A study of capillary pressure and capillary continuity in fractured rocks

Abstract

The production of oil is challenging in fractured reservoirs due to the large transmissibility contrast between matrix and fracture, and primary recovery is often low. The recovery efficiency depends on the relationship between the fracture and matrix permeabilities, and is strongly dependent on the wettability of the matrix, which reflects the imbibition potential of the reservoir. High demands and rising oil prices has increased focus on improved oil recovery from large, low recovery oil fields. Some of the world’s largest remaining oil reserves are found in oil‐wet, fractured, carbonate reservoirs. The understanding of multiphase fluid flow in oilwet fractured reservoirs has been studied in this thesis, especially the influence of capillary pressure. The presence of capillary pressure is important in recovery mechanisms like spontaneous imbibition, waterflooding and gravity drainage. The centrifuge method is a frequently used method to measure capillary pressure, and relies on establishing a stable saturation for each rotational speed. There exists no global, absolute requirement for equilibrium, and this size is often based on experience, and is strongly dependent on the sensitivity of the measuring apparatus. The benefits of using an automated, high resolution camera in volume measurements have been demonstrated, and the impact of accuracy on the time to reach equilibrium saturation at a given rotational speed is illustrated. Another difficulty when generating the capillary pressure curve using a centrifuge is the large uncertainty related to solving the integral problem associated with the calculation of the capillary pressure curve from production data. Methodologies for direct measurement of saturation to avoid this uncertainty have been proposed, eliminating the need for mathematical approximate solutions to obtain the local capillary pressure curve. The Nuclear Tracer Imaging Centrifuge (NTIC) method has the capability to measure the local water saturation during centrifugation, thus limiting the redistribution of fluids and the need to solidify phases, drawbacks associated with other methods for direct measurement of capillary pressure. Improved capillary pressure curves are presented, and the reliability and reproducibility in the NTIC capillary pressure curves have been demonstrated. The curves generally coincided with results from other existing centrifuge methods. The correct measurement of saturation as a function of capillary pressure will increase the confidence in simulations where the input multiphase controls the flow patterns and the recovery. The impact of wettability on capillary continuity in fractured rocks has been studied extensively, but is still not fully understood. Two visualization methods, to measure the in situ fluid saturation development in fractured rocks, are reviewed and illustrate the benefits of applying complimentary imaging to study the impact of fractures and wettability on multiphase flow in fractured reservoirs. Separately, each technique provided useful insights to local phenomena, but collectively, when combining the resolutions and observations made, a better explanation of observed phenomena could be obtained. The concept of wetting phase bridges observed during waterfloods in stacked waterwet homogenous chalk plugs has been extended to a heterogeneous limestone rock type with an oil‐wet wetting preference. The study shows how droplets of oil forming on the fracture surface contribute to the fluid transfer between two separated matrix blocks across an open fracture. The presence of droplets, evolving into bridges across the fracture, may be important for gravity drainage, reducing the capillary retained oil in each isolated matrix block. Droplets growth is impacted by the wettability of the interface between fracture and matrix and flow rates. Spontaneous transport of oil, i.e. transport without associated pressure increase, across the fracture was observed when there was an affinity between mobile fluid and the wettability of the fracture surface. Injection rates and pressure across the fracture controlled droplet growth and the potential for the droplets to bridge the fracture to form a continuum in the capillary pressure curve. The importance of fracture capillary pressure in waterfloods of fractured limestone rocks was demonstrated in a numerical reproduction of experimental results. The results showed not only that there was a dependency of the presence of capillary pressure in the fracture, but also there was a strong dependency of the distribution of the capillary pressure inside the fracture network on the development of waterfronts during water injection.