lodos fdo bonilla foams.pdf

Created by: Luis Fernando Bonilla C. ........................................... 1

1. INTRODUCTION

Currently, the use of foam fluids is increasing rapidly in the petroleum industry because these fluids exhibit properties that are desirable in many field operations. In drilling, the reduced density of the foam fluids, their high carrying capacity, and their minimum filtrate and circulation losses are among the desirable properties of the drilling fluid during underbalanced drilling operations. The use of foams as a fracturing fluid also presents several advantages. The high carrying capacity, the minimum amount of fluid placed in the formation, and the excellent fluid recovery after treatment are some of the advantages that foam fluids present when used during fracturing operations. Although not limited to, foams have also been used in oil and gas fields in well stimulation, clean up, and fishing operations.

Foams are complex mixtures of a gas, a liquid, and a surfactant whose rheological properties are strongly influenced by parameters like temperature, absolute pressure, foam quality, texture, foam-channel wall interactions, liquid phase properties, and type and concentration of surfactant. Therefore, the rheology of foams is more complex than that of simpler fluids.

When used in oil field operations, foam exits in an unsteady state due to constant changes in temperature, pressure, composition, and shear rates. These changes affect the flow properties of the foam fluids and may drastically alter their hydraulic behavior. Consequently, all the variable values predicted during the design process depend on the changes of the foam properties. For this reason, the calculation and prediction of the friction losses in laminar and turbulent flow for foam fluids is a major challenge in oil field operations.

In order to improve the prediction of the friction losses when pumping foams in tubular goods, a comprehensive study of the effects of liquid phase type, shear rate, pipe diameter, temperature, and quality on the rheological properties of foams was conducted using the new foam loop of the Well Construction Technology Center (WCTC) of The University of Oklahoma. Tests were run in laminar flow regime for aqueous and gelled foams at 1000 psia, for qualities ranging from 30% to 80%, and temperatures ranging from 75 °F to 175 °F using two different diameter pipes. Liquid phase rheology was also evaluated using a Couette type viscometer.

1.1 OBJECTIVES

The main purpose of this research was to conduct an integrated experimental investigation of the rheological behavior of foams in which several parameters were considered. Flow data were obtained while flowing aqueous and guar foam fluids inside pipe type viscometers in laminar flow regime. The main objectives of this study were:


• To determine wall slip effects (if any) of foams when flowing them in pipes and the influence of pipe size on their rheology.

• To analyze the effects of liquid phase rheology and gas fraction on foam viscosity.

• To study the effects of temperature on liquid phase viscosity and foam viscosity.

• To obtain empirical correlation that predicts foam viscosity as a function of liquid phase properties, gas fraction, and temperature. The new correlation will be compared with previous studies.

2. THEORY AND LITERATURE REVIEW

2.1 APPLICATIONS OF FOAMS IN THE PETROLEUM INDUSTRY

Foams have a variety of applications in oil field operations, e.g. they have been used in well stimulation, fracturing, drilling, acidizing, cleanout operations, mobility control in enhanced oil recovery, and many others specific activities where foam properties present advantages compared with other potential fluids. In general, these advantages may be summarized as follows:

• Reduced presence of liquid phase and minimum fluid loss to the formation that causes a minimum or null presence of loss control agents such as solid fines and chemical additives. This contributes to costs minimization and reduced formation damage.

• High viscosity and low friction pressure loss in pipes that reduce the required power on surface and allow to achieve higher pumping rates.

• Foam breaks rapidly after it reaches the pit or tank, so only the liquids comprising the foam solution and contaminants must be handled. Chemical not spent in the foam operation remains in the water phase and can be processed through regular field procedures.

The above advantages can be complemented with others when foams are used in specific operations.

2.1.1 Stimulation

Hydraulic fracturing and acidizing are the most common of the stimulation methods in which foamed liquids are used.


