Assessment of Permeability of Structures in Mahanagdong Using Soil Radon Gas Signatures

Transcription

1 Assessment of Permeability of Structures in Mahanagdong Using Soil Radon Gas Signatures by JW Mismanos and AA Vasquez GEOSCIENCES AND RESERVOIR ENGINEERING GROUP ENERGY DEVELOPMENT CORPORATION 38/F One Corporate Center, Pasig City, Philippines 21 September 2013

2 PRESENTATION OUTLINE INTRODUCTION Mahanagdong Radon as a potential permeability indicator METHODOLOGY Surveyed areas: MG-5 and MG-RD1 The instruments and sampling procedure RESULTS Statistical analysis (EDA and population partitioning) Correlation with MG-RD1 and MG-5 wells Radon signatures in thermal areas 2

3 PHILIPPINE GEOTHERMAL PRODUCING FIELDS 3

4 MAHANAGDONG (MG) IS ONE OF THE GEOTHERMAL SYSTEMS IN THE GREATER TONGONAN GEOTHERMAL FIELD MAHANAGDONG A Installed Gross Capacity Main Plant 120 MW TCP 12.7 MW MAHANAGDONG B Installed Gross Capacity Main Plant 60 MW TCP 6.6 MW

5 RADON SIGNATURES CAN BE USED AS PERMEABILITY INDICATORS IN A GEOTHERMAL RESERVOIR Radon is a intermediate daughter of radium in the 238 U decay chain. It occurs naturally in geothermal systems (Wollenberg, 1975) and it can be transported by HT fluids (Semprini& Kruger, 1980). Fluid convection enables radon migration to near-surface through permeable conduits (Cox, 1980). High radon concentrations in the soil can indicate rapid transport through faults/fractures. SPATIAL PATTERNS IN SOIL RADON CAN BE USED TO DELINEATE STRUCTURES OF GOOD PERMEABILITY. 5

6 THE SOIL RADON GAS SURVEY METHODOLOGY AIMS TO HELP WITH EDC S HUNT FOR PERMEABILITY This study is part of the Collaborative Research Program on Permeability. The survey was first tested in areas where knowledge of permeability is available, with the following objectives: Compare soil radon gas signatures with permeability reported from well geology and drilling data. Differentiate anomalous from background soil radon concentrations by determining threshold values. Characterize radon concentrations in thermal areas. 6

7 TWO AREAS WERE SURVEYED: EAST OF MG-RD1 & EAST OF MG-5 Survey grid was patterned with a 150-meter point spacing. MG-5: 32 points in a 0.4 km 2 area MG-RD1: 73 points within 1.5 km 2 Soil radon was also measured at thermal manifestations: Kapakuhan Banat-i Hanipolong Mahanagdong 7

8 INSTRUMENTS FROM SARAD GMBH WERE USED IN THE SURVEY The soil gas probe (SGP) houses the radon accumulation and detection chamber. Analogue radon sensor (ARS) converts SGP inputs into digital readings. The pictured instrument set-up was implemented in sampling. Radon concentration is presented as bequerels(bq)/m 3 i.e.decay events per second in 1m 3 of space. Duration of measurement: 1.5 hrs (min. time for fast-mode: 60 min.) 8

9 EXPLORATORY DATA ANALYSIS STATISTICAL PROCEDURES ARE APPLIED IN THE DATA REDUCTION OF SOIL RADON GAS SURVEY DATA 9

10 STATISTICAL & GRAPHICAL ANALYSIS WAS USED TO DETERMINE ANOMALOUS RADON CONCENTRATIONS Relative Frequency Relative Frequency PAD MGRD Soil Radon Gas (Bq/m 3 ) 0.6 PAD MGRD1 (Box-Cox) 0.5 Relative Frequency Relative Frequency log [Soil Radon Gas] Soil Radon Gas (Bq/m 3 ) PAD MG5 PAD MG5 (Box-Cox) log [Soil Radon Gas] Log [Soil Radon Gas] Log [Rn-222] R² = 1 Sample Size: 2 Log Probability Plot of MGRD1 R² = Sample Size: 65 R² = Sample Size: Relative Cumulative Frequency LOG Probability Plot Curve of Best Fit Population A (8.22%) Population B (90.41%) Population C (1.37%) Log Probability Plot of MG5 R² = Sample Size: 28 R² = Sample Size: Relative Cumulative Frequency LOG Probability Curve of Best Fit Population A (12.5%) Population B (87.50%) MG-RD1 Data distribution positively skewed (leans to the left) Three populations discerned Anomaly < 284 Bq/m 3 Anomaly 8224 Bq/m 3 MG-5 Data distribution positively skewed (leans to the left) Two populations discerned Anomaly 3098 Bq/m 3 10

