Deriving Mineralogy and Reservoir Properties in the Oil Sands Using X-Ray Fluorescence (XRF)

Transcription

1 Deriving Mineralogy and Reservoir Properties in the Oil Sands Using X-Ray Fluorescence (XRF) Tom Weedmark, Ron Spencer, Justin Besplug and Heather Wright, XRF Solutions Ltd. Introduction XRF Solutions have developed a novel approach to determine reservoir properties of heavy oil reservoirs through direct XRF measurements on core. There are several benefits to using this approach: including non-destructive data collection, high resolution data, rapid turnaround and lower cost compared to conventional laboratory analyses. This study demonstrates that spectral gamma, mineralogy (including clays), porosity, permeability, oil content, water content, and oil quality can all be derived from modelling XRF elemental data. Data produced are of comparable quality to those obtained from conventional laboratory techniques such as XRD, Dean Stark and others. These conventional laboratory analyses are destructive, costly and require significant time (weeks to months). XRF data are obtained and processed rapidly (within a few days) at a fraction of the cost and at high resolution (10 cm spacing resulting in 100 s of analyses per core). Natural Gamma and Radioactive Sands XRF Solutions determine the elemental concentrations of K, Th and U, the major contributors to natural gamma in the cores. These can be compared directly to spectral gamma from well logs or converted to total gamma and compared to gamma from well logs. There are no spectral gamma logs available in these wells and the XRF data are compared to total gamma. Gamma response in both of these cores is primarily from K and Th in clay minerals. The U content is generally below our detection limits in the 9-23 and 6-17 cores. Comparison of the XRF determined gamma with gamma from the well logs allows for accurate depth correction of the core. The XRF data from the cored intervals of these two wells have a strong correlation with the wireline gamma; therefore, no depth shifts have been applied to these cores. 1

2 Potassium Uranium Thorium XRF GR WL GR Potassium Uranium Thorium XRF GR WL GR Wt % PPM PPM Counts Counts Figure 1. XRF Spectral Gamma. XRF K, U, Th and derived gamma for two oil sands cores are displayed with depth. There are some areas of the Alberta oil sands that contain radioactive sands or hot sands. These are intervals that contain relatively clean sand, but have high gamma readings. In some cases the gamma response is at or above that of the shale units. The high gamma readings are a result of concentrations of heavy minerals in the sand. There are variable concentrations of these heavy minerals, resulting in variable gamma response. The most abundant heavy minerals we have found in hot sand intervals are zircon, monazite, rutile, ilmenite and garnet. Elevated gamma readings are a result of contributions from U and Th present in zircons and the rare earth phosphate mineral monazite. The radioactive U and Th are present as trace components of these minerals and can be difficult to detect. We use the most abundant elements within the heavy mineral assemblages such as Zr for zircon, P for monazite, Fe for ilmenite and Ti for ilmenite and rutile, to locate these hot sand intervals. These elements can be present in clay or phosphate minerals; however, they are not generally present in high quantities in clean sands. Element profiles for Zr, P, Th, Ti, Fe and U in the 9-23 core are displayed, along with the total clay content and wireline gamma (log GR), in the figure on the left below. There are low amplitude spikes in Zr and Ti below metres and corresponding low amplitude spikes in Zr, P and Ti. There is a higher amplitude spike in Zr, P, Ti and Fe just below metres and the gamma is higher relative to the clay content. There are also slight separations in portions of the clay and gamma curves from to metres that tend to correspond to increases in Zr. We interpret this core to contain low amounts of heavy minerals in the cleaner sands with little or no impact on gamma and slightly higher amounts of heavy minerals in clay-rich intervals with minor impact on gamma. We do not see indications of significant hot sand in the 9-23 core. Below (right) is an example from a third core (10-17) from this project that does contain a hot sand. XRF data were collected at lower frequency than in the other two cores. The same data groups used in the 9-23 core are shown for the Gamma below 254 metres correlates well with clay content indicating it is controlled by K and Th in clay minerals. Other elements follow these profiles indicating they are associated with clays in this interval. The gamma and clay content profiles differ substantially from 254 to 248 metres, where the gamma is substantially elevated relative to the clay content. This interval contains relatively clean, bitumen-saturated sand with a low clay content typical of a hot sand. Levels of Zr, P and Ti are clearly elevated here and the Th and U are also higher than in other low clay intervals of the core Wt % PPM PPM Counts Counts 2

