that, in an effort to address these production
issues, techniques have been developed such as "zipper fracs"-
where two horizontal wells placed in proximity to each other are
stimulated alternately-and "simul-fracs,"-where two wells with
similar setups are stimulated simultaneously. However, they
have met with only limited success.
Nagel asserts that multistage hydraulic fracturing from a single well, and multiwell completions such as zipper fracs in particular, induce a complicated, altered stress field around the
hydraulic fractures. This altered stress field can, in many field
situations, stabilize natural fractures and weakness planes, and
increase their resistance to shearing during fracturing, when in
contrast, a primary objective of fracture stimulating shales is to

"Geomechanics helps us understand
parameters essential for designing and applying

effective hydraulic fracture treatments.

shear natural fractures and weakness planes.
Nagel supports this assertion by presenting a detailed geomechanical simulation study performed to observe the interaction of a propagating hydraulic fracture with natural fractures
and weakness planes during multiwell completions.
The results of Nagel's work-like Roegiers'-highlight the importance that geomechanics (i.e., the interactions among in situ
stresses, pressures, natural fractures and weakness planes, and
well configurations), has on our ability to optimize hydraulic
fracture design. As geomechanics becomes recognized more
widely as an integral part of designing and implementing hydraulic fracture treatments, opportunities to understand what is
going on down hole can be difficult to find.
ARMA has emerged as a leader in investigating and providing information on hydraulic fracturing geomechanics. In June,
ARMA's technical committee on hydraulic fracturing held its
second workshop in San Francisco with a goal of clarifying the
fundamental physics involved with hydraulic fracturing, as well
as demonstrating model validity and diversity to capture them.
Results from the modeling session indicate most models can
capture various physics involved with fracturing, with some differences between models. The differences may be attributed to
a variety of assumptions, theoretical backgrounds, and numerical
approaches incorporated into each model. While the model comparison was not designed to evaluate the superior model, this exercise has helped to demonstrate each model's ability to capture
the recognized physics in this area.
Needless to say, a solid understanding and basis in geomechanics can help optimize a treatment. Understanding the factors that
one can't change, such as fracture height, fluid loss coefficient,
tip effects and Young's modulus, and how these impact a design
is an important part of developing that design. Additionally, understanding what one can control, such as pump rate and fluid viscosity, and how they interact with what can't be controlled, is the
rest of the story in designing a fracture treatment. ❒

JEREMY VISCOMI is the Mid-

Continent regional lead for
the Petroleum Technology
Transfer Council. He has more
than a decade of experience
in developing and organizing
technical conferences and special events, primarily in the
oil and gas industry.
OCTOBER 2017 23

American Oil and Gas Reporter - October 2017

Table of Contents for the Digital Edition of American Oil and Gas Reporter - October 2017