Foams began to be used as a fracturing fluid because of their several desirable properties. Additional to the general advantages cited previously Holcomb (1982), Ward (1986), and Penny (1993) in their papers summarized the following reasons for using foams in fracturing jobs:

• Better fracture extension due to low fluid loss with moderate in-fracture viscosity.

• Foams are good proppant transport agents. This implies a good proppant placement and low proppant settling velocity.

• The absence of solid fines in foams or chemical fluid-loss additives leaves both the formation face and the proppant bed clean and, thus, maximum proppant bed conductivity can be utilized.

• Both clean-up time and costs are reduced tremendously. The short time the fluid is on the formation reduces the time of chemical reaction with the formation and clay absorption of the water.

• Foams are energized fluids. Their energy aids in the return of the pumped fluids.

• Foams offer low hydrostatic head for works in depleted zones.

• Other advantages include: quick fracture cleanup that offers quick evaluation of results compared with the conventional fluids, limited use of onsite water storage, the number and size of pumping equipment are reduced.

In acid fracturing, a major disadvantage in the use of acid is its high leak-off rate and quick spending time. The use of a foamed acid fracturing fluid controls accelerated leak-off without the help of additional fluid loss additive.

2.1.2 Completion Work

Foams are used as underbalanced completion fluids. The inherent advantages of reduced fluid loss and increased productivity make foam a viable alternative in many completion jobs.

Foamed fluids are used for gravel packing. Gravel packs placed with foamed systems exhibit similar or better characteristics than conventional systems. These characteristics include increased gravel-carrying capacity, porosity and permeability, after pack settling of slots or wire-wrapped openings in the liner as well as the prevention of plugging them [Ward (1986)]. Advantages of foam are again clear in these jobs. Loss of circulation is reduced and generally this system costs less than conventional gravel packs. In liquid-sensitive


formations, dry foam systems help to eliminate permeability damage and clay swelling.

Matrix acidizing is also a completion process. Matrix acidizing is used to remove damage near the wellbore without creating a fracture. One benefit of using foamed acid in matrix acidizing is the use of foam energy to get a good and quick clean up of the undissolved fines created during the treatment. Additionally, foamed acid uses less acid volume than conventional matrix acidizing and deeper penetration is achieved.

2.1.3 Drilling

Foam is also utilized as a drilling fluid because it has many of the beneficial characteristics of conventional drilling fluid. Also, the popular underbalanced drilling technique can be carried out successfully using foam as a drilling fluid. Drilling with foams presents the following advantages:

• Offers low hydrostatic pressure to drill under-pressurized formations reducing lost circulation, filtrate and solid invasion.

• Foams have high carrying solid capacity to guarantee excellent transport of drilling cuttings out of the well bore.

• Reduces drill string torque and drag.

• They have excellent insulation properties, low heat capacity and poor heat conductance. These properties are desirable when drilling in severe low or high temperature conditions.

2.1.4 Foams for EOR and Control Injection Profile

Foam fluids have been used as diverting agents. The function of a diverting agent is to enlarge the coverage of the treatment across the zone of interest. The first usage of foams as diverting agents was in matrix acidizing. Because bulk foam exhibits non-Newtonian behavior, this property is used to alter the treatment fluid injection profile. The changes of foam properties with pressure and temperature are advantageous when treating intervals with more than one zone having different pressures. In the lower pressure zone, the foam will have a higher quality and potentially more diverting effects. EOR area has indicated that foams can be used to alter the fluid/gas flow contrasting permeabilities in oil and gas wells. The goal in diversion is to achieve equal flow in all zones per unit area.

Steam-foams for heavy oil and bitumen recovery is an additional use of foam systems in EOR. The steam-foam process, which consists of adding surfactants with or without noncondensible gas to the injected steam, was


developed to improve sweep efficiency of steam drive and cyclic steam process.

2.1.5 Other Uses

Foamed fluids are also used in a variety of oil field operations that include:

• Coring: The reduced hydrostatic pressure, formation invasion and damage that foams offer during drilling makes foam an excellent fluid to use during coring operations. Core will be recovered with little or no invasion of circulating fluid.

