ABSTRACT
Hydraulic fracturing has been an effective technique to stimulate
damaged wells or wells producing from low-permeability formation. It has
been established that the orientation of a hydraulic fracture is
perpendicular to the direction of the least principal stress in the
formation. Thus, most pressure transient analysis techniques are based
on the assumption that the fracture is vertical. However, it is now
generally agreed that the direction of the least principal stress is not
always parallel or perpendicular to the plane of the formation. For
this reason it is very likely that some hydraulically fractured wells
have inclined fractures.
Objectives of study:
(1) Provide background to justify the conclusion that the direction
of least principal stress might be at an orientation different from
parallel or perpendicular to the bedding plane of the formation.
(2) Develop a technique, based on the pressure derivative
concept, for interpreting pressure transient tests in wells with an
inclined hydraulic fracture. Three cases are possible:
(a) the fracture is symmetric in both lateral and horizontal directions,
(b) the fracture is asymmetric in only one of the directions, and
(c) The fracture is asymmetric in both directions.
CHAPTER 1
INTRODUCTION
1.1 Introduction
Hydraulic fracturing is an important well stimulation technique that
has been widely used in the oil and gas industry. The technique involves
creation of fractures or fracture system in porous medium to overcome
wellbore damage, to improve oil and gas productivity in low permeability
reservoirs or to increase production in secondary recovery operations.
Most of the pressure transient analysis techniques used to analyze
pressure responses of fractured wells are based on the assumption that
the fracture is either vertical or horizontal. However, hydraulic
fracture could be inclined with a certain angle with respect to the
vertical direction because it is now generally agreed that the direction
of the least principal stress is not always parallel or perpendicular
to the plane of the formation. For this reason it is very likely that
some hydraulically fractured wells have inclined fractures. This has
been proved by laboratory experiments as well as data from actual field
studies with the aid of modern technology such as surface tilt meters.
Thus, for an inclined hydraulic fracture, the vertical orientation
assumption may lead to serious errors in well test analysis especially
when the inclination angle is high. More so, there are very few studies
concerning pressure transient analysis of inclined hydraulic fracture
and there is not any applicable well test analysis procedure available
for inclined fractures. For this reason, it is important to develop well
test analysis procedures for this type of fracture in naturally
fractured reservoirs.
A naturally fractured reservoir can be defined as a reservoir that
contains fractures (planar discontinuities) created by natural processes
like diastrophism and volume shrinkage, distributed as a consistent
connected network throughout the reservoir. They present unique
challenges that differentiate them from conventional reservoirs. It
represents over 20% of the world's oil and gas reserves, but is however
among the most complicated class of reservoirs to characterize and
produce efficiently.
The purpose of this study is to develop a technique, based on the
pressure derivative concept, for interpreting pressure transient tests
in wells with an inclined hydraulic fracture in a naturally fractured
reservoir. This report will provide some background on inclined
hydraulic fracture including in-situ stress state, factors that affect
the stress state, occurrences of inclined hydraulic fractures and field
studies. Two techniques of transient pressure analysis using pressure
derivatives will be developed. They are type curve matching and Tiab’s
Direct Synthesis (TDS). Both uniform flux and infinite conductivity
models would be considered in the study. Analytical solution for each
flow regime will be analysed in details.
1.2 Technical Objectives:
· To provide background to justify the conclusion that the
direction of least principal stress might be at an orientation different
from parallel or perpendicular to the bedding plane of the formation.
· To develop a technique, based on the pressure derivative
concept, for interpreting pressure transient tests in wells with an
inclined hydraulic fracture in naturally fractured reservoirs.
Three cases are possible:
(a) The fracture is symmetric in both lateral and horizontal directions,
(b) The fracture is asymmetric in only one of the directions, and
(c) The fracture is asymmetric in both directions.
1.3 Hydraulically fractured wells
Naturally fractured reservoirs are different from conventional
(unfractured) reservoirs. They are heterogeneous in type and consist of
matrix blocks separated from one another by the fracture system. The
matrix blocks are made of the original rock that was present before
fracturing took place. The matrix is characterized by its permeability
km and porosity φm . The fracture system is characterized by its permeability kfand porosity φ f . It means a naturally fractured reservoir is a double-porosity and double-permeability reservoir.
1.4 Naturally Fractured Reservoirs
Fractures are displacement discontinuities in rocks, which appear as
local breaks in the natural sequence of the rock’s properties.It may
appear as micro-fissures with an extension of only several micrometers,
or as continental fractures with an extension of several thousand
kilometres.
