FORMATION EVALUATION PETE 663 SHALY SAND EVALUATION - B Summer 2010
SHALY FORMATION ISSUES Water Saturation
Obstacles with Shaly Sands Fresh formation water can cause conventional analysis to overestimate water saturation Salty formation water can cause low resistivity, meaning pay zones can be bypassed Thin beds may lead conventional log analysis to underestimate porosity and overestimate water saturation
SHALY FORMATION ISSUES LECTURE A Shales/clays have several origins and forms Shales/clays affect: Porosity Permeability Vshale Estimations Assumptions Log responses LECTURE B Shales conduct electricity Problems with Archie-based methods Rwa problem Sw errors
WELL LOG EFFECTS - 1 Well X Water leg OWC @ 150 ft Shaly interval 220-230 ft. Resist. increase Sonic Δ t increase Density ρ b increase
HC zone OWC @ 150 ft Shaly interval 115-130 ft. WELL LOG EFFECTS - 2 Resist. decrease Sonic Δt increase Density ρ b increase Neutron φ N increase
Without shale R o = FR w C o 1 or F C o = C w F C w
With shale C C = w + o F X C o X 1 F C w The factor X is the excess conductivity caused by the fact that the clays and shales are conductors of current
ELECTRICAL CURRENT FLOWING THROUGH BRINE CLEAN SANDSTONE Pore Current Pore Throat Grain Brine
SCHEMATIC - ELECTRICAL CURRENT FLOWING THROUGH DISPERSED CLAY COATING AND BRINE, SHALY SANDSTONE What is the effect of dispersed clay on resistivity when pores are filled with brine? Grain Brine Clay Coat Total Conductivity =
ELECTRICAL CURRENT FLOWING THROUGH DISPERSED CLAY COATING, SHALY SANDSTONE What is the effect of dispersed clay on resistivity when the porefilling fluid is oil rather than saline water? Electrical Current Grain Oil Clay Coat
SILICATE TETRAHEDRON SiO2 SILICATE MINERALS Silicates are the most abundant minerals Basic building block is the silicate tetrahedron (SiO4) Oblique View Si, + 4 O -2 Map View Modified from Grim, 1968
MONTMORILLONITE STRUCTURE 9.7 17.2 n H2O & Mg, Na, Ca Modified from Halliburton, EL-1007 n H2O & Mg, Na, Ca
Montmorillonite
MUSCOVITE STRUCTURE (Similar to Illite) Electrical Current Aluminum replaces silicon Mobile cations; Includes Helmholtz Planes From Grim, 1968
Illite
WHAT IS SHALE? Clay + silt + other Clays Plate-like form Large surface area Contain Al +3 and Si +4 Substitution by Mg +2 Negative charge results Attraction by water and cations Clay Crystal x H Absorbed Water Sodium Ion Hydration Water Water Outer Helmholtz Plane Schematic Water Molecule
DIFFERENT MODELS OF DIFFUSE LAYER Clay Crystal Cl - Na + Ionic Concentration In NaCl Solution Saline Water 0 X d Distance From Clay Surface Gouy profile of diffuse layer, thickness X d = 3.06 1 ( n) increases as salinity decreases. Clay Crystal x H Absorbed Water Sodium Ion Hydration Water Saline Water (X H / 6.18 A) O H + H + Model of exclusion layer (Helmholtz Plane)sodium ions excluded from surface layer by dielectric properties of water
SPECIFIC SURFACE AREAS OF SOME MINERALS Mineral Sand Kaolinite Illite Montmorollinite Ft^2/ft^3 4.3-8.7 thousand 15.2 million 85.4 million 274 million Clays have extremely large surface areas Surface area varies greatly among clay minerals
SURFACE AREA vs CEC
WATER SATURATION SHALY SANDS
Shaly Formation Issues Water Saturation Shales/clays have several origins and forms Vshale Estimation Assumptions Log responses Shales/clays affect formation Permeability Porosity Shales conduct electricity Problems with Archie-based methods Rwa problem Sw errors
WATER SATURATION EQUATIONS Many different water saturation equations have been developed Archie s model for a clean formation is: n w S = FR R t All other models are for shaly formations where the rock is not a perfect insulator w