11 IN MG-RD1, HIGH SOIL RADON CORRELATES WITH PERMEABLE STRUCTURES INTERSECTED BY WELLS 1. Intersection of Lower Mahanagdong and Macape Splay. Major permeable zone of MG5RD. 2. CatmonFault. Permeable zones in MG5RD, MG6RD, MG9RD. 3. CabalonanFault and CFL intersection. All wells reported poor permeability in CFL. 4. Cabalonan-A and Kinuhaan-C. Good permeability in MG7RD and MG8RD. 11

12 IN MG-5, MOST HIGH SOIL RADON CONCENTRATIONS WERE OBSERVED AT FAULT INTERSECTIONS 1. Intersection of Mantugop- C & Lumpag-A. Permeable zone in MG36D. 2. North Mamban& Lumpag Fault. Permeable in MG36D, MG32D & MG27D. Tight in MG18D & MG34D. 3. MantugopFaults and North Mamban intersection. 4. Mantugop-B and Lumpag Fault. Permeable zone in MG18D. 5. Mantugop-A and Lumpag Fault. Permeable zone in MG32D. 12

13 ONLY TWO THERMAL AREAS SHOWED HIGH RADON CONCENTRATIONS Radon concentration (Bq/m 3 ) MG-RD1 threshold value MG-5 threshold value THIS IS UNEXPECTED FOR THERMAL AREAS, WHICH ARE PERMEABLE ZONES CONDUCTING GEOTHERMAL FLUIDS 13

14 SUMMARY A preliminary soil radon gas survey was executed in Mahanagdongto test for the correspondence of radon signatures with reported permeability. Anomalous radon was determined to be 8,224 Bq/m 3 and 3098 Bq/m 3 for MG-RD1 and MG-5, respectively. Good correlations were observed in MG-RD1 while MG-5 demonstrated fair correlations. Thermal areas registered low soil radon contents. THESE PRELIMINARY RESULTS WILL HELP SHAPE THE BASIS FOR PERMEABILITY PROSPECTING IN MG-RL 14

15 IT IS HOPED THAT AN EVALUATION OF THIS PROCEDURE WILL BE ACHIEVED AFTER SURVEYS AT OTHER EDC SITES The preliminary results still warrant the study of factors that affect radon concentration in soil or reservoir fluid degassing. Results of the soil radon survey in MG-RL must be integrated with results from geological/structural mapping and geophysical imaging. Survey programs shall also be conducted in MAGBU this year and in NIGBU next year. Conducting simultaneous investigations of other soil gases/substances, e.g.co 2 & Hg, can reinforce the relationship of surface anomalies with permeability at depth. THIS MAY BE ESTABLISHED AS A GOOD PERMEABILITY-PROSPECTING METHOD FOR EDC S OPERATING AND EXPLORATION FIELDS. 15

16 END OF PRESENTATION THANK YOU VERY MUCH! For questions, please

17 STATISTICAL & GRAPHICAL ANALYSIS WAS USED TO DETERMINE ANOMALOUS RADON CONCENTRATIONS PAD MGRD Log Probability Plot of MGRD1 Relative Frequency Relative Frequency Soil Radon Gas (Bq/m 3 ) 0.6 PAD MGRD1 (Box-Cox) log [Soil Radon Gas] Log [Soil Radon Gas] R² = 1 Sample Size: 2 R² = Sample Size: 65 R² = Sample Size: Relative Cumulative Frequency LOG Probability Plot Curve of Best Fit Population A (8.22%) Population B (90.41%) Population C (1.37%) 17

18 A PILOT SOIL RADON SURVEY WAS CONDUCTED IN MAHANAGDONG TO TEST OUT THE TECHNIQUE PHASE 1 Experimentation Verified Structures Knowledge on Permeability Available MG-5 MG-RD1 PHASE 2 Actual Prospecting Most are Inferred Structures Knowledge on Permeability Unknown MG-RL 18

19 STATISTICAL & GRAPHICAL ANALYSIS WAS USED TO DETERMINE ANOMALOUS RADON CONCENTRATIONS PAD MG Log Probability Plot of MG5 Relative Frequency R² = Sample Size: Soil Radon Gas (Bq/m 3 ) 0.6 PAD MG5 (Box-Cox) 0.5 Log [Rn-222] 3.00 R² = Sample Size: 28 Relative Frequency Relative Cumulative Frequency log [Soil Radon Gas] LOG Probability Curve of Best Fit Population A (12.5%) Population B (87.50%) 19