C8 hydraulic fracturing

 Objective: to create highly conductive paths some distance away from the wellbore into the reservoir. o Execution of a hydraulic fracture involves the injection of fluids at a pressure sufficiently high to cause tensile failure of the rock. o At the fracture initiation pressure, often known as the breakdown pressure“, the rock opens. o As additional fluids are injected, the opening is extended and the fracture propagates. o A properly executed hydraulic fracture results in a path, connected to the well, that has a much higher permeability than the surrounding formation.

5/9/2014 Designed & Presented by Mr ĐỖ QUANG KHÁNH, HCMUT 03/2014 Đỗ Quang Khánh – HoChiMinh City University of Technology Email: dqkhanh@hcmut.edu.vn or doquangkhanh@yahoo.com Content & Agenda Ref:  Recent Advances In Hydraulic Fracturing, John L Gidley, Stephen A Holditch, Dale E Nierode & Ralph W Veatch Jr.,1991  Reservoir Stimulation, 3e – Economides & Nolte  Petroleum Production Systems - Economides et al., 1994  Production Operations: Well Completions, Workover, and Stimulation -Thomas O Allen, Alan P Roberts,1984 5/9/2014 Introduction  Objective: to create highly conductive paths some distance away from the wellbore into the reservoir o Execution of a hydraulic fracture involves the injection of fluids at a pressure sufficiently high to cause "tensile failure" of the rock o At the fracture initiation pressure, often known as the "breakdown pressure“, the rock opens o As additional fluids are injected, the opening is extended and the fracture propagates o A properly executed hydraulic fracture results in a "path," connected to the well, that has a much higher permeability than the surrounding formation Introduction o Minimum hydraulic fracturing candidate well selection screening criteria 5/9/2014 LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o Every hydraulic fracture can be characterized by its: – length ; – conductivity; – related equivalent skin effect o In almost all calculations, the fracture length, which must be the conductive length and not the created hydraulic length, is assumed to consist of two equal half‐lengths, xf, in each side of the well - beside, consider the penetration ratio: Ix = xf / xe LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o The dimensionless fracture conductivity: CfD = kf W / k Xf = (Ability of fracture to deliver oil/gas to well)/(Ability of formation to deliver gas into the fracture) > 30 (Infinitely Conductive Fracture) xf w o -Related to Prat’s a (called the relative capacity): CfD = л/2a where:k is the reservoir permeability, k f is the fracture permeability, and w is the propped fracture width o Fracture skin effect varying with fracture conductivity (Cinco-ley and Samaniego, 1981) 5/9/2014 LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o Equivalent skin effect, sf, & Improve Productivity Index J: o The equivalent skin effect, sf: the result of a hydraulic fracture of a certain length and conductivity & can be added to the well inflow equations in the usual manner.=> sf is pseudo skin factor used after the treatment to describe the productivity:  2kh   2kh    J D J      B  ln[ re ]  0.75  s  B  f rw o Prats (1961): the concept of dimensionless effective wellbore radius r’wD in a hydraulically fractured well: LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT  for small values of a, or high conductivity fractures, the r’wD is equal to 0.5, leading to r’w = xf /2; which suggests that for these large-conductivity fractures the reservoir drains to a well with an effective wellbore equal to half of the fracture half-length  Since the effective wellbore must be as large as possible, values of ”a” larger than unity m ust be avoided because the effective wellbore radius decreases rapidly => hydraulic fractures should be designed for a < or CfD > 1.6  for large values of a, the slope of the curve is equal to 1, implying a linear relationship between r’w and a that is approximately r’w = kf w/4k; Which suggest that for low conductivity fractures, the increase in r’w does not depend on fracture length but instead on fracture permeability-width product,which must be maximized 5/9/2014 LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o What length or fracture permeability is desirable in hydraulic fracturing?  • Low‐permeability reservoirs, leading to high‐conductivity fractures, would benefit greatly from length  • Moderate‐ to high‐permeability reservoirs, naturally leading to low‐conductivity fractures, require good fracture permeability (good quality proppant and nondamaging fracturing fluid) Notation rw wellbore radius, m (or ft) r'w Prats’ equivalent wellbore radius due to fracture, m (or ft) f  s f  ln xf rw Cinco-Ley-Samanieggo factor, dimensionless sf the pseudo skin factor due to fracture, dimensionless rw xf Prats' dimensionless (equivalent) wellbore radius But JD is the best 5/9/2014 Pseudo-steady state Productivity Index q  Jp Production rate is proportional to drawdown, defined as average pressure in the reservoir minus wellbore flowing pressure Circular:  2kh   J D p q   B    JD  r  ln  e    s  rw  Drawdown Dimensionless Productivity Index Pseudo-skin, equivalent radius, f-factor J 2kh   r B ln 0.472 e  s f  rw   J or 2kh  r  B ln 0.472 e  r 'w   Prats f (C fD ) J 2πkh  0.472re  x  Bμ ln   s f  ln f  xf rw     2πkh  0.472re  Bμ ln  f xf   Cinco-Ley 5/9/2014 Dimensionless Productivity Index, sf and f and r’w JD  ln 0.472 re  sf rw or JD  ln 0.472 re r 'w Prats f (C fD ) 1 JD   0.472re x  0.472re  f ln   s f  ln f  ln x xf rw  f  Cinco-Ley Factor f (after Cinco-Ley and Samaniego, 1981) 5/9/2014 LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT oExample: 5/9/2014 Proppant placement into formation  We can use the propping agent to increase fracture length or width  Tip screenout (TSO) techniques:  fracture width can be increased without increasing the fracture extent How should we select the optimum fracture length and width under the constraint that the proppant volume is given? Fracture half length & CfD,opt the optimum CfD,opt = 1.6 is a given constant for any reservoir and any fixed amount of proppant 5/9/2014 Optimum fracture dimensions  Once we know the volume of proppant that can be placed into one wing of the fracture, Vf, we can calculate the optimum fracture dimensions as  Moreover, since  and yopt - 0.75 = 0.869, we obtain Fracture Orientation & In situ stress Least Principal Stress Horizontal fracture Least Principal Stress Vertical fracture The fracture will be oriented at a 90-degree angle to the least principal stress 10 5/9/2014 Vertical Profile of Minimum Stress  The effective stress, s’, is the absolute stress minus the pore pressure (p) weighted by the poroelastic constant (a):  minimum effective horizontal stress  total horizontal stress 1) Poisson ratio changes from layer to layer 2) Pore pressure changes in time -500 Ground Surface Critical Depth 977 m -1000 -500 -1500 -1000 -2000 -1500 -2500 -2000 -3000 20x106 40x106 60x106 Stress, Pa Current Depth , m Depth from original ground surface, m Crossover of Minimum Stress -2500 80x106 14 5/9/2014 Stress Gradients Overburden gradient gradient Slope of the Vertical Stress line  1.1 psi/ft Frac gradient  Basically the slope of the minimum horizontal stress line 0.4 - 0.9 psi/ft  Extreme value: 1.1 psi/ft or more STRESS oExample: 15 5/9/2014 Fracturing Pressure  Fracture Initiation Pressure or breakdown pressure is the peak value of the pressure appearing when the formation breaks down and a fracture starts to evolve Usually it is approximated by where smin is the minimum horizontal stress, smax is the maximum horizontal stress, T is the tensile stress of