Enhanced Formation Evaluation and High-Resolution Facies Determination Using Spectroscopy Logs in the Nile Delta, Egypt
- Marie Van Steene (Schlumberger Logelco Inc.) | Adel Farghaly (RWE Dea) | Ahmed Abu El Fotoh (Schlumberger) | Susan Herron (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- North Africa Technical Conference and Exhibition, 14-17 February, Cairo, Egypt
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 5.5.2 Core Analysis, 1.6 Drilling Operations, 5.2 Reservoir Fluid Dynamics, 1.14 Casing and Cementing, 1.6.9 Coring, Fishing, 3.3.2 Borehole Imaging and Wellbore Seismic, 5.6.2 Core Analysis, 5.6.1 Open hole/cased hole log analysis, 5.5.11 Formation Testing (e.g., Wireline, LWD), 4.1.5 Processing Equipment, 1.2.3 Rock properties, 2.4.3 Sand/Solids Control, 6.5.4 Naturally Occurring Radioactive Materials, 5.1.1 Exploration, Development, Structural Geology, 5.1.5 Geologic Modeling, 1.11 Drilling Fluids and Materials
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Computing clay volume using elemental neutron capture spectroscopy logs in combination with a multimineral solver for the complex, shaly sand reservoirs of the Nile Delta reservoir improved accuracy over using the mineral fractions output from the spectroscopy model alone. It was also found that the aluminium log from direct aluminium yield measurement leads to a better clay volume estimation, as opposed to using the aluminium log from the aluminium emulator algorithm.
Combining the spectroscopy data with borehole image data generated a high-resolution lithofacies column that provides an accurate stratigraphic interpretation. Applying cutoffs to generate a high-resolution sand count enabled us to sort the reservoir units from the poorest to the best quality sands and improved our understanding of the distribution of the best reservoir quality in the well. This approach provides a unique solution to characterize thinly bedded reservoirs in wells drilled with oil-based mud.
A wide variation in rock quality exists in the sands and shaly sands of the Nile Delta. The mineralogy is complex, including the presence of feldspars, calcite, heavy minerals, and several different clay types. To improve evaluation of these gas-bearing reservoirs, neutron capture spectroscopy data is routinely acquired. Neutron capture spectroscopy logs provide continuous elemental concentrations of silicon, calcium, iron, sulfur and aluminium (either computed from other elements or measured). These elements are then combined through the Spectrolith model (Herron et al., 2002; Herron and Herron, 1996) to provide a mineralogical model of the formation, including the mineral concentrations of quartz, feldspars, and mica (QFM), carbonate (calcite and dolomite), clay (sum of all the dry clay minerals), pyrite, and siderite. The objective of acquiring spectroscopy logs was to obtain quantitative lithology and clay volume evaluation. However, doubts were raised about the amount of clay from the spectroscopy model, especially in the reservoir sections, which were thought to be cleaner than what the spectroscopy showed.
Core samples were analyzed by the dual-range Fourier transform infrared (DRFT-IR) technique, which indeed confirmed that the clay volume computed by the spectroscopy model was overestimated.
We evaluated two methods to obtain a petrophysical model more consistent with the core results. The first method used a linear relationship directly derived from the core data. The second method used a mineral solver to model the formations. The mineral solver estimated lower amounts of clay in the matrix, more closely modeling actual reservoir properties.
An example demonstrates how spectroscopy data can be useful in optimizing the wireline formation testing program.
Finally, we combined microresistivity borehole images data with spectroscopy data to produce high-resolution lithofacies. We used the lithofacies to perform a facies-based high-resolution sand count that allowed us to evaluate the quality of sand facies from wells drilled with oil-based mud in thinly bedded reservoirs.
All example wells in this project were drilled in the Nile delta. The well used to illustrate the formation evaluation process was drilled with a KCl water-based barite mud. The upper reservoir is constituted of channel fills while the lower reservoir (approximately 500 m deeper than the upper reservoir) was deposited in a lacustrine environment and is constituted of washover fans and lacustrine shales.
The wells used to illustrate the wireline formation tester program optimization and the high-resolution lithofacies technique were all drilled with oil-based mud. The reservoirs were composed of channel fills and distal turbidite sheets.
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