• Remedial work: Foam fluids have been used in stimulation of wells in combination with other chemicals or thermal energy, fishing operations, circulation of inner liners, pulling, and reinstalling liners, clean outs [Hutchinson (1969 and 1972), Beyer (1972)].

• Handling H2S and Steam. Foam containing an excess of alkali metal base has been used to neutralize dangerous produced gases, such as hydrogen sulfide. Also, the low heat conductivity of foams made them desirable circulating fluids in high temperature steam injection wells.

2.2 LIMITATIONS OF FOAM SYSTEMS

Use of foams also include some disadvantages:

• Its low hydrostatic head may not be sufficient in deep wells to keep the pump pressure within safe working limits.

• Difficulties when carrying high proppant concentration.

• Proper tubing and casing design must be implemented to allow for the application of any stimulation treatment.

• Usually higher cost. Nitrogen requirements above 4000 SCF/bbl of liquid could exceed the practical job cost limits.

• There could be instability of the foam, the foaming agents, and other surfactants at well conditions (high pressures, temperatures, and time).

• There are still a lot of uncertainties associated with the properties of foams.

2.3 THEORY OF FOAMS

For the purpose of this investigation foams are defined as a special kind of colloidal dispersion in which a gas is dispersed in a continuous liquid phase. Although the colloidal systems have particle sizes ranging from 1 to 1000


microns, the gas bubbles in general may vary in size and in many practical occurrences foam bubbles sizes may exceed this range. The gas phase is also known as the internal or dispersed phase, and the liquid phase is also known as the external or continuous phase. For the petroleum industry foams may contain not only gas and liquid phase, but also solid particles, and even other liquid drops such as oil.

Preparation of foams in the petroleum encompasses the use of three basic components: Liquid phase, gas phase, and a foaming agent. Water, brines, hydrocarbons, acids, alcohol, and polymers (gels) are among the fluids used as the continuous phase. Air, nitrogen, carbon dioxide, and a combination of nitrogen and carbon dioxide have been used as the gas phase.

The primary foaming agents used to generate foams are surfactants. Surfactants consist of molecules having a hydrophilic group attached to a long hydrophobic tail. They are classified according to the nature of the hydrophilic group. Ionic, cationic, nonionic, or amphoteric surfactants can be used as the foaming agent. Surfactant molecules orient themselves so that the hydrophilic group is in an aqueous environment and the hydrophobic tail is in a non-aqueous environment. Hence they will concentrate at the liquid-gas interfaces of foams. In this manner, they may increase or decrease the surface tension of the liquid and may strengthen or weaken the bubble walls. So, not all surfactants will act as foaming agents; some will act as defoamers.

2.3.1 Foam Quality and Foam Texture

Quality and texture are two specific terms used to characterize foam systems. Quality is defined as the volume percent or fraction of gas in a foam under a specified set of conditions of pressure and temperature. It is commonly expressed as a percentage. For two-phase systems foam quality is calculated using the following equation:

100),(),(

),( xVV

VQ

PTlPTg

PTg

+= ............................ (2.6)

In three-phase systems, when foams contains solid particles, the definition of foam quality has the following form:

100),(),(

),( xVVV

VVQ

sPTlPTg

sPTg

++

+= .................... (2.7)

Foam texture has been considered an important parameter that affects the rheology of a foam fluid. It is a qualitative rather than a quantitative value that gives a description of foam according to its bubble size, shape, and


distribution within the foamed system. Foam quality, absolute pressure, foam generation technique, chemical composition, and shear rate are among the factors that influence foam texture.

