Use of Carbon/Oxygen Logs Run in Open Hole in Wells Drilled With Oil-Based Muds
- M.J. Sullivan (Chevron Overseas Petroleum Inc.) | D.L. Belanger (Chevron Overseas Petroleum Inc.) | W.F. Stewart (Chevron Overseas Petroleum Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- February 2002
- Document Type
- Journal Paper
- 60 - 67
- 2002. Society of Petroleum Engineers
- 3.1.6 Gas Lift, 4.1.5 Processing Equipment, 5.6.1 Open hole/cased hole log analysis, 5.4.1 Waterflooding, 1.6 Drilling Operations, 6.5.2 Water use, produced water discharge and disposal, 5.8.5 Oil Sand, Oil Shale, Bitumen, 5.5 Reservoir Simulation, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 1.11 Drilling Fluids and Materials, 3.3.1 Production Logging, 5.2 Reservoir Fluid Dynamics, 2.2.2 Perforating, 3.3 Well & Reservoir Surveillance and Monitoring
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Carbon/oxygen (C/O) logs have been run in open hole to assess zones that are potentially swept with fresh injection water in several infill wells offshore Cabinda, Angola.
Interpretation of these logs has been challenging because of the invasion of oil-based mud (OBM) filtrate, mixed sand/carbonate lithology, variable water salinity, large boreholes, and lack of clean wet zones in which to normalize. We discuss the impact of these factors and our experiences in managing them. An example is included in which the results of time-lapse logging suggest that the invasion of OBM filtrate in water zones dissipates slowly or not at all. We show comparisons between saturations derived from the resistivity and the C/O ratio (COR) for these various conditions.
The Angola Block 0 concession comprises new and mature reservoirs in an offshore operating environment. Waterflood operations are being used in several of these reservoirs to enhance recovery. Because the injected water has a relatively low water salinity (33 kppm NaCl equivalent seawater), detection of zones experiencing injection-water breakthrough is difficult when using traditional pulsed neutron capture (PNC) logging tools, which respond to the chloride ions in saline formation waters. One technique frequently employed in these circumstances is the use of C/O logging tools. These tools measure the relative abundance of carbon and oxygen, thereby distinguishing oil from water independent of salinity. These tools are usually run in cased hole in existing wells that have been on production for some time. In some fields, however, active infill drilling programs are under way in which new wells are drilled through producing horizons that may already have been swept with low-salinity injection water. These swept layers can be difficult to distinguish from oil-bearing intervals using conventional resistivity logs. Recently, C/O logging tools have been run in open hole as part of the standard openhole logging program. This has proven to be an effective means of distinguishing oil from low-salinity water before casing the well, despite the influence of mud-filtrate invasion.
Peripheral waterflooding is under way or planned for many of the active developments. Most of the current waterflood activity is within the Vermelha formation, which is a complex lithology reservoir with variable amounts of sand, shale, and carbonate (dolomite with minor amounts of calcite and siderite) contained in the rock matrix.
Reservoir surveillance in the early life of these fields centered around monitoring injection-well profiles and identifying water breakthrough in producers with production logs and PNC logs. Layer pressures are tracked with wireline formation tester data whenever an infill well is drilled. These data are used with fullfield reservoir simulations to track and predict water movement in the reservoir and develop infill drilling strategies. PNC logs provide accurate water-saturation information until low-salinity injection water reaches a logged well. Arrival of low-salinity injection water can be identified by a drop in produced water salinity, but it is impossible to tell which interval contributes the low-salinity injection water. Production-profile logs continue to provide valuable information after floodwater breakthrough occurs in the perforated intervals, but water saturation from PNC logs is no longer reliable. When low-salinity injection-water breakthrough is suspected (in perforated or unperforated sections), a C/O log is substituted for the PNC log to provide a salinity-independent means of determining water saturation.1-4 Despite the efforts to track the flood front and drill infill wells in unswept areas of the reservoir, there is a chance that breakthrough has already occurred in some of the proposed completion intervals. Most Block 0 wells are on gas lift, so minimizing water production is vital to maximizing oil production. The best way to minimize water production is to identify floodwater breakthrough during the openhole log analysis and complete only intervals that have not been swept by the waterflood. Standard Archie water-saturation analysis, based on resistivity and porosity, requires knowledge of the formation water resistivity (Rw). In a waterflood environment, the water resistivity can be variable, which adds considerable uncertainty in the computed water saturation. A salinity-independent means of determining water saturation, or at least discriminating fresh water from oil, has a significant impact on perforation selection. We were initially skeptical that reasonable C/O measurements could be made in an open hole with OBM because the borehole signal is an order of magnitude greater than the formation signal. When using OBM, small variations in the borehole size could produce a C/O response roughly equivalent to the response from oil in the formation.5 To date, we have run and confidently interpreted C/O data recorded with the 3.63-in.-diameter Pulsed Spectral Gamma Tool in 11 open and cased wellbores drilled with OBMs.
Impact of Uncertainty in Water Salinity on Net Pay and Perforation Decisions
The salinity of the Vermelha formation water is 200 kppm NaCl equivalent. Rw=0.025 ohm-m at 140°F (60°C). The formation water resistivity is higher in intervals in which floodwater breakthrough has occurred because of the presence of fresh floodwater. A conservative strategy for avoiding water production would be to assume floodwater might be present in any interval and compute Sw with a formation water resistivity that matches the floodwater. All intervals that still meet the Sw cutoff criterion are definitely pay, and there is no risk of producing water unexpectedly. The downside of this strategy is that some intervals that could have contributed water-free production will be eliminated. These intervals would pass the Sw cutoff criterion if 200 kppm water were actually present but would not pass the Sw cutoff criterion if 40 kppm water were present. An example of the impact of this is summarized in Table 1. The net pay calculated for Well TK-E10 is 375 ft assuming 200 kppm water and 250 ft assuming 40 kppm water. The difference of 125 ft represents 11.2 million bbl of oil in place (Sw=0.30, porosity=0.30, Bo=1.17). The value of the C/O log (or any log that can confirm the presence of fresh water) is tied to how much of this possible fresh water can be identified and avoided when perforating.
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