PETROPHYSICAL EVALUATION CORE COPYRIGHT. Saturation Models in Shaly Sands. By the end of this lesson, you will be able to:

Transcription

1 LEARNING OBJECTIVES PETROPHYSICAL EVALUATION CORE Saturation Models in Shaly Sands By the end of this lesson, you will be able to: Explain what a shale is and how to distinguish between a shaly sand and a sandy shale Describe the effect of shale on resistivity logs and Neutron logs Tell about the difference in the Archie Equations and Shaly Sand models Discuss why CEC (Cation Exchange Capacity) is important List three types of clay minerals and tell which has the highest CEC Describe three modes of shale distribution 1

2 WHAT ARE SHALY SANDS? Shaly sands: Reservoir sands containing 10% to 35% Vshale (clay minerals plus other components) Shale is defined by grain size. Clay size particles are those less than 4 microns SHALY SANDS: WHAT ARE EFFECTS? Alters calculated S w Alters porosity calculated from Neutron log Due to excess conductivity. The use of Archie s equation can underestimate HC: use one of the shaly sand equations for evaluation Increase actual S w of pay sand Can significantly decrease the effective porosity and permeability Can increase sensitivity of reservoir to commonly used well bore fluids Due to H+ & OH ions and CBW in clay minerals. CEC, Do not use Neutron log use Density Allows sand to produce water free at higher S w (e.g.50 70%) When clay minerals occur in pores and pore throats Drilling filtrate Completion/workover fluids 2

3 SHALY SAND RESISTIVITY Archie equation assumes rock framework has no electrical conductivity True for clean sandstones and clean carbonates Not true for rocks that contain shale/clay mineral Clay minerals are conductive in water and brine The presence of clay minerals (common in sandstones, less common in carbonates) adds a conductivity contribution Cause Sw over-estimation in shaly rock using Archie equation a major problem in shaly sandstone reservoir evaluation ADDITIONAL CONDUCTANCE MODEL More lanes less traffic jam Equally: More conductance paths less resistivity Hence, clay conductance leads to decrease in resistivity Additional conductance is expressed with Q v (or CEC). Q v is related to the amount and type of clay 3

4 SHALY SAND RESISTIVITY EQUATIONS All shaly sand models are based on Archie for limit of no shale content All are constructed with the concept of two parallel resistances of: The pore brine The shale components The general equation takes the form: 1 R t S 2 w = + X, F R w where X is the conductivity contribution of the shale element SHALY SAND RESISTIVITY EQUATIONS Two families of equations: Traditional Family: Shale is considered a homogeneous conductive medium and resistivity equations are based on V sh. These equations use an R sh term for shale resistivity Technically Correct Family: The conductivity of the shale component is a function of the cation exchange capacity (CEC) of the various types and abundances of clay minerals 4

5 MAJOR SHALY SANDSTONE EQUATIONS Poupon et al (1954) Poupon and Leveaux (1971) Hossin (1960) Schlumberger (1972) Simandoux (1963) Waxman and Smits (1968) Barton and Pied (1969) (modified Simandoux) SHALY SANDSTONE ANALYSIS Log R o None Clay Solids Clavier et al (1977) Juhasz (1981) Archie versus Waxman Smits * * * ** * ** * Log R w Water S w Points plot straight H C Shale Effect Log R o None Clay Solids c l a y * * * * *** * * Log n d C B W S wt t Water R w H C Shaly sand points plot off straight line Total porosity from neutron log Total porosity from density log 5

6 OLDER EQUATIONS BASED ON VSH, RSH. Physical basis for these equations is incorrect These log-based V shale models don t account for shale/clay minerals existing in differing morphologies Note that clay mineralogy may differ between sandstones and associated shales However, equations can provide reasonable approximations of S w especially when equation parameters are adjusted to conform with local data from S w measured in cores or production tests These equations are widely used today because of: Simplicity Limited requirement for input parameters 1 S 2 = w + X R t F R w Basic form of the shaly sandstone equation has two components: Archie equation Shale component, X The two parameters of X are: Vsh (volume of shale) Rsh (shale resistivity) Both are problematic and result in uncertainty Gamma Ray Resistivity Simandoux (1963) is most commonly used. 1 R t = S 2 w F x R w + V sh x S w R sh Schematic for Estimation of V sh and R sh 6

