Hydraulic Fracturing Tight Reservoirs: Rock Mechanics and Transport Phenomena

Abstract

Conventional reservoirs have been fracture stimulated using acid fracturing and proppant fracturing.
Acid fracturing is performed to improve well productivity in acid-soluble formations such as
limestone, dolomite, and chalk. Hydrochloric acid is generally used to create an etched fracture,
which is the main mechanism for maintaining the fracture open during the life of a well. Proppant
fracturing is an alternative option that has been applied in carbonaceous and siliceous formations.
There is no quantitative method to provide an answer of whether acid fracturing or proppant
fracturing is an appropriate stimulation method for a given carbonate formation. How rock
mechanics can be applied to decide on what method is more effective? Laboratory experiments have
been performed to simulate acid etched to study the effect of elastic, plastic and viscoelastic rock
behavior and their effects on fracture conductivity. Comparison of acid vs. proppant fracturing
conductivity in carbonate formation is presented.
Fracturing low permeability reservoirs is totally different than fracturing tight formations. The
fracture geometry required in low permeability reservoirs need to be planar, conductive and
penetrating deep in the reservoir. Fracture complexity in these reservoirs is to be avoided for
optimum stimulation treatment. However, in fracturing tight formation, a complex fracture network
is desirable for better recovery. Creating multiple fractures in horizontal wells without the use of
mechanical intervention, is becoming essential especially in tight gas reservoirs. We have learned
how to initiate hydraulic fractures into a specific direction and place as many fractures as desired in
horizontal wells but with casing and perforation. The challenge now is to initiate weak point across
the horizontal well such that fracturing fluid will initiate a fracture there. How rock mechanics has
been applied to achieve this objective? We are fracturing tight gas sand in harsh environment, at
depth more than 18000 ft, of temperature close to 400 °F, and one can figure out the extreme in-situ
stresses relevant to this depth.
When the reservoir pressure decreases, the elastic displacement in response to the increase in
effective stress will cause natural fractures to close leading to a decline in reservoir productivity.

The matrix medium feeds the natural tensile fractures which carry the fluids to the wellbore. The
decline in conductivity with increasing effective stress should follow a logical declining rate to
support a given production rate. How the concept of effective stress has been applied to understand
the stress-dependent conductivity of various conductive components of a given reservoir? Rock
mechanics testing of these stress sensitive reservoirs becomes vital to optimize fracturing tight
formations.
Economical production from tight reservoirs, including shale gas and shale oil formations, requires
horizontal well drilling and massive proppant hydraulic fracturing stimulation. The stimulation
involves generating sufficient fractures network or stimulated reservoir volume (SRV), which is
achieved by placing optimized stimulation treatments along the horizontal section of wellbores
ideally drilled from multi-well pads to increase the production rate and ultimate recovery. Hydraulic
fracturing in naturally fractured formations is characterized by generating a fractures’ network that
should be designed for in extremely low permeability of unconventional reservoirs. Fractures should
extensively reach shale matrix to achieve commercial gas production. Therefore, production rate and
ultimate recovery depend on the size of the created SRV.
The transport phenomena controlling fluid flow through tight formation is no longer sufficient to be
modeled by Darcy’s flow. Diffusion and imbibition are important transport mechanisms. The concept
of osmosis and flow through a semi-permeable membrane component are critical. Additionally,
diffusion and a special case of molecular flow due to Knudson effect will be discussed. Conventional
reservoir simulation collapses when trying to simulate fluid flow through tight reservoirs. Numerical
studies on a hydraulically fractured well to simulate the dynamic processes during fracturing
injection, following well shut-in (soaking), and production are discussed.