With shale C C = w + o F X C o X 1 F C w The factor X is the excess conductivity caused by the fact that the clays and shales are conductors of current
Commonly used formulas to account for shale: Vsh Models Simandeaux (better with saline fm water) Indonesia (developed for fresher fm water) Double-layer model (Attempt to avoid using Vsh) Waxman-Smits Dual water
CO vs CW in Shaly and Clean Sands Non-Linear Region (Indonesian Equation) Linear Region (Simandoux Equation) CO Slope = 1 / F CW Modified from Halliburton, EL-1007
Cw/Co Variation with Cw and Vcl Clean Sand, F = aφ -m CW CO CW Modified from Halliburton, EL-1007
Archie n w m e w t S A C C φ = ( ) sh sh n w sh n w m e w t C V S V A S C C 1 1 + = φ Simandeaux (better with saline water) All V sh models are similar: total C = clean C + shale C Indonesia (better with fresh water) 2 / 2) / ( 1 2 / n w sh V sh n w w t S C V S F C C sh + =
Waxman-Smits model C t = C ' w φ m t A S n wt Where: C ' = C + w w BQ S v wt Note independent conduction paths by free water and bound water
New terms BQ V : conductivity of bound water Q V : cation exchange capacity (meq/gm dry clay) 1 meq = 6E20 atoms measures how many cations are present different clays have different CEC s kaolinite 0.03 to 0.06 chlorite 0 to 0.1 illite 0.1 to 0.4 montmorillonite 0.8 to 1.5 B is: specific counterion conductivity (mho/m per meq/cc) Counterions are the charge-balancing Na cations B is a per unit measure Measures how effectively cations conduct electricity
Waxman-Smits Swt obtained by iteration S wti+ 1 = 1 R w + ( BQ S ) wti where Swt 0 is the initial guess, Swt 1 is the next guess, etc., and F B Note: R w in B equation is at 75 F. F R t A = φ m t [ ( 0. 5 R )] = 1 0. 83 e w B max v 1 n
Maximum Equivalent Conductance of Sodium-Exchange Ions, λ NA or B max vs Temperature λ NA or B max, mho - cm -2 mca -1 0.25 0.20 0.15 0.10 0.05 0 20 40 60 80 100 120 140 160 180 200 Temperature, C
Graphing values of B max vs. temperature ( R and/or F) on various types of graph paper, one finds that B max vs. log (T R) is more or less linear: B max = (51.31)ln T R ( ) 317. 2
Approximate values for Q v are: Very shaly Q v =1.5 Moderate shale Q v =1.0 Medium shale Q v =0.5 Low shale Q v =0.25 No shale Q v =0 CEC or Q v should, however, be lab measured Q v may correlate with logs (e.g., GR)
Dual water model C t = C w m t φ A S n wt where C w = S S b wt C wb + 1 S S The dual-water model is a more general form of the Waxman-Smits model. The free water salinity can be different than the salinity of the claybound water. To determine Sw, use iterative method, like W-S b wt C wf
Case study 1. Pennsylvanian Cottage Grove Sandstone The Cotton Grove is identified as the interval from 6542 to 6620. Sample shows calcareous shaly sandstone. Calculate Sw at 6566 and 6680 ft. Conventional Analysis Rw = 0.05 @ Tf φ e = 2 D φ + φ 2 2 N Shaly sand F = 0.81/ф2 Rmf = 0.47@Tf φ D = ρ ρ ma ma ρ ρ b fl ρf = 1.0 gm/cc ρma = 2.68 gm/cc
At depths 6566 and 6580 6566 6680
Calculate Sw at 6566 and 6680 ft. Conventional Analysis (Archie s method) gives the results below Sw Results are high (60%), which could make one cautious about developing the well Depth φ d φ n φ xp ρ b φ d Sw 6566.135.145.14 2.47.125 0.66 6580.15.145.14 2.45.137 0.61
sh sh app corr V φ φ φ = 2 2 2 Ncorr Dcorr corr φ φ φ + = ρ ρ ρ ρ φ = ma fl ma b D MIN MAX MIN SH GR GR GR GR I = 1) 0.33(2 2* = sh I V sh Shaly Sand Analysis - Steps 1. 2. 3. 4. + = SH SH SH SH t w e e w we R V R V R R R S 2 2 2 * 5*.4* 0 φ φ 5. Simandoux Equation
