WELL STIMULATION STIMULATION TECHNIQUES PCB 4323 OVERVIEW OVER VIEW OF W ELL STIMU LA LATION TION TECH NIQUES
WELL STIMULA STIMULATION TION TECHNIQUES By
Ms. Haizatul Hafizah Hussain
[email protected] haizatulhafizah.hus@ petronas.com.my .my Office Ext. : 7373 Office Room : J3-02-25
LESSON OUTC OUTCOMES OMES At the end of this lecture, students should be able to;
Define well stimulation technique
Describe reasons for stimulation
Describe the relationship of skin and formation damage
Identify different types of well stimulation techniques
LECTURE OUTLINES
Definition
Reasons for stimulation
Describe the relationship between well stimulation,
formation damage and skin
DEFINITION A
treatment performed to restore or enhance the productivity of a well by
improving the flow of hydrocarbons. Stimulation treatments fall into two main groups, hydraulics fracturing treatments and matrix treatments. Fracturing
treatments are performed above the fracture pressure of the
reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix
treatments are performed below the reservoir fracture pressure and
generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area.
CONVENTIONAL, TIGHT GAS AND UNCONVENTIONAL RESERVOIRS?
CONVENTIONAL, TIGHT GAS AND UNCONVENTIONAL RESERVOIRS? Conventional Reservoir
A reservoir in which buoyant forces keep hydrocarbons in place below a sealing caprock. Reservoir and fluid characteristics of conventional reservoirs typically permit oil or natural gas to flow readily into wellbores.
Tight
Gas Reservoir
Natural gas reservoirs locked in extraordinarily impermeable, hard rock, making the underground formation extremely "tight."
Unconventional
Reservoir
Any reservoir that requires special recovery operations outside the conventional operating practices, requires assertive recovery solutions such as stimulation treatments.
REASONS FOR STIMULATION Increase production
efficiency or flow capacity
Overcome formation damage Enhance production from low
permeability wells
Increase effective drainage area Produce
complex reservoirs (e.g. discontinuous sand bars)
Increase wellbore stability To increase
ultimate economic recovery
WELL STIMULATION
The damage of formation near the wellbore zone means the reduction of permeability at the area.
This reduced permeability degrades the productivity of the wells and hence, need to
be
stimulated
to
optimize
the
production.
This involves pumping of acids, energized fluids, and various other chemical to improve formation flow characteristics.
WELL STIMULATION During drilling and completion operations, formation damage are caused by: Solid particulates blocks pore throats by mud solid invasion which leads
to
blocked pore throats – permeability reduced. For water based mud, if it invades the clay zone, it causes swelling which
also
reduces the size of the pore throats – also reduced the permeability. Scale – due
to pressure and temperature changes. Salts in formation water
becomes over saturated and precipitate. Wax Asphaltenes –
precipitate due to pressure drop
WELL STIMULATION From Darcy’s
equation,
=
excess flowing pressure drop
∆ , the ∆
∆ created
by the presence of volume rock which has suffered formation damage near well bore area can be reduced by matrix treatment and acidizing. Obviously,
the damaged zone has lower
permeability than original.
WELL STIMULATION The well stimulation techniques include : Hydraulic
fracturing
Matrix acidizing / acid fracturing Perforation
The stimulation treatments increase well productivity by pumping a specially formulated fluid The fluid is designed to remove the formation damage
WELL STIMULATION =
( − ) . ( + )
Steady-state, radial flow equation describes the well inflow. Well inflow, can be improved by
Increase permeability – height factor OR
Decrease the factors : S, , and
1
WELL STIMULATION =
∆
=
ln
The removal of this formation damage will restore the natural well productivity.
The relationship between the skin, permeability and the radius of the damaged and undamaged can be presented by the Hawkins formula.
This is a convenient tool for analyzing the influence of varying levels and depth of formation.
WELL STIMULATION =
ln
3
Hawkins equation can be used to determine whether the optimum skin value is achieved or not. This calculates the formation damage skin (Sd) resulting from a cylinder of reduced permeability around the wellbore.
The equation shows that the formation damage skin increases as the permeability damage ratio increases or the radius damage rd increases.
SKIN
Skin is a dimensionless variable, which can indicate the extent of formation damage or permeability improvement at the wellbore.
Skin Factor can be positive (i.e. formation damage) or negative (i.e. stimulation)
SKIN
Impact of Skin on Production
SKIN =
+
+
+
4
The value of the Total Well Skin (Stotal) measured during a production test has many other sources other than formation damage.
It is very important to be able to identify the formation damage component (Sd), since this can be reduced by better operational practices or possibly, be removed by a stimulation treatment
The well geometry skin, completion skin and production skin have a common cause which is the disturbance of the fluid flow streamlines normal to the well
FORMATION DAMAGE SKIN, =
+
+
+
The figure shows the resulting producing pressure profile and compares it with the equivalent pressure profile for an undamaged well.