3 205 Clay Log GR Zr P Th Ti Fe U Figure 2. Radioactive Hot Sand Indicators. Hot sand indicators for two oil sands cores are displayed with depth. Heavy mineral concentrations that produce hot sands can be difficult to find through conventional methods. Identification of hot sands is important in oil sands reservoirs as the concentration of heavy minerals throws off not only the gamma log, but also other well logs which are used in reservoir property and reserve estimates. Mineralogy 0 Wt % API PPM PPM PPM PPM PPM PPM SUNCOR 1AA/ W4 Clay Log GR Zr P Th Ti Fe U 0 Wt % API 200 PPM PPM PPM PPM PPM PPM XRF Solutions employ principles of mineralogy phase theory to construct normative mineral algorithms. In essence this uses the mathematical constraints for solving simultaneous equations; the number of independent equations must be equal to the number of unknowns in order to obtain a unique mathematical solution. The amounts of each mineral present are the unknowns and there is a mass balance equation for each element in the minerals. We use both major and trace elements in our mass balance equations. The most abundant minerals in the Formation oil sands reservoirs determined from XRD and SEM analyses are the framework grain minerals quartz and feldspar (mostly K-spar with minor plagioclase) and the clay minerals kaolinite and illite; minor amounts of smectite and trace amounts of chlorite are also present in the clay mineral assemblage. Carbonate minerals are Fe-rich with variable amounts of Ca and/or Mg; Fe-rich carbonates are present as nodules, concretions or cements in shale. Minor amounts of pyrite are also present in the reservoirs. Bulk mineralogy data for the 9-23 and 6-17 cores obtained from the XRF analyses are shown as continuous curves in the Figures below. Clay mineral assemblages (not shown) are also determined through XRF normative mineral algorithms. The 9-23 core contains a relatively clean sand reservoir below metres with an increase in shale above this. The 6-17 core has abundant shale-rich intervals throughout. Red diamonds are data for 10 XRD determinations from these cores. We find that in general XRF determinations yield higher amounts of clay and lower amounts of framework grains than XRD determinations in samples where bitumen was removed prior to the XRD analyses. This is likely a result of the loss of fines during bitumen removal. 3

4 Clay Quartz Feldspar Mg-Ca Carb Fe-Ca Carb Pyrite Clay Quartz Feldspar Mg-Ca Carb Fe-Ca Carb Pyrite Weight Percent Weight Percent Figure 3. Bulk XRF Mineralogy. Mineralogy determined from XRF on two oil sands cores are displayed with depth. Reservoir Properties This project was a test by the client to compare the quality of the XRF determined reservoir properties to conventional laboratory determinations. In this case the client was interested in the mineral assemblage provided above, but also in the porosity, permeability, oil content, water (or gas) content and the quality of the oil. All the XRF determined reservoir properties are determined using the XRF mineralogy and/or trace elements in the same manner for both wells. The permeability is dominantly controlled by the grain size of the sediments and clay content, which can be determined through XRF analyses. The weight percent S per volume of oil is used as an indicator of oil quality. These are summarized in the figures below for each well. For each well the continuous curves display XRF determined values for quartz plus feldspar, porosity, oil content, water content, oil saturation, permeability, and S/V oil. Conventional laboratory determined porosity, oil content, water content and oil saturation from Dean Stark analyses are displayed as purple squares, permeability as yellow squares and oil viscosity as black squares. XRF determined properties are in excellent agreement with these conventional laboratory measurements. Differences are largely a result of sample size and frequency. XRF values are from discrete, cm-scale spots, conventional laboratory determinations are from cm homogenized samples. XRF analyses allow for determination of higher frequency variation, primarily from the presence of thin shale layers. The 9-23 core contains a relatively clean sand reservoir and the 6-17 core has abundant shale intervals. 4

5 Clay Qtz+Fld Porosity Oil Water Oil Sat Perm S/Voil Clay Resistivity Qtz+Fld Porosity Oil Water Oil Sat Perm S/Voil Vol % Vol % Vol % Vol% Frac D Vol % Relative Figure 4. Reservoir Properties. Reservoir properties from two oil sands cores are displayed with depth. The 9-23 core contains a continuous oil column with a high oil content and low water cut, especially through the lower 30 metres. The oil content decreases and water content increases upward as the clay increases. The oil quality increases upward through the oil column. This is likely a result of biodegradation near the base of the oil column; removing lighter ends causing an increase the S/V oil and viscosity. The profile above this is consistent with diffusion through the oil column. The 6-17 core contains a discontinuous oil column with a variable oil and water content as a result of multiple shale-rich intervals that contain little to no oil and a high water cut. The oil quality near the base is quite poor likely a result of biodegradation near the base of the oil column removing lighter ends and increasing the S/V oil and the viscosity. Unlike the 9-23 core, the shale rich intervals do not allow a smooth diffusion through the oil column. The very high S/V oil in the lower few metres is an indication of extremely degraded or dead oil. The permeability (grain size) profiles for both cores show significant changes in the clean portions of the reservoir, which do not appear to be related to increases in clay mineral content. There is a general positive correlation between increased permeability and increased S/V oil. It is likely that increased permeability allows for more interaction between the formation waters and the bitumen; therefore increased biodegradation resulting in elevated S/V oil Vol % Vol % Vol % Vol% Frac D Ohm*M Vol % Relative 5