Using Simandoux Equation for shaly sand analysis we have: Depth φ d ρ b GR I GR V cl 6566.125 2.47 45 0.33 0.19 6580.137 2.45 45 0.33 0.19 Depth φ nc φ dc φ e Swe 6566 0.092.119.106 0.51 6580.067.135.107 0.45 Comparison of Sw values: with shaly sand analysis Sw = 0.45-0.51 without shaly sand analysis Sw = 0.61 0.66
Case Study 2. Permian Basin, Spraberry Sandstone, Midland Basin Deep Spraberry sandstone was encountered at a depth of 7720 to 7750. The formation is not very clean, as is evident from the log sandstone
Log Analysis of Depths 7724,7732,7738 7724 7732 7238
Conventional log analysis produced the following results: ρ f = 1.0 gm/cc ρ ma = 2.68 gm/cc Depth φ d φ n φ xp ρ b φ d Rt Sw 7724 0.18 0.22 0.20 2.39 0.17 2.6 0.66 7732 0.23 0.23 0.23 2.32 0.21 2.9 0.50 7738 0.24 0.25 0.245 2.30 0.23 2.0 0.55 The saturations obtained are 0.50 to 0.66, which is fairly high
Shaly sand analysis produced the following: Depth ρ b φ d GR LOG I GR V SH 7724 2.39 0.17 75 0.48 0.31 7732 2.32 0.21 68 0.42 0.26 7738 2.30 0.23 65 0.39 0.24 Depth φ nc φ dc φ e Swe 7724 0.136 0.127 0.132 0.477 7732 0.160 0.186 0.173 0.383 7738 0.185 0.199 0.192 0.459 Comparison of saturation values: Without shaly sand analysis - range 0.50-0.66 With shaly sand analysis - range 0.38-0.48
Detection of Secondary Porosity
Vertical, Mineralized Fracture: 1U Payzone Shackelford 1-38A
Mineralized Fracture: 1U Payzone Shackelford 1-38A
Shackelford 1-38A (1-U in the Upper Spraberry) water saturation with different m & n compared with measured water saturation from whole core analysis. Sharp contrast between pay and non-pay is observed, by fluorescence, at a depth of 7092 ft. 1 Water saturation (Sw) 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Pay Sw (a=0.81, m=2, n=2) Sw (a=1, m=1.66, n=1.46) Sw (core) Non-pay 0 7083 7084 7085 7086 7087 7088 7089 7090 7091 7092 7093 7094 7095 7096 Depth, ft.
Fractured Zone Identification 0.3 Fractured Zone (pay) Non-fractured zone (Non-pay) 0.2 Porosity (neutron) Porosity 0.1 Porosity (sonic) Porosity (core) 0 7083 7084 7085 7086 7087 7088 7089 7090 7091 7092 7093 7094 7095 7096 Depth, ft.
Fracture Detection
Sponge Core - 5U Zone O Daniel #37
FMI O Daniel #37 5U Zone
Detection of Secondary Porosity
SHALY SAND ANALYSIS - INDONESIA EQUATION WELL X EXAMPLE
WELL X EXAMPLE Shale layer : 0-12 feet Clean layer: Approx from 190 220 feet Required : Water saturation at 225 feet and 47 feet using the Indonesia equation The matrix is sandstone
INDONESIA EQUATION C t = C F w S n / 2 1 ( V / 2) n / 2 w + V sh sh C sh S w
Porosity Estimation Using Vsh 1 φ corr = φ app V φ sh sh Effective porosity = φ corr Apparent porosity, matrix adjusted = φ app Apparent porosity in shale = φ sh Example...
SHALY SAND ANALYSIS - INDONESIA EQUATION WELL X EXAMPLE Depth Rhob Ø D Ø NLS Ø NSS GR Vsh Ø D (corr) Ø N (corr) Rt Sw 010 2.39 15.5 37.5 41.5 88 100 - - 2.7-225 2.27 23 23 27.5 20 12.8 20 19.6 0.5 0.95 047 2.17 29 24 28 20 12.8 26.2 20.5 30 0.1
SHALY SAND ANALYSIS - INDONESIA EQUATION WELL X EXAMPLE Depth ρ b φ D φ NLS φ NSS CGR V SH φ D corr φ N corr R t S w 010 2.39 15.5 37.5 41.5 88 100% 2.7 225 2.27 23.0 23 27.5 20 19% 20.0 19.6 0.5 0.95 047 2.17 29.0 24.0 28.0 20 19% 26.2 20.5 30 0.10
New terms W-S and Dual Water models depend on CEC Without CEC, have to use nearby shale S b - bound water saturation S b = f(cec, C wf ) S b =V sh φ sh /φ t C wb - bound water conductivity C wb = g(cec, S b )
SUMMARY Clays are conductive complex resistivity responses, Sw determination Two types of Sw models for shaly sand Vsh models (e.g., Simandeaux) Double-layer models Vsh models Empirical Assume shale properties same as nearby shale All shales have same effect Double-layer models Do not use Vsh Use electrical properties of clays (CEC)