Development of Aphron Drilling Fluids (Desarrollo de Fluidos de Perforacion de
Afrones). Frederick B. Growcock, Gerard A. Simon and Anthony B. Rea.
Development of Aphron Drilling Fluids (Desarrollo de Fluidos de Perforacion de Afrones) Frederick B. Growcock, Gerard A. Simon and Anthony B. Rea V SEFLU y CEMPO, Margarita, May 24-28, 2004
ABSTRACT A drilling fluid was recently introduced that contains specially designed micro-bubbles, or “aphrons”. This microbubble technology has been employed effectively worldwide, and especially in Latin America, to seal problematic formations that have very high permeability and microfractures. Currently there are three aphron drilling systems in various stages of development: APHRON ICSTM, EMS2100, and POLYPHRON ICSTM. Each fluid is specifically designed to improve filtration control, reduce ECD’s and mitigate lost circulation. A polymer/surfactant package was recently added to the water-based systems to increase the longevity and pressure resistance of the bubbles, while the oil-based system has an added surfactant to achieve like results. In this paper, the authors will present the design and properties of aphron drilling fluids, and discuss how the fluid characteristics work together to provide superior performance in drilling operations that have a high risk of lost circulation. INTRODUCTION Aphron drilling fluids have been successfully used in 300+ applications worldwide to drill depleted reservoirs in mature oil and gas fields, high-permeability formations and microfractured rock. These fluids serve as a successful and cost-effective alternative to UBD for avoidance of whole fluid loss and differential sticking.1-4 There are two chief attributes of these fluids that permit a decrease of fluid invasion and damage to the formation. First, the properties of the base fluid are such that upon entering a loss zone, the flow rate decreases dramatically. This occurs because the bulk fluid is very highly shear-thinning and possesses a very high LSRV (Low-Shear-Rate Viscosity). Second, very tough and flexible microbubbles are incorporated into the bulk fluid with conventional mud mixing equipment. These highly stabilized bubbles, or “aphrons,” are essential to sealing the problem area by forming an internal bridge that acts as a loss circulation material. Currently, there are two water-based aphron drilling fluids, APHRON ICSTM and EMS-2100, and one oil/synthetic-based aphron drilling fluid, POLYPHRON ICSTM.
In this paper, we discuss the general
characteristics of the three aphron systems, as well as recent advances in the technology.
THE APHRON STRUCTURE Water-based aphrons, as found in APHRON ICSTM and EMS-2100 systems, consist of two essential elements: a spherical core of air and a protective outer shell.5 Water-based aphrons have a trilayer surfactant shell, Fig. 1, enabling it to be much more robust to temperature and pressure than a standard air bubble, which is stabilized by only a monolayer of surfactant. Around the air core, an inner surfactant film is enveloped by a viscous water layer, and an outer bilayer of surfactants provides rigidity and low permeability to the structure while imparting some hydrophilic character to. Under quiescent conditions, the structure is compatible with the aqueous bulk fluid, but when enough shear or compression is applied to the aphron, e.g. when bridging a pore network, the outermost shell layer is stripped, rendering the aphron hydrophobic.5 Oil-based aphrons (in POLYPHRON ICSTM systems) are similar in structure, as shown in Fig. 2, but do not contain the outermost surfactant layer: around the air core, an inner surfactant film is overlayed by a viscous water layer and a single outer layer of surfactant.6 Aphrons act as a unique bridging material, forming a micro-environment in a pore network or fracture that appears to behave in some ways like a foam, and in other ways like a solid, but flexible bridging material. As is the case with any bridging material, concentration and size of the aphrons are critical to the mud’s ability to seal thief zones. Aphrons are created and entrained in the bulk fluid with standard mud mixing equipment, which reduces the safety concerns and costs associated with high-pressure hoses and compressors commonly utilized in air or foam drilling.7 Although each application is customized for the individual operator’s needs, the mud system is generally designed to contain 12-15% by volume air. Aphrons are thought to be sized or polished at the drill bit to achieve a size of 15-100 µm diameter, which is typical of many bridging materials DRILLING PROPERTIES OF APHRON-BASED FLUIDS Tables 1a, 1b, and 1c illustrate typical APHRON ICSTM, EMS-2100, and POLYPHRON ICSTM formulations, respectively. The major difference between the two water-based systems lies in the type of primary viscosifier, which is polymer-based in the APHRON ICSTM system and clay-based in the EMS-2100 system. Both systems consist of viscosifiers, pH control additives, aphron generator and stabilizer, and filtration-control additives. Table 1c illustrates a typical formulation for the POLYPHRON ICSTM system. This oil/synthetic-based system can utilize either organophilic clay or polymer as the primary viscosifier, and it also contains an aphron generator and stabilizer. Standard API rheological properties for the APHRON ICSTM and EMS-2100 systems (Table 2, 70°F, hot-roll 150°F) and for the POLYPHRON ICSTM system (Table 3, 70°F, hot-roll 250°F) were acquired in the laboratory. Also included are two properties important for all aphron drilling fluids: LSRV (Brookfield Viscometer, 70°F,