In geological terms, a fracture is any planar or curvi-planar
discontinuity that has formed as a result of a process of brittle
deformation in the earth’s crust. Naturally fractured rocks can be
geologically categorized into three main types, based on their porosity
systems:
(1) Intercrystalline-intergranular;
(2) Fracture-matrix ; and
(3) Vugular-solution
ABSTRACT
Hydraulic fracturing has been an effective technique to stimulate
damaged wells or wells producing from low-permeability formation. It has
been established that the orientation of a hydraulic fracture is
perpendicular to the direction of the least principal stress in the
formation. Thus, most pressure transient analysis techniques are based
on the assumption that the fracture is vertical. However, it is now
generally agreed that the direction of the least principal stress is not
always parallel or perpendicular to the plane of the formation. For
this reason it is very likely that some hydraulically fractured wells
have inclined fractures.
Objectives of study:
(1) Provide background to justify the conclusion that the direction
of least principal stress might be at an orientation different from
parallel or perpendicular to the bedding plane of the formation.
(2) Develop a technique, based on the pressure derivative
concept, for interpreting pressure transient tests in wells with an
inclined hydraulic fracture. Three cases are possible:
(a) the fracture is symmetric in both lateral and horizontal directions,
(b) the fracture is asymmetric in only one of the directions, and
(c) The fracture is asymmetric in both directions.
CHAPTER 1
INTRODUCTION
1.1 Introduction
Hydraulic fracturing is an important well stimulation technique that
has been widely used in the oil and gas industry. The technique involves
creation of fractures or fracture system in porous medium to overcome
wellbore damage, to improve oil and gas productivity in low permeability
reservoirs or to increase production in secondary recovery operations.
Most of the pressure transient analysis techniques used to analyze
pressure responses of fractured wells are based on the assumption that
the fracture is either vertical or horizontal. However, hydraulic
fracture could be inclined with a certain angle with respect to the
vertical direction because it is now generally agreed that the direction
of the least principal stress is not always parallel or perpendicular
to the plane of the formation. For this reason it is very likely that
some hydraulically fractured wells have inclined fractures. This has
been proved by laboratory experiments as well as data from actual field
studies with the aid of modern technology such as surface tilt meters.
Thus, for an inclined hydraulic fracture, the vertical orientation
assumption may lead to serious errors in well test analysis especially
when the inclination angle is high. More so, there are very few studies
concerning pressure transient analysis of inclined hydraulic fracture
and there is not any applicable well test analysis procedure available
for inclined fractures. For this reason, it is important to develop well
test analysis procedures for this type of fracture in naturally
fractured reservoirs.
A naturally fractured reservoir can be defined as a reservoir that
contains fractures (planar discontinuities) created by natural processes
like diastrophism and volume shrinkage, distributed as a consistent
connected network throughout the reservoir. They present unique
challenges that differentiate them from conventional reservoirs. It
represents over 20% of the world's oil and gas reserves, but is however
among the most complicated class of reservoirs to characterize and
produce efficiently.
The purpose of this study is to develop a technique, based on the
pressure derivative concept, for interpreting pressure transient tests
in wells with an inclined hydraulic fracture in a naturally fractured
reservoir. This report will provide some background on inclined
hydraulic fracture including in-situ stress state, factors that affect
the stress state, occurrences of inclined hydraulic fractures and field
studies. Two techniques of transient pressure analysis using pressure
derivatives will be developed. They are type curve matching and Tiab’s
Direct Synthesis (TDS). Both uniform flux and infinite conductivity
models would be considered in the study. Analytical solution for each
flow regime will be analysed in details.
1.2 Technical Objectives:
· To provide background to justify the conclusion that the
direction of least principal stress might be at an orientation different
from parallel or perpendicular to the bedding plane of the formation.
· To develop a technique, based on the pressure derivative
concept, for interpreting pressure transient tests in wells with an
inclined hydraulic fracture in naturally fractured reservoirs.
Three cases are possible:
(a) The fracture is symmetric in both lateral and horizontal directions,
(b) The fracture is asymmetric in only one of the directions, and
(c) The fracture is asymmetric in both directions.
1.3 Hydraulically fractured wells
Naturally fractured reservoirs are different from conventional
(unfractured) reservoirs. They are heterogeneous in type and consist of
matrix blocks separated from one another by the fracture system. The
matrix blocks are made of the original rock that was present before
fracturing took place. The matrix is characterized by its permeability
km and porosity φm . The fracture system is characterized by its permeability kfand porosity φ f . It means a naturally fractured reservoir is a double-porosity and double-permeability reservoir.
1.4 Naturally Fractured Reservoirs
Fractures are displacement discontinuities in rocks, which appear as
local breaks in the natural sequence of the rock’s properties.It may
appear as micro-fissures with an extension of only several micrometers,
or as continental fractures with an extension of several thousand
kilometres.
In geological terms, a fracture is any planar or curvi-planar
discontinuity that has formed as a result of a process of brittle
deformation in the earth’s crust.Naturally fractured rocks can be
geologically categorized into three main types, based on their porosity
systems:
(1) Intercrystalline-intergranular;
(2) Fracture-matrix ; and
(3) Vugular-solution