7 SHALE CONDUCTIVITY To overcome the problem related to Rsh and Vsh determination, the Waxman Smits equation use a solution based on ionic double layer theory. 1 R t = S 2 w F * x R w BQvS w F* CEC is a function of the various clay mineral species and the abundance of each. CEC DEFINITION Clay-rich sands with dispersed shale have excess conductivity due to the presence of cations bound at the clay surface The concentration of Na+ can be measured by chemical means and is called: Cation Exchange Capacity (CEC) + CEC is the most important property of clay in shaly sand evaluation 7

8 SHALY SANDS/SANDSTONES Different clay mineral species have different measured values of CEC. Each species will have a different affect on logs Smectite (fine grained clay) Illite Chlorite Kaolinite (coarse grained clay) CEC LAB: MEASUREMENT METHODS Membrane potential (recommended) 80 to 150meq/100gm 10 to 40meq/100gm 0 to 20meq/100gm 3 to 5meq/100gm Wet Chemistry method: Conductometric titration is usual contractor method Sample is crushed, placed in barium solution, and titrated Multiple salinity Absorbed water (not recommended) 8

9 UNDERSTANDING CLAY CHEMISTRY: BOUND/FREE WATER Clay Free ions Sand grain Sand grain Negatively charged clay surface creates additional conductance path Q V DETERMINATION: WET CHEMISTRY TITRATION METHOD Crushed sample BaCI 2 wash Ba + clay titrated with MgSO 4 (BaSo 4 + Mg + clay) titrated Gives CEC Negatively charged clay surface Gouy Positive layer ions moving along clay surface QV = CEC (1 Phi) GD 100 Phi 9

10 PROBLEMS IN USE OF WAXMAN SMITS MODEL FOR SW Waxman Smits model requires representative core samples in a laboratory, on which to measure CEC Requires actual rock samples from reservoir Difficult to obtain reliable values of CEC from rock samples using standard laboratory methods For these reasons double ionic layer models are often considered to be impractical Consequently, Waxman-Smitts model modified to use substitute data determined from logs using surrogate variables Juhasz equation (1981 technical paper): SATURATION MODELS: DIFFERENT SHALE DISTRIBUTIONS Clean Sand Laminar Shale Structural Shale Dispersed Shale 10

11 SATURATION MODELS: RECOGNIZING SHALE DISTRIBUTIONS Sand with a pore volume 100% filled with dispersed clay SATURATION MODELS: RECOGNIZING SHALE DISTRIBUTIONS Density/Neutron Crossplot End point for laminated clean sand-shale, 100% shale. 11

12 Example of a Petrophysical Evaluation Summary Log Net (TV) Kb md md md md MDT Spls MD (16131 TV) 9366 psia (11.2 ppg) 171 o F 27.9 o API 1135 GOR MD (16197 TV) 9386 psia (11.2 ppg) 172 o F 26.7 o API 1150 GOR NET NET TV SAND Top-TV TOP BASE GROSS SAND N/G PAY Hole Net HC POR SWA Perm FT-TVD FT-MD FT-MD FT-MD FT-MD FT-MD Angle PAY TYPE % % insitu H OIL H OIL H OIL H OIL LEARNING OBJECTIVES M1 Sand MD Gross 153 MD Net 81 MD (78 TV) N/G 0.53 Por. 29.6% Sw 37% Kb 350 md Hole Angle 14 o Model: Thomas Steiber (N/G & Sand Porosity) Waxman Smits Saturation H-C LAM Resistivity Model Permeability Correlation w/sidewalls Ka Rw =.03 (from wet sand) m, n = 1.75 You are now able to: Explain what a shale is and how to distinguish between a shaly sand and a sandy shale Describe the effect of shale on resistivity logs and Neutron logs Tell about the difference in the Archie Equations and Shaly Sand models Discuss why CEC (Cation Exchange Capacity) is important List three types of clay minerals and tell which has the highest CEC Describe three modes of shale distribution 12