L3 spindle, 0.06 sec -1) and “Half-Life” (3 hr). The Half-Life is a relative measure of the stability of the aphrons; a simple procedure for calculating Half Life is given in Appendix A. The rheological properties of the two water-based systems are very similar, although the LSRV and half-life are higher in the APHRON ICSTM system, while the fluid loss is lower in the EMS-2100 system. APHRON CHARACTERIZATION STUDY The U.S. Department of Energy recently awarded a two-year grant to MASI Technologies
LLC,
a joint venture
8
between M-I LLC and ActiSystems, Inc. This two-phase project will determine how aphron drilling fluids seal permeable and fractured wellbore rock with minimal risk of formation damage. Various techniques are being used in this project to better understand and characterize aphrons. These include (1) Acoustic Bubble Spectrometry,9 a technique which enables bubble analysis in opaque fluids; (2) Sight Flow Pressurization, which allows for visualization of aphrons while varying pressures; (3) Environmental Scanning Electron Microscopy, to visualize flow of aphrons in a pore network; (4) Pressure Transmissibility, which permits measurement of the pressure loss and speed of pressure transmission through an aphron micro-environment; (5) Interfacial tension and contact angle goniometry, to quantify the hydrophobic nature of the aphron shell that is revealed when aphrons aggregate in a fracture or pore network; (6) Air Diffusivity, which enables determination of the rate of loss of air from aphrons at elevated pressures; and (7) Triaxial Loading Core Leak-Off, to test at elevated temperature and pressure the sealing capability of aphron drilling fluids through various types and lengths of rock core and the formation damage potential of the fluids.10 HOW APHRON DRILLING FLUIDS MINIMIZE FLUID LOSSES Aphron drilling fluids reduce fluid invasion both rheologically and mechanically. The base fluid has a very high LSRV, as stated earlier, yet its viscosity at high shear rates is unusually low. Thus, equivalent circulating density (ECD) is quite low and the potential for fracture initiation and propagation is also low. In addition, the base fluid has relatively low thixotropy, as evidenced by the similarity between the 10-min and 10-sec gel strengths. Thus, when fluid suddenly enters a low-shear-rate region, viscosity builds very rapidly. In contrast, typical clay-based fluids are very thixotropic, as evidenced by highly progressive gels, and minutes( if not hours) are required for viscosity of a clay-based fluid entering a loss zone to build to the high level required to stem fluid invasion. Mechanical stabilization of the wellbore is also of paramount importance for aphron drilling fluids. Aphrons can form bridges within the loss zone that act as an internal seal to complement the rheological properties of the base fluid. For aphrons to be effective, they must be stable. Aphron stability is accomplished
through control of the size, collision rate and mechanical properties of the microbubbles. The bubble size can be controlled by the amount of shear energy put into the system, along with the surfactant type and concentration. The collision rate is inversely proportional to the bulk fluid viscosity, so that an increase in bulk viscosity decreases the rate of coalescence among aphrons. Not only does the high viscosity of the base fluid itself slow fluid invasion in the loss zone, it also reduces the rate of coalescence and aggregation of the bubbles until they reach the pore throat or fracture tip, at which point they are forced together into a large bubble complex of deformed bubbles. Thus, an internal seal is formed, as shown schematically in Fig. 3.2 This seal may have properties not unlike that of a non-adhering foam, i.e. the bubbles do not wet the pore/fracture walls; consequently, the bubbles are easily flushed back out via formation fluids during production.2 SUMMARY Aphron-based drilling fluids have been used in numerous applications worldwide to successfully control fluid losses in weak and low-pressure zones. These zones are stabilized rheologically and mechanically: the lowthixotropy highly shear-thinning base fluid generates an intrinsic low rate of fluid invasion, while the aphrons form internal seals across pore networks and microfractures. The surface chemistry of these aphrons is such that the internal seal is capable of being broken by flow-back of produced fluids. ACKNOWLEDGMENTS The authors thank the managements of M-I
LLC
and MASI Technologies
LLC
for permission to present this
paper. NOMENCLATURE ECD = equivalent circulating density UBD = underbalanced drilling LSRV = low-shear-rate viscosity HTHP = high-temperature high-pressure REFERENCES 1. Montilva, J., Ivan, C.D., Friedheim, J. and Bayter, R.: “Aphron Drilling Fluid: Field Lessons From Successful Application in Drilling Depleted Reservoirs in Lake Maracaibo,” OTC 14278, presented at the 2002 Offshore Technology Conference, Houston, May 6-9, 2002.
