Proppant Transport & Screenout Behavior. R.D. Barree

Transcription

1 Proppant Transport & Screenout Behavior R.D. Barree

2 In this session Look at traditional proppant transport and its assumptions Look at common remedies for early screenout What factors actually affect proppant transport?

3 Proppant Transport & Settling Simple models assume 1 D flow and model single particle settling (Stokes Law) Fluid and particle velocity profiles are much more complex in 3 D flow. Slurry density gradients cause gravity under running, fluid shear, and non homogeneous concentration profiles. Fluid properties and leakoff cause transverse particle migration Lateral particle motion changes transport and leads to screenout

4 Traditional Prop Transport Suspended proppant slurry (uniform concentration) Clean pad fluid to create w=3-6xd Frac height (assumed to be constant) Fracture half-length Settled sand bank

5 Common Assumptions: Fluid Loss/Transport/Screenout Proppant is homogeneously distributed Vertically, laterally, transversely Sand and fluid travel together Pad is required to open width for sand Pad is depleted by leakoff Screenouts caused by prop bridging Prop concentration increased by leakoff False assumptions lead to failed remedies

6 Common Remedies for Early Screenout (If caused by pad depletion and bridging) Pump more pad volume Increase pump rate Use higher viscosity fluids Use smaller proppants Use fluid loss additives Sometimes they work, and sometimes NOT!

7 Factors Affecting Proppant Transport Particle velocity profile in fracture Concentration distribution across fracture width Slurry viscosity increase with solids addition Single particle Stokes settling velocity Hindered particle settling Convection from slurry bulk density gradients Proppant holdup Proppant bridging

8 Velocity Distribution of Particles Between Parallel Plates 1.2 Cum. Particle Count For a uniform particle distribution, the velocity profile is given by the cumulative frequency plot Particle Velocity, cm/sec

9 Particle Velocity Profiles Normalized to Fluid Velocity 1.2 Cum. Particle Count Cv=0% Cv=10% Cv=25% Cv=35% Cv=55% Relative Particle Velocity

10 Proppant not Homogeneously Distributed Velocity, cm/sec Particles at low concentration Particles at high concentration Normalized Slot Width

11 Single Particle Settling Velocity Predictions Terminal settling velocity for a single particle in an infinite fluid body: Stokes Law for laminar flow Allen s Equation for transition flow Newton s Equation for turbulent flow Terminal velocity can be modified for multiple particle interactions. Wall effects can be considered for narrow channels.

12 Single Particle Terminal Settling Velocities Stokes laminar flow regime Allen transition flow regime v t v t = g 0.20d = ( ρ ρ ) s 18μ 1.18 l d 2 ( g( ρ ρ ) ρ ) ( μ ) ρ l s 0.45 l l 0.72 Newton turbulent flow regime v t = 1.74d 0.5 g ( ρ ρ ) s ρ l l 0.5

13 Single Particle Settling Rates in a 1.0 cp Newtonian Fluid 100 Settling Velocity, cm/sec Mesh Size Stokes Allen Newton Actual Particle Diameter, inches

14 Single Particle Settling Rates in a 55.0 cp Newtonian Fluid 100 Settling Velocity, cm/sec Mesh Size Particle Diameter, inches Stokes Allen Newton Actual

15 Slurry Settling Experiments in a Vertical Slot Model Parallel plate model 5 feet x 6 inches x 0.25 inches 30% and 40% PEG solutions 30/50 mesh and 95 mesh silica sand slurries Volumetric concentrations from 0 55% solids Slurry settling velocity compared to Stokes velocity

16 Slurry Settling Rates Controlled by Bulk Density Gradients 100 Settling Velocity, cm/sec Vmeas(95) Vmeas(40) Stokes(40) Stokes(100) Vcalc Solids Concentration, Cv

17 Proppant Movement by Bulk Flow or Convection Convection[Phys]: Transmission of energy or mass by a medium involving movement of the medium itself. McGraw Hill Dictionary of Scientific and Technical Terms, Fourth Edition v s w 2 = 12 μ a ( ) Δρ gh + P g f For fluid flow between parallel plates. z f c

18 Thin Fluid Transport is Different From Suspension Transport Proppant drops out of fluid quickly. All solid transport is in a thin traction carpet. Bank height builds to an equilibrium based on fluid velocity. A clear fluid layer is maintained above the settled bank. The bank advances by dune building.