2.3.2 Foam Expansion Factor

Some authors use the expansion criterion rather than quality to characterize foam fluids. The expansion factor is the ratio of the foam volume produced to the liquid volume used to make the foam. Its value will be always greater than unity, and mathematically is expressed as:

l

PTf

V

VEF ),(= ............................................. (2.8)

2.3.3 Foam Stability

It is the ability of foams to resist bubble breakdown resulting from bubble collapse or coalescence. The most common method to quantify foam stability is the half-life criterion. The half-life is the time required for the foam to drain half of its liquid volume. The method of foam generation, surfactant type, surfactant concentration, liquid phase viscosity, and foam quality are among the factors that affect the foam stability. The experimentally measured half-life is also function of the height of the foam column.

2.3.4 Bulk Foam

The term bulk foam refers to any foam for which the length scale of the confining space is greater than the length scale of the foam bubbles. In other words, bubble sizes are negligible compared to the characteristic diameter of the flow path. This work was made under the concept of macroflow of bulk foams.

2.3.5 Foam Structure

Figure 2.1 [Schramm (1994)] is a two dimensional sketch that shows the general structure and components of a common foam system. The thin liquid films that separates one bubble from the other is called lamellae. The plateau border is the region of transition at which thin fluid films are connected to other thin films or mechanical supports such as a solid surface. In high quality foams, where the gas bubbles are in strong contact, the connection of three lamellae, at an angle of 120°, is known as the Plateau border.

Several investigators agree that foam structure is quality dependent. In low quality foams, the foam bubbles are spherical and do not suffer any kind of deformation [See Figure 2.2, Blauer et al (1974)]. When the foam quality increases, because of the high-volume fraction of the gas phase, the gas


bubbles are distorted in the form of polyhedra separated by thin films. Blauer et al (1974) divided the range of foam quality into regions on the basis of bubble interactions. Figure 2.2 schematically depicts in two-dimensions the structure changes of foams in the three quality regions.

Figure 2.1 Components of a general foam system. Liquid phase, gas phase, lamellae, and plateau border. (After Schramm, 1994)

Figure 2.2 Foam Structure at Various Qualities (After Blauer et al, 1974)

Below a quality of 52%, spherical bubbles may be well dispersed and not in contact with each other. Between the qualities 52% and 74%, bubbles become closely packed and may contact each other during flow, causing bubble interference. However, there is still sufficient liquid phase available to prevent bubble shapes deviating from a spherical shape. Foams in quality range between 74% and 95% are thermodynamically driven to minimize system energy and deform from their spherical state to achieve maximum stability. When the quality of foam is increased above 95%, the foam no longer remains stable and becomes a mist. Therefore, the structure of foams depends on the volume fraction of the dispersed phase and varies from dispersed spheres to close-packed spheres and dodecahedral foam structure as the volume fraction of the gas phase increases. In addition to the volume fraction, the structure of


the foam also depends on the contact angle between the thin films and the adjacent Plateau borders.

2.3.6 Foam Generation Techniques

The main techniques of foam generation used in the petroleum industry can be grouped into three methods:

• Additions of the gas phase to a flowing stream of liquid/surfactant mixture, which then passes through some type of inline mixing device. This mixing device has taken the form of a length of pipe packed with sand, core material, screens or steel wool.

• Injection of the gas phase through a restricted diameter pipe (foam generator) into a flowing stream of the liquid mixture. The shear associated with the restricted gas flow through the foam generator is believed to be responsible for foam generation.

• Addition of the gas phase to a circulating loop containing the liquid/surfactant mixture under pressure until the desired quality is obtained. The shear imparted by the inline pump and the tubing are believed to be responsible for the generation of foam.

The last foam generation technique mentioned above was used during the present investigation. Although the gas bubbles in the foamed fluids may vary in size from approximately 50 µm to 1200 µm in pipe flow conditions, Harris (1985) considered that foam texture is shear dependent and will reach equilibrium at a particular shear rate where the foams appear to have uniform size bubbles.