The extra pressure drop has to be compensated for either by a reduce pressure drop across the choke or by a smaller production rate
WELL GEOMETRY SKIN, =
+
+
+
The well geometry skin reflects geometrical considerations which alter the skin value occur due to the well design. These include:
Positive Skin
Limited entry – well not perforated across the complete reservoir height and/or well not fully penetrating the reservoir
Well not placed in the centre of the drainage boundary
Negative Skin
Well is slanted through the formation. As well deviation increases their longer exposure to the producing formation show an increasing well productivity.
COMPLETION SKIN, =
+
+
Insufficient perforation
Too short or too narrow shape, incorrect phasing
High density of long/wide perforations - increase in the inflow
Gravel packing
+
Packed with a high permeability gravel - positive well skins
Fractures
Can either occur naturally or are artificially created propped hydraulic fractures. They lead to an increased inflow and negative skin as they form a high permeability pathway from deep in the formation to the wellbore
PRODUCTION SKIN, =
+
+
+
A rate dependent skin often observed in high rate gas wells. This is due to non-Darcy or turbulent flow. In presence can be a useful indication that the well is a potential stimulation candidate
Producing well with a
Flowing bottom hole pressure below the bubble point or
Retrograde condensate fluid so that there are two phases at the perforations instead of gas only
The presence of the extra phase reduces the effective permeability to the major phase – positive skin
FORMATION DAMAGE SKIN
Skin factor increases with a larger radius of damage and damage ratio, k o/k d
Effect of formation damage on well production : reduces the productivity
OVERVIEW OF WELL STIMULATION TECHNIQUES Well Fracturing Acid
Stimulation
Acid
Fracturing
Perforation
OVERVIEW OF WELL STIMULATION TECHNIQUES Well Fracturing The
process of hydraulic well fracturing pressurizing the well until a
fracture propagates through the reservoir rock. Acid A
Stimulation
stimulation treatment used to remove damage near the wellbore.
Involves the injection of a reactive fluid into the reservoir rock at a pressure below the fracturing pressure.
OVERVIEW OF WELL STIMULATION TECHNIQUES Acid
Fracturing
Common
use in carbonate formations, whereby acid is injected into the
formation at a sufficiently high pressure to create fractures or widen existing natural fractures. Perforation Can
be used in areas where perforation collapse, water or gas
breakthrough has occurred.
EXERCISE 1 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION Assume that a well has a radius rw of 0.328ft and a penetration of damage 3ft beyond the well (ie, rd=3.328ft). 1. What would be the skin effect if the permeability impairment results in equal to 5 and 10, respectively?
2. What would be the required penetration of damage to provide the same skin effect as the second case but with = 5
EXERCISE 1 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION Given =
When = 5
ln
When = 10
3.328 = ln 0.328
3.328 = ln 0.328
= .
= .
EXERCISE 1 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION If S = 20.9 and k/k s = 5, then the penetration, is ?
. =
=
.
= . =
.
EXERCISE 2 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION A Production Engineer is contemplating whether going for cased hole completion or open hole completion in completing Well-1 in Gelama Merah reservoir. The reservoir and the completions properties are as follow; Formation Permeability Drainage Area
375 mD 60 acres ( =
Wellbore Radius
) 7
If cased hole completion is considered, the pay zone will be perforated using through tubing gun with 14 inches penetration depth. The available mud types for drilling are as follow; Mud
A
B
Permeability in damaged zone (mD)
100
10
Depth of invasion (ft)
2.3
0.9
EXERCISE 2 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION Which mud system do you recommend if you decided to perform; 1. Open hole completions 2. Cased hole completions Calculate the damage skin for both mud system using Hawkin’s formula. M u d A : = 3.44 M u d B : =
.
For open hole completions, use Mud A For cased hole completions, use Mud B
EXERCISE 3 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION An initial well test in a reservoir gave a stabilized oil flow rate of 5780 bopd for a stabilized flowing bottomhole pressure of 1524psi with a skin factor of zero. After 18 months of production, the flowing bottomhole pressure was 1250 psi to maintain the same initial production rate. Consider following data: Oil permeability, k o
120 mD
Formation thickness, h
80ft
Oil viscosity, μ o
1.3 cp
External radius, re
1181 ft
Wellbore radius, rw
0.49 ft
Oil formation volume factor, Bo
1.21 res bbl/stb
Reservoir pressure, pe
2566 psi
Assume the flow is steady state
EXERCISE 3 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION Calculate the mechanical skin factor for this well after 18 months’ production.
. = ( ) Re-arrange the steady-state equation gives :
= ( ) .
EXERCISE 3 – PERMEABILITY IMPAIRMENT VERSUS DAMAGE PENETRATION = ( ) . =
()() ( ) . ()(. ) (. ) . = .
HAVE YOU ACHIEVED THE LEARNING OUTCOMES TODAY?
ANY THOUGHTS?
