The first Borehole to Surface Electromagnetic (BSEM) pilot field survey in the Kingdom of Saudi Arabia (KSA) was successfully executed to identify oil and water bearing reservoir layers in a carbonate oilfield water injection zone. Maximizing recovery factor by means of detailed mapping of hydrocarbon accumulations in the reservoirs is a key requirement for oil producing companies. This mapping is done routinely by accurate measurements of fluid distribution at the wells' locations, but a knowledge gap exists in the inter-well volumes, where typically only density-based measurements are available (seismic and gravity). Such technologies are not always effective to discriminate and quantify the fluids in the porous space (especially when difference in fluid densities is small, such as oil and water). On the contrary, when high electrical resistivity contrasts exist between hydrocarbons and water, electromagnetic (EM) based technologies have the potential to map the distribution of the fluids and monitor their movement during the life of the field, hundreds of meters or kilometers away from the wellbores. The objective of a BSEM survey is to obtain fluid sensitive resistivity and induced polarization maps. These are based on an acquisition grid at the surface, a few kilometers around the EM transmitting well, which reveal oil and water bearing zones in the investigated reservoir layers. In this pilot field test, BSEM showed the potential to map water-front movements in an area of about 4km from the single well surveyed, evaluate the sweep efficiency, identify bypassed/ lagged oil zones, and eventually monitor the fluid displacements, if surveys are repeated over time. The data quality of the recorded signals is highly satisfactory. Fluid distribution maps obtained with BSEM are coherent with production data measured at the wells' locations, filling the knowledge gap of the interwells area.Three key R&D objectives for this BSEM pilot are achieved. Firstly, the capability to record at the surface EM signals generated in the reservoir, secondly, the capability of BSEM to discriminate between oil and water saturated reservoir zones, and finally obtain resistivity maps and a fluid distribution estimate plausible and coherent with the information obtained from well logs, crosswell EM, production data and reservoir models. In addition to reservoir monitoring, BSEM can be very useful in non-diagnosed areas like exploration fields for hydrocarbon exploitations.
In reservoir surveillance, gas saturation is routinely monitored both in gas reservoirs for reservoir performance and in saturated oil reservoirs to prevent gas coning or to optimize infill drilling well placement. This paper presents a new pulsed neutron technology and method that enable the quantitative monitoring of the gas saturation variations to address these reservoir management issues. One of the key features of the newly designed pulsed neutron tool is the new type of Lanthanum Bromide (LaBr3) detectors. The extra-long spacing of the far detectors provides a larger volume of investigation that is more representative of the actual reservoir condition. The quantitative aspect of the measurement is achieved by using the ratios of the detector counts, so that the rock matrix effects are diminished, as opposed to the traditional sigma measurement, which can be influenced significantly by the rock matrix properties. This new tool and data interpretation methodology have been tested in both clastic and carbonate reservoirs with encouraging results. This paper presents an overview of the technology and some field application examples.
Due to the shallow depth of investigation of logging tools such as Nuclear Magnetic Resonance (NMR), the signal interpretation of the flushed zone must be performed carefully. Understanding invasion effects on the logs is an important prerequisite for any petrophysical evaluation. While it is relatively easier to evaluate and correct for the effect of filtrate invasion in basic logs, such as triple combo, special care must be taken for advanced logging techniques such as NMR. For example, it is generally assumed that the volume of remaining wetting fluid in the flushed zone equals to the volume of micropores that do not contribute to flow when the well is produced. The amount of these immobile fluids is estimated using the NMR bound fluid log, a key input for the prediction of rock quality and well performance, especially in complex clastic and carbonate pore systems. In certain formations, NMR bound fluid logs exhibit some differences between adjacent wells drilled with oil-based (OBM) and water-based muds (WBM). This paper summarizes the lessons learnt from a laboratory NMR study of oil-based mud filtrate (OBMF) invasion as a function of rock mineralogy and microstructure, mud chemistry and displacement/flow pressure. In this work, we studied the effect of a commercial surfactant usually added in OBM formulations. We investigated the effect of different surfactant concentrations on the fluid-fluid interfacial tension (IFT) properties and on the fluid-solid interaction properties, using contact angle measurements on both sandstone and carbonate model surfaces. Furthermore, we investigated the effect of the additive on the capillary pressure properties and remaining water saturations on sandstone and carbonate rocks. To maximize the generality of the results we used two very different driving mechanisms for the fluid displacement: centrifuge and flow-through. The data showed that carbonate and clastic rocks behaved differently over a wide range of flow mechanisms and water saturations, proving that mineralogy plays a crucial role in the fluid displacement. Under the measurement uncertainties, the irreducible water saturation, however, remained constant regardless of the OBMF composition or driving mechanism. We showed how sandstone and carbonate rocks behave in respect to wettability alteration due to a surfactant used in OBM formulations. The systematic difference, whatever the driving mechanism is, strongly suggests that the differences in NMR responses between sandstone and carbonate originate from chemical composition and surface properties rather than microstructural differences between sandstone and carbonate rocks.
Remaining Oil Saturation (ROS) is a crucial input to reservoir development projects. Determination of such a time-dependent parameter assists the evaluation of the sweep efficiency, provides calibration points for simulation models and also sets the basis for future workover and EOR projects. Quantification of ROS in waterflooded areas has always been a known challenge to resistivity-based techniques. The mixed water salinity environment, due to differences between original connate water and injected water, and the impact of the imbibition process on the saturation exponents are the main hindrances. Over the past few years, data acquisition strategies have been deployed to overcome these challenges. The strategies include in-situ measurements that are insensitive to water salinity and to fluid displacement processes, such as Nuclear Magnetic Resonance in Log-Inject-Log mode, Carbon/Oxygen, and Dielectric logs. Extensive wireline formation testing programs usually follow to confirm the findings from the aforementioned measurements. Depending on the well's criticality, special coring programs might also be included such as sponge coring or liquid trapper. Special planning and standardization efforts are necessary for such extensive data acquisition programs to ensure data quality and consistency. This paper presents Saudi Aramco data acquisition strategies for ROS applications. It also highlights the main challenges and the technologies deployed to resolve them. Advantages and limitations of these technologies are reviewed. Lastly, our ongoing efforts to reduce the uncertainties of such analyses are presented.
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