2. Growcock, F.B., Simon, G.A., Rea, A.B., Leonard, R.S., Noello, E. and Castellan, R.: “Alternative AphronBased Drilling Fluid,” IADC/SPE 87134, presented at the 2004 IADC/SPE Drilling Conference, Dallas, Mar. 2-4, 2004. 3. Brookey, T., Rea, A. and Roe, T.: “UBD and Beyond: Aphron Drilling Fluids for Depleted Zones,” presented at IADC World Drilling Conference, Vienna, Austria, Jun. 25-26, 2003. 4. Growcock, F.B., Simon, G.A., Guzman, J., and Paiuk, B.: “Applications of Novel Aphron Drilling Fluids,” AADE-04-DF-HO-18, presented at the AADE 2004 Drilling Fluids Conference, Houston, TX, Apr. 6-7, 2004. 5. Sebba, F.: Foams and Biliquid Foams – Aphrons, John Wiley & Sons Ltd, Chichester (1987). 6. Growcock, F.B., Khan, A.M. and Simon, G.A.: “Application of Water-Based and Oil-Based Aphrons in Drilling Fluids,” SPE 80208, presented at SPE International Symposium on Oilfield Chemistry, Houston, Feb. 5 – 8, 2003. 7. White, C.C., Chesters, A.P., Ivan, C.D., Maikranz, S. and Nouris, R.: “Aphron-Based Drilling Fluid: Novel Technology for Drilling Depleted Formations,” World Oil, vol. 224, no. 10 (Oct. 2003). 8. United States Department of Energy press release, “New 'Smart Drilling' Projects Promise Lower Cost, More Reliable Gas Drilling,” Sept. 8, 2003. 9. Duraiswami, R., Prabhukumar, S. and Chahine, G.L.: “Bubble Counting Using and Inverse Acoustic Scattering Method,” J. Acoust. Soc. Am., vol. 104 (1998) 2699. 10. Ivan, C.D., Growcock, F.B. and Friedheim, J.E.: “Chemical and Physical Characterization of AphronBased Drilling Fluids,” SPE 77445, presented at the 2002 SPE Annual Technical Conference and Exhibition, San Antonio, Sept. 29 – Oct. 2, 2002. SI Metric Conversion Factors bbl
X 0.159
= m3
cP
X 1.00
= mPa-s
°F
X (°F-32) X 5/9 = °C
ft
X 0.3048
=m
gal
X 0.00379
= m3
in
X 0.0254
=m
lb
X 0.454
= kg
lb/bbl
X 2.853
= kg/m3
lb/gal
X 119.8
= kg/m3
lb/gal
X 0.120
= Specific Gravity (SG)
lbf/100 ft2
X 0.478
= Pa
psia
X 6.895
= kPa
APPENDIX A – HALF-LIFE OF ENTRAINED AIR2 The Half-Life method serves as a simple way to determine bubble stability of aphron-based drilling fluids. The calculation assumes that the rate of loss of entrained (undissolved) air follows standard first order kinetics, as in the case of a true foam. Although aphrons are better characterized as dispersed bubbles rather than foams and their rate of decay is not strictly first order, experience indicates that the Half-Life is a fair trend indicator of bubble stability. First determine the “initial” amount of entrained air in the mud using the following expression: % Airi = [(dt – di)/dt] x 100 where dt is the theoretical density of the air-free mud and di is the initial density after the aphron generation step. Determine the “final” amount of entrained air in the mud after some arbitrary period of time, tf, e.g. 3 hr, 24 hr: % Airf = [(dt – df)/dt] x 100 The rate coefficient for loss of air from the mud, KAir, is given by KAir = (tf – ti)-1 ln (% Airi/%Airf) = tf-1 ln (dt – di)/(dt – df) where ti = 0 and df is the “final” mud density after the desired waiting period. Note that the Half-Life for decay of the entrained air, τ1/2, is simply equal to τ1/2 = ln 2 / KAir = 0.693/KAir
