Impacts on oil recovery from capillary pressure and capillary heterogeneities

Abstract

This thesis summarises the findings made in NFR-funded project “Capillary Pressure and Capillary Heterogeneities”. The focus has been to determine the impact on oil recovery from wettability and fractures in carbonate rocks. Secondly a new method for measuring capillary pressure has been developed. The results of this work are reported in this thesis. The work presented in this thesis has improved the understanding of the interaction between wettability and fractures on the production mechanisms in carbonate reservoirs, in different directions. Figure 1 gives an overview of the project “Capillary Pressures and Capillary Heterogeneities”, its fundamental building blocks and the interaction between them to better understand how main conclusions in this study may be drawn; Building block 1: A reproducible method for altering wettability has been developed, and continuously improved throughout the study. Feed-back from the large scale block experiments have actively been used to improve the wettability alteration technique, in particular when it comes to radial and lateral wettability heterogeneity. This is further described in Paper 1 and 2. Building block 2: The study of fracture crossing mechanisms at different wettabilities has improved the understanding of the production mechanisms in fractured chalk. The observations in the large scale block experiments have been used to understand the results of the fracture crossing experiments and vice versa. This is further described in Paper 4. Building block 3: Capillary pressure and relative permeability curves have been measured at different wettabilities. The capillary pressure curves in particular, have also been measured using different centrifuge methods. The experimental results have actively been used in numerical simulations. The numerical simulations in return have been used as QC for the experimental results, especially when measuring the spontaneous and forced imbibition curves. This is described in Paper 1 and 6. Building block 4: Large scale waterflood experiments have been performed at different wettabilities, using different fracture networks, measuring the in-situ saturation development. This has led to an improved understanding of the production mechanisms in this type of reservoirs. Further details can be seen in Paper 3 and 5. Building block 5: Numerical simulations have actively used the experimental data obtained from the large scale block experiments and the capillary pressure/relative permeability, and at the same time provided an improved understanding on the potential impacts of changing the parameters. It has also helped us to improve the interpretation of the results. Paper 3 and 5 treats this aspect in detail. Building block 6: The outlines of a method for measuring in-situ capillary pressures at different wettabilities have been presented, and a feasibility study performed. If this method is further developed and successful, it could be possible to measure in-situ capillary pressure curves using live crude oil at reservoir conditions. This work is reported in Paper 1, 6 and 7. The main conclusions drawn from this thesis are; 7 scientific papers are published on a broad variety of subjects, and describes in detail the experiments and research treated in this thesis. Scientific research has been performed, investigating the subjects of capillary pressure and capillary heterogeneities from different angles. This thesis discusses the findings in this study and aims to illustrate the benefits of the results obtained for further development of other experiments, and/or even the industrial benefits in field development. The methods for wettability alteration have developed throughout the work. From producing heterogeneous wettability alterations, the methods have improved to giving both radial and lateral uniform wettability alterations, which also remains unaltered throughout the duration of the experimental work. The alteration of wettability is dependent on initial water saturation, flow rate, aging time and crude oil composition. Capillary pressure and relative permeability curves have been measured for core plugs at different wettabilities using conventional centrifuge methods. The trends observed are mostly consistent with theory. The production mechanisms of strongly and moderately water wet chalk has been investigated. At strongly water wet conditions in fractured chalk; the flow is governed by capillary forces, showing strong impact from the fractures. At moderately water wet conditions, the impact of the fractures are absent, and a dispersed water front is observed during the displacement. The oil recovery is about the same, at the two wettabilities. Fracture crossing mechanisms at the same wettability conditions have been mapped. And the observations are consistent with those of the water floods. During strongly water wet displacement, the fracture crossing is occurring once the inlet core has reached endpoint of spontaneous imbibition. At moderately water wet conditions the fracture crossing is less abrupt, and creation of wetting phase bridges is observed. The water may pass the capillary discontinuity before inlet core is at endpoint for spontaneous imbibition. The observations of the water flood experiments have been validated using numerical simulators Eclipse and Sensor. Experimentally measured capillary pressure and relative permeability curves have been used to history match the observed production of the waterfloods. The observed variations in production mechanisms at wettability change are confirmed. Direct measurement of saturation methods for measuring capillary pressure scanning curves have been investigated and compared to conventional centrifuge techniques. The same trends are observed for curves measured at different wettabilities, and the capillary pressure curves measured using DMS methods have also been validated in numerical simulations of type Eclipse and Sensor. A feasibility study to develop a new method of measuring capillary pressure at various wettabilities has been performed with encouraging results. The conclusion is that the work should be further developed. The method has potential to enable capillary pressure measurements using live crude oil at reservoir conditions. All in all, several experimental methods applicable in future SCAL synthesis have been presented. The observations are consistent and underline the production mechanisms of fractured chalk reservoirs, and will serve as inspiration in the future evaluations of tertiary oil recovery processes. An innovative approach to the measurement of capillary pressure is suggested.