2.4 LITERATURE REVIEW

Mitchell (1969) has been one of the leading investigators of foam rheology applied to petroleum industry. He studied foamed fluids prepared using water as the liquid phase and air as the gas phase. Foam flow properties were investigated using capillary viscometers of various diameters at an average pressure of 1000 psi. In his foam rheology investigation he reported that foam viscosity was independent of shear rate for quality less than 54%. This means that foam behaves like Newtonian fluid in this range. For qualities greater than 54%, foam viscosity was dependent on shear rate and quality and the foam fluid behaved like Bingham Plastic fluid in this range. For quality greater than 97% the system converts to slug flows or mist flow. He also found that slippage at the wall does not occur for the tested foamed system. Foam viscosity was related to its quality and to the viscosity of the liquid phase by,


( )Qlf 6.31+= µµ ...................................... (2.9)

where,

µf = Foam viscosity, cp.

µl = Liquid viscosity, cp

Q = Foam quality, fractional.

Mitchell determined values for yield point and plastic viscosity that increase with foam quality. These are shown in Figure 2.3.

0

2

4

6

8

10

12

0 0.2 0.4 0.6 0.8 1

Quality, fraction

App

. Vis

cosi

ty, c

P

0

0.5

1

1.5

2

2.5

Yiel

d Po

int,

lbf/f

t²

Apparent Viscosity

Yield Point

Dispersed Bubbles

Foam Region

Mist Region

Bub

ble

Def

orm

atio

n

Bub

ble

Inte

rfer

ence

Figure 2.3 Plastic Viscosity and Yield Point of Foam as a Function of Quality (After Mitchell, 1971)

Harris et al (1987 and 1990) did high-temperature rheological studies of foam fracturing fluids. They prepared foams using nitrogen in the gas phase and HPG from 0 to 80 lb/Mgal in water in the liquid phase. Their rheometer was a recirculating foam flow-loop with 0.305-in. and 0.18-in. ID stainless steel tubing. Temperature ranged from 75 °F up to 300 °F and pressure of 1000 psi. They came up with the following conclusions: Foam fluids did not thin as rapidly as base gel fluids when temperature was increased. The tested foamed fluids behave as yield pseudoplastic fluids that can be described using


the Herschel-Bulkley model. The foam yield stress τo (in lbf/ft2) was found to be a function of foam quality and the viscosity of the base fluids.

For low viscosity base fluids with n > 0.9,

If quality > 0.6 then,

( )Qo 9exp00008.0=τ ................................. (2.10)

If quality ≤ 0.6 then,

Qo 03.0=τ ............................................... (2.11)

For higher viscosity base fluids with n ≤ 0.9,

6.007.0 ≤= QforQoτ ........................... (2.12)

and

( ) 6.09exp0002.0 >= QforQoτ ............. (2.13)

The foam consistency index (K) is a function of consistency index of the liquid phase, the quality, and empirical constants.

( )( )[ ] ( )21275 75.0exp75018.0exp QQCTQCKK f +−−= …….(2.14)

( ) 8.1751 4 nC = ............................................. (2.15)

( )[ ]752 31.3exp nC +−= .............................. (2.16)

The flow behavior index (n) of the foamed system was assumed to be the same as that of the liquid phase, but varied with temperature. These temperature variations of the flow behavior index were found to be a function of quality.

( )( )[ ]750019.00028.0exp75 −−= TQnn f ..... (2.17)

where,

C1 = Constant in foam consistency index.

C2 = Constant in foam consistency index.

T = Foam temperature, °F.


Q = Foam quality, fraction.

n75 = Flow behavior index of liquid phase at 75 °F.

nf = Foam flow behavior index.

K75 = Consistency index of liquid phase at 75 °F, lbf.sn/ft2.

Kf = Foam consistency index, lbf.sn/ft2.

τo = Foam yield stress, lbf/ft2.

No wall slip effects were found comparing their experimental data from 0.18-in. ID pipe with mathematical model derived when 0.305-in. ID pipe was used. Their empirical equations were said to describe foam rheological behavior well from 75 °F to 300 °F, 0 to 80 % quality, and containing 0 to 80 lb/Mgal of HPG in water.

