TAD M. SMITH VERITAS EXPLORATION SERVICES FLUID SUBSTITUTIONS AND FRAME PROPERTIES; EFFECTIVE USE OF A MODEL-BASED APPROACH TAD M. SMITH VERITAS EXPLORATION SERVICES

WELL MODELING Water Depth: 3200 ft Middle Miocene Is this pay?

FLUID SUBSTITUTED CURVES

MODELED GATHERS INSITU (gas) OIL BRINE SAND 1 = BLUE SAND 2 = RED TOP SAND 1 TOP SAND 2 INSITU (gas) OIL BRINE SAND 1 = BLUE SAND 2 = RED

GASSMANN’S EQUATION (Gwet = Gdry) GASSMANN’S EQUATION RELATES THE SATURATED PROPERTIES OF ROCK TO ITS FRAME AND MATRIX PROPERTIES. BEST AVAILABLE TOOL FOR PERFORMING FLUID REPLACEMENT MODELING. (Gwet = Gdry)

TOPICS Uncertainties in application of Gassmann Constraints on “dry frame” values Using frame property models to solve (partially) difficult problems shaley sands tight gas sands invasion corrections porosity modeling When do the models fail?

SOURCES OF UNCERTAINTY Porosity Fluid saturation in the formation (Sw) Fluid properties in the formation Matrix (i.e., lithology) Fluid saturation in the invaded zone (Sxo) Fluid distribution (homogeneous or “patchy”) INCREASING UNCERTAINTY

OTHER SOURCES OF UNCERTAINTY Quality of the measurements Frequency dependence Depth of investigation (convolving tools with different DOI’s and resolutions) Well-bore condition Circulation time Tool configuration Processing

CALCULATING THE “DRY” FRAME (Kfr) Calculate Algebraically eliminate Empirical estimates/direct measurement* *most core measurements for clean quartz sands, indicate that the ratio of Kfr/G  1 (PRfr = 0.125) 2 Two problems: 1) end up not calculating K*, which provides some valueable information, and 2) porosity is in the denominator.

CALCULATE EXPLICIT CRITICAL POROSITY (Nur, 1992) Krief et al., 1990

CALCULATED FRAME PROPERTIES Remember, for clean quartz sands, Kfr/G  1 (PRfr = 0.125)

CALCULATED FRAME PROPERTIES Calculated using CP Explicit calculation Remember, for clean quartz sands, Kfr/G  1 (PRfr = 0.125)

RATIOS MOST THEORETICAL MODELS PREDICT SIMILAR RESULTS EXPLICIT CALCULATION OF Kfr CAN BE PROBLEMATIC DIRECT APPLICATION OF FRAME MODELS IS PROBLEMATIC WHAT DO THE MODELS TELL US ABOUT RATIOS? Krief et al. (1990) Critical porosity MOST THEORETICAL MODELS PREDICT SIMILAR RESULTS (dry frame PR) IS INDEPENDENT OF POROSITY

VP calculated using Gassmann and pseudo Kfr Pseudo Kfr (red curve) VP calculated using Gassmann and pseudo Kfr Kfr (calculated; green curve) 2 COMPONENT SYSTEM VRH MIX OF END-MEMBERS

DO CORE MEASUREMENTS SUPPORT THIS MODEL?

PRfr = 0.25 PRfr = 0.125 Kfr (GPa) PRfr = 0.08 G (GPa)

APPLICATIONS

FLUID SUBSTITUTION IN SHALEY SANDS

CORRECTING FOR NEGATIVE POISSON RATIOS Negative PR values are commonly observed in tight gas sands NEGATIVE PR VALUES DUE TO CRACKS IN THE ROCK MATRIX? Effective medium modeling shows that Vp/Vs ratios in dry rock decrease as porosity becomes more “crack-like” (Katahara, 1999)

CORRECTING FOR NEGATIVE POISSON RATIOS

INVASION CORRECTIONS DRY GAS SAND, DRILLED WITH OBM

UNDERSTANDING COMPLEX LITHOLOGY NOTE THE LARGE VARIATION IN PRfr Note that Kfr/G ratios vary from ~1 - >3

ASSUME A QUARTZ MATRIX MODELED VP LOWER THAN MEASURED VP LOW Kfr/G RATIOS < 1 FOR “CLEAN” ZONES

THIS MIGHT EXPLAIN THE WIDE RANGE IN PR* Feldspar? Quartz sand? CROSS-PLOTTING AND MULTIMINERAL ANALYSIS INDICATES THAT FELDSPARS MIGHT BE COMMON. THIS MIGHT EXPLAIN THE WIDE RANGE IN PR*

SCENARIO 2 ALLOW FOR COMPLEX LITHOLOGY RECOMPUTE VP NOTE HIGH Kfr/G RATIO!

SOME EXPECTATIONS FOR SEDIMENTARY ROCKS NOTE: Plagioclase feldspars are non-radioactive, and have a N-D response nearly identical to quartz “DRY” FRAME POISSON’S RATIO NOTE: for rocks with >65% quartz, PRfr values should be < ~0.2 1 .9 .8 .7 .6 .5 .4 .3 .2 .1 FRACTION OF QUARTZ

UNDERSTANDING COMPLEX LITHOLOGY KEY POINT: VARIATIONS IN PRfr MAY BE INDICATIVE OF VARIABLE COMPOSITION IN THE ZONE OF INTEREST

OTHER USES Kdry G POROSITY MODELING PREDICTION OF BIOT COEFFICIENT + NOTE: IT IS NOT SUFFICIENT TO SIMPLY ADJUST POROSITY IN GASSMANN’S EQUATION

STRESS ANISOTROPY (Xu, 2002) SANDS AT LOW NET EFFECTIVE STRESS FAILURE MODES CARBONATES STRESS ANISOTROPY (Xu, 2002) SANDS AT LOW NET EFFECTIVE STRESS EFFECT OF GRAIN CONTACTS?

Kfr (GPa) G (GPa) PRfr = 0.125 PRfr = 0.29 PRfr = 0.08 Increasing Effective Stress Kfr (GPa) G (GPa)

CONTACT MODELS Winkler Hertz-Mindlin (PRfr = 0.008) Walton - for pure quartz Walton (PRfr = 0.250)

OBSERVATIONS Kfr/G ratios are sensitive to the composition of a rock, and may vary from ~1 to 3 It is not appropriate to simply assign a constant dry rock PR (or Kfr/G) ratio when applying Gassmann. Will vary with composition Rocks with high PRfr values are not uncommon Utilize the explicit calculation of Kfr May provide insight into the composition of the rock High PRfr values may not be wrong

OBSERVATIONS Use model-based Kfr/G ratios as a quality check against the explicit calculation for Kfr Frame property models can be used to help solve difficult problems. fluid substitutions in shaley sands velocity problems in low porosity gas sands p-wave invasion corrections porosity modeling Biot Coefficient