Table 1a. Formulation of APHRON ICSTM System Function
Concentration
Component Fresh water/brine
Continuous phase
0.97 bbl
Soda ash GO DEVIL II ACTIVATOR I
Hardness Buffer Viscosifier System Stabilizer
3 lbm/bbl 5 lbm/bbl 5 lbm/bbl
ACTIVATOR II BLUE STREAK Biocide EMI-779 EMI-780 EMI-802 ACTIGUARD
pH control Aphron Generator Biocide Aphron Stabilizer Aphron Stabilizer Aphron Stabilizer Mud Conditioner and Shale Stabilizer Oligomer* Defoamer *Optional Component
0.5 lbm/bbl 2 lbm/bbl 0.05 gal/bbl 0.5 lbm/bbl 0.5 lbm/bbl 0.3 lbm/bbl 1 lbm/bbl As Needed
Table 1b. Formulation of EMS-2100 System Component
Fresh water/brine Soda ash Caustic Soda EMI-781 EMI-782
Function
Continuous phase Hardness Buffer Alkalinity Control Agent Viscosifier Filtration Control System Stabilizer BLUE STREAK Aphron Generator Biocide Biocide EMI-779 Aphron Stabilizer EMI-780 Aphron Stabilizer EMI-802 Aphron Stabilizer ACTIGUARD Aphron Enhancer and Shale Stabilizer Oligomer* Defoamer *Optional Component
Concentration
0.97 bbl 0.25 lbm/bbl 1.5 lbm/bbl 25 lbm/bbl 2 lbm/bbl 0.5 lbm/bbl 0.05 gal/bbl 0.5 lbm/bbl 0.5 lbm/bbl 0.3 lbm/bbl 1 lbm/bbl As Needed
Table 1c. Formulation of POLYPHRON ICSTM System Function
Component
Oil/Synthetic Fluid TriVis TRIVIS CP+ MICRODYNE Water
Continuous phase Viscosifier Secondary Viscosifier Aphron Generator Polar Activator
Concentration
0.97 bbl 25 lbm/bbl 2 lbm/bbl 3 lbm/bbl 10 lbm/bbl
Table 2. Standard Properties of Typical APHRON ICSTM and EMS-2100 Systems Additive Water Soda Ash Caustic Soda GO DEVIL II ACTIVATOR I EMI-781 EMI-782 ACTIVATOR II BLUE STREAK EMI-779 EMI-780 ACTIGUARD Std API Viscosity 600 300 200 100 6 3 PV YP Gel 10 sec Gel 10 min
Units mL/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl g/lab bbl
LSRV (0.06 sec-1) API Fluid Loss Half-Life
APHRON ICSTM 338 3
EMS-2100 331 0.3 1.5
5 5 25 2 2 1 0.5 0.5 1
0.5 0.5 0.5 0.2
° ° ° ° ° ° cP lb/100ft2 lb/100ft2 lb/100ft2
94 78 70 60 39 36 16 62 39 58
84 63 53 42 23 22 21 42 30 46
cP mL/30 min hr
192,000 7.1 152
130,000 14 108
Table 3. Standard Properties of Typical POLYPHRON ICSTM System Before HotAfter HotAdditive Units Rolling Rolling, 185 F Diesel lab bbl 0.97 0.97 TRI-VIS g/lab bbl 15 15 TRI-VIS CP+ g/lab bbl 2 2 MICRODYNE g/lab bbl 3 3 Water lab bbl 0.03 0.03 Std API Viscosity 600 ° 34 41 300 ° 28 25 200 ° 16 19 100 ° 11 13 6 ° 6 8 3 ° 3 6 PV cP 6 16 2 YP lb/100ft 22 9 2 Gel 10 sec lb/100ft 3 6 2 Gel 10 min lb/100ft 3 6 -1 LSRV (0.06 sec ) cP 63,500 72,800 HTHP Fluid Loss* mL/30 min -2 Half-Life hr -5 Electrical Stability V 984 1,130 * 185 deg F
Weakly Bound Surfactant
WaterWater-Based Mud
Air
Surfactant BiBi-Layer
Viscosified Aqueous Layer
Figure 1. Schematic of Water-Based Aphron
Surfactant Molecules
OilOil-Based Mud
Air
Viscosified Aqueous Layer
Figure 2. Schematic of Oil-Based Aphron Annulus Annulus
Formation TransitionZone Zone Transition
HigherPressure PressureZone Zone Higher
pp
Lower Pressure Zone
Aphrons Aphrons or or Polyphrons Polyphrons
Figure 3. Formation Invasion by Aphron Drilling Fluid