3. DATA PROCESSING AND ANALYSIS

For the liquid data obtained from Bohlin viscometer and the liquid and foam fluid data gathered in the foam loop, the standard procedure for rheological analysis was followed. This procedure was taken from the API RP 39: Standard Procedures for Evaluation of Hydraulic Fracturing Fluids (1983).

3.1 RHEOLOGICAL AND FRICTION LOSS DATA FOR LIQUID PHASE AND FOAM FLUIDS FROM LOOP

Data obtained from loop tests included time, differential pressure, temperature, fluid density, absolute pressure, and flow rate. Only data obtained under steady state conditions were used in the calculations. The time reading was used as a reference to determine the steady state conditions and screen the useful data from the raw data. Since the gathered experimental values represent the properties of the fluid under circulation at a given conditions of pressure, temperature, and flow rate, formulas and principles may be applied to estimate the flow properties at the flow conditions.

For the case of liquid phases under conditions of steady, fully developed flow through the pipes, shear rate at the wall can be calculated using the following equation,

dv

w8

=γ ..................................................... (4.1)

and shear stress at the wall is obtained from,


LPd

w 4∆τ = ................................................... (4.2)

where,

τw = Shear Stress at the wall

γw = Shear rate at the wall

v = Fluid Velocity

d = Pipe Diameter

∆P = Differential Pressure.

L = Pipe Length.

For the case of foam fluids, the raw data processing begins with the calculation of foam quality based on the known conditions of temperature, pressure, and fluid density. The flow loop is a closed system; hence, the total mass in system remains constant in every test. The total mass in the foamed system is equal to the sum of the mass of liquid and the mass of gas, and the total volume of foamed fluid is equal to the sum of the liquid volume and the gas volume.

During this investigation the nitrogen gas was used as the internal phase of the foamed fluid. As a gas, nitrogen density is a function of the average pressure and temperature of the system. The equation of state was used to calculate gas density. For the case of nitrogen, density can be expressed as,

ZRTMP

N =2

ρ ................................................. (4.3)

Nitrogen z-factor was calculated using a table given by Eilerts et al (1948). This table shows the z-factors as a function of pressure and temperature. It was considered that the volume of gas entering into solution with the liquid phase was negligible.

Water, the foaming agent, and the gelling agent compose the liquid phase of the foam. Due to both the small concentration of foaming agent and foaming agent density value close to water density, the effect of the surfactant on liquid density was not considered when calculating liquid phase density. Hence, the liquid density was obtained from a weighted average of the water and gelling agent densities, which can be summarized as follows,


Guarw

GuarwL VV

mm++

=ρ ....................................... (4.4)

The Micromotion flow meter determined the fluid density of the flowing fluid. Hence, foam density was obtained directly from the reading given by the flow meter. Because the value of foam density is used to calculate the foam quality, it is convenient to recall the equation that defines foam density,

2

2

NL

NL

f

ff VV

mmVm

+

+==ρ .................................... (4.5)

Foam quality is defined as the percentage of gas volume contained within the foam.

100),(),(

),( xVV

VQ

PTlPTg

PTg

+= ............................ (4.6)

Using the definition of density to express the mass of liquid and nitrogen in terms of volume, Eq. 4.5 can be written as,

( )222 NNLLNLf VVVV ρρρ +=+ ..................... (4.7)

Solving Eq. 4.6 for liquid volume, we get

( )2

1NL V

QQV −

= ......................................... (4.8)

Substituting Eq. 4.8 into Eq. 4.7, and solving for foam density,

( ) QQ NLf 21 ρρρ +−= .................................... (4.9)

Solving Eq. 4.9 for foam quality, the following expression is obtained for foam quality as a function of liquid, foam and nitrogen density at a given condition of pressure and temperature:

1002

xQNL

fL

ρρρρ

−

−= ........................................ (4.10)

lodos fdo bonilla foams.pdf

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