19 Video of Slick Water Sand Transport

20 Proppant Bridging and Screenouts Proppant particles bridge in a circular orifice 3 6x the particle diameter Particles bridge in a slot when the gap equals the largest particle diameter Stable Bridge Unstable-Flow Dismantles Bridge

21 Variable Width Slot Apparatus Model Width=18 Variable Slot Width Fracture Channel Width = 0.3 Slurry Inlet 4-12 ppa ΔP Slurry Outlet Viewing direction in video

22 Slot Bridging Video Flow is left to right Borate x linked Guar fluid 8 ppa 20/40 Ottawa sand slurry Black 16/30 ceramic markers Slot width equals maximum (16 mesh) particle diameter

23 Proppant Bridging Video Copyright B&A

24 Summary of Bridging Studies Bridge stability in holes and slots is different Slurries up to 16 ppa were pumped through a slot 1+ particle diameter wide Proppant bridges are permeable and transmit fluid pressure Slight opening of fracture width releases bridge Bridging alone is a temporary and ineffective screenout mechanism

25 Annular Flow Apparatus Bottom - Fluid In Top Fluid Out Frac fluid pressure Leakoff path Internal pressure Outer wall Annular gap Inner wall Fluid loss

26 Proppant Transport in the Presence of Fluid Leakoff Concentration profile across slot Transverse velocity from fluid loss Migration of entrained particles Force balance on particles at the wall

27 Particles Held Dynamically at the Fracture Wall Particles are pulled to the leakoff site Transverse fluid velocity generates lift and drag Leakoff velocity imposes stabilizing gradient

28 Sand Node Formation Flow is from bottom to top Fluid is typical of crosslinked guar Velocity is 1 2 fps Prop concentration is 1.5 ppa Fluid efficiency is >90%

29 Node Formation at Fracture Leakoff

30 Sand Accumulation: Low Cv Leakoff Volume * Injected ppa Mass of Sand in Node

31 Stable channel flow Node grows in length Fluid velocity in channel erodes sand Channel dimensions become stable Sand held in place dynamically Note effects of inhibiting leakoff

32 Dynamically Stable Channel

33 Proppant Holdup in Fractured Systems Early Injection Slurry injection at low concentration builds islands or nodes of packed sand

34 Proppant Holdup in Fractured Systems Continued Injection Nodes interconnect and leave open channels for all injection minimal pressure rise noted at inlet

35 Proppant Holdup in Fractured Systems Incipient Screenout Entire fracture is packed except for narrow flow channel <1. Screenout occurs suddenly without warning

36 Interaction of Fissure Opening Mechanisms: PDL, Holdup and Storage W/ P PDL and Storage Q=T P/ L V p V f W/ P~YME

37 Effects of Proppant Holdup First proppant in: accumulates at high leakoff sites becomes immobile at the frac wall Later injected fluid: flows in localized high velocity channels is less subject to heat up, aging, breaking remains near injected prop concentration Tracer surveys show first proppant injected remaining at wellbore. Does this indicate localized high leakoff?

38 Effects of Holdup (cont.) Leads to short propped length Yields a non uniform proppant distribution Can cause near well screenouts and perforation plugging Can be linked to proppant induced pressure increases Can substantially affect final conductivity and well performance

39 Example of Proppant Induced Pressure Increase Modeling A 8000 GOHFER Bottom Hole Pressure (psi) GOHFER Slurry Rate (bpm) GOHFER Bottom Hole Pressure (psi) A B A GOHFER Surface Pressure (psi) GOHFER Surface Prop Conc (lb/gal) GOHFER Surface Pressure (psi) A C A B 16 C :00 00:10 00:20 00:30 1/2/1970 Time Proppant Holdup Factor = :40 Customer: Job Date: Ticket #: Well Description: UWI: GohWin v Mar-01 15:21 1/2/1970 0

40 Mitigating Proppant Holdup High viscosity gels Minimize fluid loss Deep invasion of fracture system Possible severe productivity damage Particulate fluid loss additives Must bridge natural fractures Requires low permeability to stop leakoff Can minimize invasion of fractures Altered design philosophy Use clean, non damaging fluids Stay below critical sand input concentration

41 Leakoff Control with 100 mesh

42 Fluid Requirements for Transport Pad volume: Not determined by tip screenout criteria or fluid efficiency Unnecessary in water fracs and slick water jobs How much is enough? Fluid stability How much viscosity do you need? How long should the fluid remain stable What temperature profile should be used in break test design? What are the implications on cleanup and production?

43 Variable Fluid Rheology Leads to Channel Flow and Proppant Bypass High-Leakoff Fluid mobility decreases in areas of high prop conc and low shear Stagnant Fluid Fluid at frac tip is new and cold Fluid travels through small channels at high rate with little residence time or formation contact

44 Modeling Proppant Transport Model time dependent fluid rheology, especially low shear viscosity, during break Track local shear rates, fluid composition, age, and solids concentration Input proppant size and density and track slurry bulk density Check local, time dependent bridging constraints Determine pressure distribution from mobility distribution Feed back pressure distribution to fracture width profile, shear, and velocity profile

45 Closing Comments When jobs are difficult to place it is usually because of proppant transport and impending screenouts The mechanisms that lead to screenout must be understood and evaluated In many cases early screenouts are near well events caused by prop holdup and fissure leakoff These cases can be diagnosed and predicted and appropriate design changes can be made Making design changes based on incorrect models of the screenout process fails to provide solutions and may make things worse Don t be too hasty to blame narrow frac widths or low fluid viscosity