the rock material, a is the poroelasticity constant and po is the pore pressure  Fracture Propagation Pressure is the stabilized value of the injection pressure for a longer period of time during which the fracture is evolving Detection of formation breakdown from a steprate test 16 5/9/2014 Fracturing Pressure (MiniFrac)  Fracture Closure Pressure After a fracture calibration treatment, which is carried out without injecting proppant material, the fracture volume gradually decreases because of leakoff (and also because of possible back flow, if the injected fluid is flowed back through the well) (1) breakdown pressure; (2) fracture propagation pressure; (3) instantaneous shut-in pressure; (4) closure pressure; (5) fracture reopening pressure; (6) closure pressure from flow-back; (7) asymptotic reservoir pressure; (8) rebound pressure Leakoff  Fluid leakoff is controlled by a continuous build-up of a thin layer, or filter cake, which manifests an ever-increasing resistance to flow through the fracture face  The leakoff velocity, VL , is given by the Carter equation: uL  CL t Where CL is the leakoff coefficient (length/time0.5) and t is the time elapsed since the start of the leakoff process The ideas behind Carter's leakoff coefficient are that: o if a filter-cake wall is building up, it will allow less fluid to pass through a unit area in unit time; and, o the reservoir itself can take less and less fluid if it has been exposed to inflow 17 5/9/2014 AL uL  CL t VLost = S p  2CL t AL units : m  mm Lost volume per unit surface, m Fluid Loss in Lab 0.007 0.006 0.005 0.004 0.003 y = 0.0024 + 0.000069x 0.002 Sp 0.001 0 CL Sp 10 2CL 20 30 40 50 60 Square root time, t1/2 (s1/2) m s unit : m unit : or m3 m2 s Description of leakoff through flow in porous media and/or filtercake build-up  Concept of leakoff coefficient uL  m m / s1 /  1/ s s Where are those “twos” coming from?  Integrated leakoff volume: CL t VL  AC L t  Leakoff Width wL  What is the physical meaning? VL  2CL t AL m mm 18 5/9/2014 Definition of injection rate, fracture area and permeable height Width Equations Perkins-Kern-Nordgren (PKN) Kristianovich-Zheltov-Geertsma-DeKlerk (KGD) 19 5/9/2014 Comparison of PKN and KGD width equations  The crossover occurs approximately at the point at which a "square fracture" has been created, i.e., when  For the small fracture extent, the physical assumptions behind the KGD equation are more realistic  For the larger fracture extent, the PKN width equation is physically more sound Radial (Penny-shaped) Width Equation 20 5/9/2014 No-leakoff Behavior of Width Equations Perkins-Kern-Nordgren model Geertsma and deKlerk model Types of Fluids   Water-Base Fluids  natural guar gum (Guar)  hydroxypropyl guar (HPG)  hydroxyethyl cellulose (HEC)  carboxymethyl hydroxyethyl cellulose (CMHEC) Oil-Base Fluids   Acid-Base Fluids   Used in limestones or dolomitic formations Emulsions   Lease oil and gelled oils Mixtures of oil and an aqueous material (either water or acid) Gas/Foam Fluids  Specialized emulsions using nitrogen or carbon dioxide gas as the inner phase of an aqueous mixture 21 5/9/2014 Fracturing Additives Bacteria control agents Gypsum inhibitors Breakers N2and CO2 gases Clay-stabilizing agents Scale inhibitors Demulsifying agents Sequestering agents Dispersing agents Sludge inhibitors Fluid loss additives Surfactants Foaming agents Temperature-stabilizing agents Friction loss reducers Water blockage-control agents Proppant Pack Permeability & Fracture Conductivity  Proppant duties:  Be capable of holding the fracture faces apart  must be long lasting  be readily available, safe to handle, and relatively inexpensive 22 5/9/2014 Types of Proppants Two major categories:  Naturally occurring sand  White Sand ("Ottawa" sand)  Manufactured proppants  Sintered Bauxite  Intermediate Strength Proppants  Resin