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TA-GOGD is thermally assisted gas-oil gravity drainage suited for heavy oil in highly fractured formations. Steam is injected into the fractures to serve two purposes: to apply a gas gradient across the matrix blocks so that the oil in the matrix drops down by gravity and to heat the carbonate matrix blocks so that the reduced viscosity oil drips out faster. The reservoir has two fluid systems, which are gas/oil/water levels in the fractures and gas/oil/water contacts in the matrix blocks that are separated by the fractures. Reservoir surveillance requires logs of the remaining oil saturation to confirm the recovery, which is dependent upon the heterogeneities of fracture intensity and vertical permeability. The changing fluid saturations in the matrix are primarily oil replaced by hydrocarbon gas, but six fluids could be present and change: methane and steam, hot low-viscosity and cold high-viscosity oil, formation water, and condensed steam. The important zone is above the fracture gas/oil contact, where drained oil is replaced by the secondary gas cap or steam. Time-lapse pulsed neutron capture logs during the pilot phase did not provide the required matrix fluid saturations due to interference from variable annular fluids in the poorly cemented casing that masked the reservoir response. There is no logging tool available that is capable of measuring the matrix oil saturation change without being influenced by the other fluids in the matrix or the fracture-controlled fluid levels in the casing annulus, or characterized for an open gas-filled borehole, and built to withstand 247°C. The physics of nuclear magnetic resonance (NMR) provides the best chance to fulfill this reservoir surveillance requirement. This paper recounts the decision process that preceded this conclusion, and suggests a method of building a high-temperature non-metallic and non¬magnetic flask to facilitate NMR time-lapse logging in dedicated openhole observation wells in TA-GOGD developments.
TA-GOGD is thermally assisted gas-oil gravity drainage suited for heavy oil in highly fractured formations. Steam is injected into the fractures to serve two purposes: to apply a gas gradient across the matrix blocks so that the oil in the matrix drops down by gravity and to heat the carbonate matrix blocks so that the reduced viscosity oil drips out faster. The reservoir has two fluid systems, which are gas/oil/water levels in the fractures and gas/oil/water contacts in the matrix blocks that are separated by the fractures. Reservoir surveillance requires logs of the remaining oil saturation to confirm the recovery, which is dependent upon the heterogeneities of fracture intensity and vertical permeability. The changing fluid saturations in the matrix are primarily oil replaced by hydrocarbon gas, but six fluids could be present and change: methane and steam, hot low-viscosity and cold high-viscosity oil, formation water, and condensed steam. The important zone is above the fracture gas/oil contact, where drained oil is replaced by the secondary gas cap or steam. Time-lapse pulsed neutron capture logs during the pilot phase did not provide the required matrix fluid saturations due to interference from variable annular fluids in the poorly cemented casing that masked the reservoir response. There is no logging tool available that is capable of measuring the matrix oil saturation change without being influenced by the other fluids in the matrix or the fracture-controlled fluid levels in the casing annulus, or characterized for an open gas-filled borehole, and built to withstand 247°C. The physics of nuclear magnetic resonance (NMR) provides the best chance to fulfill this reservoir surveillance requirement. This paper recounts the decision process that preceded this conclusion, and suggests a method of building a high-temperature non-metallic and non¬magnetic flask to facilitate NMR time-lapse logging in dedicated openhole observation wells in TA-GOGD developments.
Further development of the fractured Natih field requires a good understaliding of the historical contributiolis from gas/oil gravity drainage and water/oil displacement. A history match of the field has been made using a fractured reservoir simulator, which apart from dual porosity, can model dual permeability and block-blook interaction. Several studies aimed at describing the reservoir in detail were required before simulation could start. The history match shows that gas/oil gravity drainage has been about twice as effective as water/oil displacement. This figure is in good agreement with the gas and water sweep data derived from recent gas saturation and water saturation measurements in the field. Prediction runs show that promoting gas/oil gravity drainage by lowering the fracture oil rim is an attractive way to further develop the Natih field.
Understanding the movement of gas in oil reservoirs is important because premature breakthrough or high gas-oil ratios significantly increase costs and can reduce ultimate recovery. The Prudhoe Bay and Kuparuk River oil fields, located on the North Slope of Alaska, have an expanding gas cap and a gas storage area, respectively. Significant gas movement in both reservoirs has occurred, since discovery, and the vertical and areal extent are not as well defined as desired. The current methods of monitoring are expensive and increasingly less reliable because of well conditions. In a search for better methods to monitor gas movement, borehole and surface gravity techniques were identified. The Borehole Gravity Meter (BHGM), because of its large radius of investigation, can predict the true gas-oil contact, irrespective of a localized gas cone and other near wellbore completion problems. Bypassed lenses of oil and intervals of gas under running oil can be identified. The data from the BHGM can provide the vertical distribution of gas and oil at a well. To better understand the areal distribution of gas, high resolution surface gravity data can potentially be used. As the gas cap propagates outward and/or downward, a negative gravity anomaly is created due to the displacement of oil by gas. The results show that significant gravity anomalies have occurred in the Prudhoe Bay and Kuparuk River oil fields due to gas replacing oil. The gravity anomalies can be measured with the borehole gravity and in certain situations with surface gravity instruments. New surveying techniques are needed to obtain increased vertical and lateral resolution for reservoirs buried as deep as the North Slope fields. Global Positioning Systems (GPS) is an economically viable method for obtaining accurate surface positions for locating surface gravity stations. Surface gravity data resolution can be improved further by stripping and downward continuation. The newly developed elevator sonde for the borehole gravity meter, may significantly improve the vertical resolution of the tool in deep deviated wells. Additional developments to improve deviation and size limitations of the BHGM, data acquisition and processing of surface gravity, and testing of GPS in Arctic latitudes are required. Plans to field test these techniques and provide data to design new techniques and tools are on going.
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