POROSITY LOGGING CORE COPYRIGHT. Density Log. By the end of this lesson, you will be able to:

Transcription

1 LEARNING OBJECTIVES POROSITY LOGGING CORE Density Log By the end of this lesson, you will be able to: Describe the logging tool configuration in the borehole and the basic physics of the Density Log and Litho-Density Logs Discuss the units and the scale used for Density log data Explain how Density log data are used to calculate porosity and what are the required inputs Describe the effect of free gas saturation on the bulk density (rb) log and the impact on calculated porosity if 100% liquid is assumed Discuss how the correction curve and caliper curve are used together to quality check bulk density log data 1

2 Density Log POROSITY FROM DENSITY AS SINGLE SENSOR Several applications Porosity estimation Lithology estimation Prediction of hydrocarbon density Input to synthetic seismograms (density x velocity) Formation mechanical properties and rock physics TYPICAL DENSITY TOOL CONFIGURATION Formation Density Compensated (FDC) Density Tool responds to electron density of the formation in front of the tool. Density Log response; combination of matrix density and density of fluids in the invaded zone in their relative proportions. For simplicity, we have considered ρ w = ρ mf. 2

3 TYPICAL DENSITY TOOL CONFIGURATION The Density Log measures bulk density of formations in open-hole (g/cc). In a known lithology, the bulk density is used to determine formation porosity. Also an important reference for detecting light hydrocarbons. The Density Log has earned its reputation as the most reliable and useful porosity log. GAMMA RAY INTERACTIONS PEF Cesium 137 Emission Limit of GR detection 3

4 Density Log LOGGING TOOLS AVAILABLE Wireline Density Measurements LDT = Litho-Density Tool, Schlumberger ZDEN = Litho-Density Tool, Baker Atlas SDLT = Litho-Density Tool, Halliburton MPD = Litho-Density Tool, Weatherford LWD Density Measurements ALD = Litho-Density, Sperry (Halliburton) ADN & CDN = Litho-Density, Anadrill (Schlumberger) MDL = Litho-Density, Baker-Huges Inteq AZD = Litho-Density, Weatherford DENSITY CORRECTION BY SPINE AND RIBS (mm) (6.35) (12.7) (19.05) (mm) (6.35) (12.7) (19.05) 4

5 LOG OF CORRECTED BULK DENSITY ( m) ( m). VS. CALIPER CURVES (454 kg) ( m) ( m) ( kg/m 3 ) 5

6 Density Log VS. CALIPER CURVES (454 kg) ( m) ( m) VS. CALIPER CURVES Good qualitative indication of log quality Should be ~ 0 in smooth borehole Barite mud exception Can be good fracture indicator as well (454 kg) ( m) ( m) A. This hole was drilled with an 8-½ inch (18 mm) drill bit. 6

7 DENSITY TOOL RESPONSE EQUATION b = Φ [S xo w + (1-S xo ) h ] + (1 - Φ) ma b ma f w h Φ S xo : Formation bulk density : Matrix density : Fluid density : Water density : Hydrocarbon density : Formation porosity : Invaded zone water saturation Rearrange to calculate porosity, Phi = ma ma - b - f Quartz Calcite Dolomite DENSITY VALUES IN FORMATION EVALUATION (G/CC) Reservoir matrix Quartz (sandstone) 2.65 Calcite (limestone) 2.71 Dolomite 2.87 Pore fluids Fresh water 1.00 Salt water (200 g/l (200 kg/m 3 )) 1.13 Fresh water with 30% residual oil Fresh water with 30% residual gas g/cc 2.71 g/cc 2.87 g/cc Other minerals Halite (rock salt) 2.03 Anhydrite

8 Density Log CORE POROSITY CALIBRATION Core porosity (%bv) Apparent fluid Density fl Log density (g/cc) DENSITY LOG RESPONSE CHART Porosity = 25% Core matrix density ma 8

9 PEF (OR PE) CURVE Original Essentially density porosity log was independent developed in the 1960s Typical Measured values bulk density Sand = 1.8 Called Formation Density Compensated Limestone = Log 5.1 (FDC) Dolomite = 3.1 Photoelectric Salt = 4.7Factor Curve (PEF) Anhydrite was later = 5.1 developed Density log was renamed Litho-Density Tool (LDT) Remember: Single Sand (quartz curve lithology rich) indicator Limestone (calcite rich) LITHO-DENSITY TOOLS: PEF (OR PE) CURVE Photoelectric Factor (PEF) Incident low energy gamma ray absorbed by electron and electron ejected from the atom PEF = (Z/A)exp 3.6 Where: Z = atomic no. A = atomic weight Measures different part of decay spectrum Can be highly affected by barite muds Single curve lithology indicator 9

10 Density Log PE VS. POROSITY: LITHO-DENSITY TOOLS Chart Use: Read PEF Read Bulk Density Determine Porosity % SUMMARY: DENSITY AS SINGLE SENSOR Porosity = 20% Source for porosity and lithology information Litho-density (PEF) provides additional lithology information Pad contact tool Rugose hole implications Barite mud implications Fracture identification implications Can be combined with other porosity devices to define lithology, porosity, shale content Depth of investigation shallow (< 50 cms) Vertical resolution ~ 20 cms 10

11 LEARNING OBJECTIVES Describe the logging tool configuration in the borehole and the basic physics of the Density Log and Litho-Density Logs Discuss the units and the scale used for Density log data Explain how Density log data are used to calculate porosity and what are the required inputs Describe the effect of free gas saturation on the bulk density (rb) log and the impact on the calculated porosity if 100% liquid is assumed Discuss how the correction curve and caliper curve are used together to quality check bulk density log data 11