Novel Application in Determining Oil Shale Porosity Using a Borehole Gravimeter

Transcription

1 Novel Application in Determining Oil Shale Porosity Using a Borehole Gravimeter Shell Exploration & Production Katia Pronina Robert Pimentel Matt Holman (presenter) 3/17/2010 File Title Copyright: SIPC 29 th Oil Shale Symposium Golden, Colorado October 20, 2009

2 Colorado Oil Shale Location Piceance Basin Drainage Basin Boundary Proposed 160-Ac R&D Tract RIO BLANCO CO GARFIELD CO GARFIELD CO MESA CO DeBeque Parachute Rio Blanco Colorado oil shale is the largest and richest contiguous deposit of its kind in the world Piceance Basin is a Laramide basin with oil shale interval in the Eocene Green River Formation Oil shale is a fine-grained sedimentary rock rich in kerogen

3 Shell Development Option for Colorado Oil Shale In-situ Conversion Process Electric resistance heaters inserted into holes to gradually heat shale subsurface Applicable to oil shale and heavy oil/bitumen Technology accelerates the natural maturation of kerogen by gradual heating in oil shale Results in high recoveries and light hydrocarbon products yielding high quality transportation fuels

4 The object of investigation - vugs (solution cavities) Vugs are cavities formed after halite/nahcolite dissolution. Vug size in Piceance basin ranges from millimeters to several feet. Example: well B (at R-4 Zone/L-4 Aquifer interval) - core is not very informative: missing core at MD - video log reveals a large cavity

5 Q: How to estimate vugular porosity? Two groups of methods: 1. Small scale: logs and core (near wellbore readings): - NMR porosity - visual estimates of core 2. Large scale: potential fields methods - seismic / acoustic velocity attenuation study - borehole gravimetry (BHGM)

6 Can we use core data? Not systematically, because of poor sampling: core size is 2-3 inches, while vugs can be 3 ft and larger Distribution of nahcolite aggregates in Piceance Basin [1]: [1] Geology of the nahcolite deposits and associated oil shales of the Green River Formation in the Piceance Creek Basin, Colorado by John R. Dyni; PhD thesis, University of Colorado, Department of Geological Sciences, 1981

7 What is borehole gravimetry? Borehole gravimetry = BHGM - acceleration due to gravity g changes with distance from centre of mass (elevation or depth) and is affected by surrounding masses (densities) - hence, accurate measurement of change in g can give us density Sir Isaac Newton Johann van Popta, KSEPL; Stephen Adams, SIPM "Gravity Gains Momentum", Middle East Well Evaluation Review, 1992; also available on

8 LaCoste & Romberg gravity meter Work principle: LaCoste & Romberg gravity meter is fundamentally a very sensitive spring balance in which the weight of a hinged beam with a small mass on its free end is balanced by the tension of a spring (see figure). As the gravitational acceleration - and hence the weight of the mass - changes, the spring tension must be changed to hold the beam in a stationary horizontal position. The spring tension is calibrated in gravity units. Precision of method: Under normal conditions, measurements can be repeated to within a standard deviation of about 6 µgal (three parts in 10 9 of the Earth's gravitational field): that corresponds to approximate change in density ~ g/cc Johann van Popta, KSEPL; Stephen Adams, SIPM "Gravity Gains Momentum", Middle East Well Evaluation Review, 1992; also available on

9 How do we interpret gravimetry data? 1. Compare gravimetry density against standard density log (gamma-gamma density) 2. Areas where densities mismatch indicate density anomalies away from the well bore 3. Hence, for interpretation good quality density logs are a must! Density anomaly away from well bore Example from Micro-g Lacoste survey

10 Gravimetry density vs. gamma-gamma density Gamma-gamma tool: measures electron density Gravity tool: measures acceleration due to gravity None of these two methods measures density directly hence, there is an intrinsic difference between them (not important for most minerals except salts and water)

11 Acquisition Planning cross-section W e s t BHGM validation E a s t BHGM survey P ic e a n c e C r e e k U in ta F o r m a tio n M a h o g a n y L ea ch ed Z o n e SSa al lt t S Sa la tl t S alin e Z o ne ( U nl e a ch e d) m i. H o riz o n t al 1. Validation in MDP location where we don t have vuggy zone (BHGM and gamma-gamma density should match) 2. BHGM program in area with vuggy zones (BHGM should be lower than gamma-gamma density in vuggy zones) S c a le

12 BHGM acquisition, Fall Validation in MDP location where we don t have vuggy zone (BHGM and gamma-gamma density should match) 2. BHGM program in area with vuggy zones (BHGM should be lower than gammagamma density in vuggy zones) Gravimeter operator:

13 First look at BHGM density data October 2008: acquisition program is finished December 2008: Micro-g Lacoste completed processing and delivered computed densities First impression: data don t make any sense! 1. Poor match in non-vuggy zones 2. In vuggy zones BHGM densities either the same or HIGHER than log densities. (In case of present water-filled voids we expect gravimetry densities to be LOWER than density log) Legend: Continuous high-resolution curves density logs; Blocked curves computed bulk densities from gravimetry (shown with error boundaries)

14 Second look and path forward 1. BHGM data look consistent; data processing is QC-ed 2. Problem with standard density logs (against which BHGM data is compared) is discovered. Gamma-gamma densities are not reliable if logging tools are not routinely calibrated in certain cases 10% of density value error is observed. 3. Further petrophysical work is needed for well data analysis and interpretation L3(B) L L L

15 Petrophysical workflow Data Preparation Log QC / Blocking 1. Construct Bad Flag from Caliper and manually edit. 1. Compile original log data 2. QC / Merge / Combine / Depth shifted raw core and log data 3. Determine petrophysical model to use 2. Remove Bad hole sections from density log. 3. Block Logs by Sums and averages. 4. Correlate wells and modify tops 5. Normalize density logs PP Calculations / Interpretation 1. Calculate apparent matrix density 2. Compute additional vugular porosity (not seen in the wellbore) 3. Calculate total porosity 4. Plot data & interpret φ vug = ρ bulk,log ρ ρ bulk,log bulk, BHGM ρ brine

16 Petrophysical results summary 1. BHGM densities in the most zone closely match density logs (after logs are normalized and QC-ed) That validates a few conclusions: - occurrence of large cavities is rare - present small vugs are evenly distributed in stratigraphic layers and adequately probed by gamma-gamma density logging 2. In cases where BHGM and log density are different we rely more on BHGM data, because log density readings are affected by wellbore conditions or may not be fairly represented

17 Conclusions 1. Gravimetry data is valid and useful: - BHGM is one of the very few methods that reads very deep into formation (~1000ft) while majority of the wells only reads a few centimeters. - Gravimetry is not affected by hole condition and near well-bore surroundings, thus it is a good independent measurement of bulk properties of formation - BHGM resolution is acceptable for large-scale characterization 2. Gravimetry immediate product is updated bulk densities, however: - with good supplemental data (such as gamma-gamma bulk density) they can be converted to change in total porosity (between near and far from wellbore);