Coated Proppants A typical proppant selection guide 23 5/9/2014 Design Logics  Height is known (see height map)  Amount of proppant to place is given (from NPV)  Target length is given (see opt frac dimensions)  Fluid leakoff characteristics is known  Rock properties are known  Fluid rheology is known  Injection rate, max proppant concentratrion is given  How much fluid? How long to pump? How to add proppant? Key concept: Width Equation  Fluid flow creates friction  Friction pressure is balanced by injection pressure  Net pressure is positive  Fracture width is determined by net pressure and characteristic dimension (half length or half height)  The combination of fluid mechanics and solid mechanics 24 5/9/2014 Two approximations:  Perkins-Kern-(Nordgren)  Vertical plane strain  characteristic half-length ( c ) is half height, h/2  elliptic cross section  Kristianovich-Zheltov - (Gertsmaa-deKlerk)  Horizontal plane strain  characteristic half length ( c ) is xf  rectangular cross section Width Equations (consistent units) Perkins-Kern-Nordgren PKN  width: w, wo, wwell,o  viscosity:   inj rate (1 wing): qi  half-length: xf  plain-strain modulus: E'  height: hf Vf = w(h f x f )  qi x f ww,0 = 3.27  E' 1/    w  0.628ww,0 Kristianovich-Zheltov Geertsma-De-Klerk KGD  qi x 2f ww = 3.22  E' h f  w  0.785ww 1/     25 5/9/2014 PKN Power-Law Width Equation  With equivalent viscosity at average shear rate, the maximum width at the wellbore is: ww, = 9.15 2n2  3.98 n 2n2 n n 1 n 1  2.14n  n  2 n   qi h f x f K    n E'   2n2    Power Law fluid K: Consistency (lbf/ft2)·sn n: Flow behavior index ww,0 Material balance +Width Equation Vf = w(h f x f ) Vf = w A 2qi Vi = qi t e xf Vfe = Vi - Vlost Average w(xf) qi hf A Lost: spurt +leakoff 26 5/9/2014 Pumping time, fluid volume, proppant schedule: Design of frac treatments  Pumping time and fluid volume: • Injected = contained in frac + lost • length reached, width created  Proppant schedule: • End-of-pumping concentration is uniform, • mass is the required Given: Mass of proppant, target length, frac height, inj rate, rheology, elasticity modulus, leakoff coeff, max-possible-proppant-added-conc Pumping time, slurry volume (1 wing) Calculate the wellbore width at the end of pumping from the PKN (Power Law version) ww,0 = 9.15 2n2  3.98 n 2n2 n n 1 n 1  2.14n  n  2 n   qi h f x f K    n E'    2n2    Convert max wellbore width into average width Assume a k = and solve the mat balance for inj time, (selecting sqrt time as the new unknown) Calculate injected volume  qi  h x  f f Calculate fluid efficiency Vi  qi te e = we  0.628ww,0   t  2κ C   V fe Vi  L  t  (we  2S p )  h f x f we Vi 27 5/9/2014 Proppant schedule calculation   e  e Calculate the Nolte exponent of the proppant concentration curve Calculate the pad volume and the time needed to pump it The required max proppant concentration, ce should be (mass/slurry-volume) ce  The required proppant concentration (mass/slurry-volume) curve Convert it to “added proppant mass to volume of clean fluid” V pad  Vi t pad  te M eVi  t  t pad   c  ce  t t   e pad  cadded   c 1 (mass/clean-fluid-volume) c  propp Gross and Net Height 2qi Vi = qi te Vfe = Vi - Vlost 2D design: hf is given A hf hp Lost: spurt +leakoff rp= hp /hf 28 ... properly executed hydraulic fracture results in a "path," connected to the well, that has a much higher permeability than the surrounding formation Introduction o Minimum hydraulic fracturing candidate... CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o What length or fracture permeability is desirable in hydraulic fracturing?  • Low‐permeability reservoirs, leading to high‐conductivity fractures, would... well selection screening criteria 5/9/2014 LENGTH, CONDUCTIVITY, & EQUIVALENT SKIN EFFECT o Every hydraulic fracture can be characterized by its: – length ; – conductivity; – related equivalent