Fracture ballooning usually occurs in naturally fractured reservoirs and is often mistakenly regarded as an influx of formation fluid, which may lead to misdiagnosed results in costly operations. In order to treat this phenomenon and to distinguish it from conventional losses or kicks, several mechanisms and models have been developed. Among these mechanisms under which borehole ballooning in naturally fractured reservoirs take place, opening/closing of natural fractures plays a dominant role. In this study a mathematical model is developed for mud invasion through an arbitrarily inclined, deformable, rectangular fracture with a limited extension. A governing equation is derived based on equations of change and lubrication approximation theory (Reynolds’s Equation). The equation is then solved numerically using finite difference method. Considering an exponential pressure-aperture deformation law and a yield-power-law fluid rheology has made this model more general and much closer to the reality than the previous ones. Describing fluid rheology with yield-power-law model makes the governing equation a versatile model because it includes various types of drilling mud rheology, i.e., Newtonian fluids, Bingham-plastic fluids, power-law, and yield-power-law rheological models. Sensitivity analysis on some parameters related to the physical properties of the fracture shows how fracture extension, aspect ratio and length, and location of wellbore can influence fracture ballooning. The proposed model can also be useful for minimizing the amount of mud loss by understanding the effect of fracture mechanical parameters on the ballooning, and for predicting rate of mud loss at different formation pressures.
Farzaneh, Seyed Amir (Heriot-Watt University) | Dehghan, Ali Akbar (Sharif University of Technology) | Ghazanfari, Mohammad H. (Sharif University of Technology) | Kharrat, Riyaz (Petroleum University of Technology)
In this work, a series of solvent- and water-injection scenarios were conducted on horizontal five-spot glass micromodels that were saturated initially with heavy oil. Sandstone and limestone rock look-alike and network patterns with different pore structures were used in the experiments. The results show that the ultimate oil recovery of a water-alternating-solvent (WAS) scheme was greater than that of a simultaneously water-alternating-solvent (SWAS) scheme, and the efficiency of a solvent-soak scheme also offers a greater recovery. Likewise, the WAS scheme resulted in greater oil recovery when compared with continuous solvent injection (CSI), with the same amount of solvent consumption. Furthermore, some pore-scale phenomena, such as viscous fingering, diffusion of solvents into heavy oil, and localized entrapment of oil and solvent because of heterogeneity and/or water blockage, are also illustrated. The results of this work can be helpful for better understanding and verification of flow transport and pore-scale events during different solvent-based-injection scenarios in heavy-oil reservoirs.
Ghoodjani, Eshragh (Sharif University of Technology) | Kharrat, Riaz (Petroleum University of Technology) | Vossoughi, Manouchehr (Sharif University of Technology) | Bolouri, Seyed Hamed (U Of Shahid Bahonar Kerman)
Heavy oil in Middle East fractured carbonate reservoirs account for 25-30% of the total oil in place in the region. Production of heavy oil from such reservoirs is thought to play an important role in the future of the ever-growing world's energy consumption in which Iran's recoverable heavy oil is more than 85 billion barrels. The offshore Ferdows field in Iran is reportedly on the order of 30 billion barrels of oil and holds perhaps the greatest promise to add significant future carbonate heavy oil production within the region.
With depletion of conventional petroleum reserves and increase of hydrocarbon fuel demand, there is no doubt that there will be a tremendous demand on the development of heavy oil reservoirs in the coming decades. Despite its strategic importance, recovery of heavy crude from fractured carbonate reservoirs has found limited applications due to the complexity of such reservoirs. As most of the oil is stored in matrix due to its higher storage capacity than fracture network, reservoir development plans will aim at maximizing the matrix oil recovery. For reservoirs with high recovery factor, minimizing matrix residual oil saturation is a critical issue to extend the life of the reservoir. For reservoirs with low recovery factor, accelerating the production rate is more vital. For each of these reservoir types, different Enhanced Oil Recovery (EOR) methods should be considered and implemented accordingly.
In this study, a comprehensive review is conducted to figure out the feasibility of heavy oil recovery from fractured carbonate reservoirs by use of Cyclic Steam Stimulation (CSS), Steam injection, In-Situ Combustion (ISC), Steam Assisted Gravity Drainage (SAGD), Vapor Extraction (VAPEX) and Expanding Solvent-Steam Assisted Gravity Drainage (ES-SAGD).
Carbonate reservoirs introduce great challenges due to their complex fabric nature (low matrix permeability, poor effective porosity, fractures) and unfavorable wettability. These challenges are further displayed when combined with increased depth and low grade oil (high density and viscosity). A huge amount of oil is contained in such reservoirs without any technological breakthrough for improving the recovery efficiently (Briggs et al. 1992).
Until recently, heavy oil reserves did not attract much interest. The lowest oil profitability, the low price of the oil barrel in the international market, the difficulties involved in its extraction and its refining, and the large amount of light and medium oils to be explored could not justify the investments. Maturity of light and medium oil fields and the significant increase in oil price placed that source of energy under a new perspective. It is possible to increase heavy oil recovery in some of these reservoirs with the help of enhanced oil recovery processes, thus enhancing oil field productivity and profitability. Screening criteria have been proposed for all enhanced oil recovery (EOR) methods by SPE (Taber et al. 1997) for conventional reservoirs.
The most proven approach to produce heavy-oil reservoirs is through thermal methods, specifically speaking steam injection. Yet, the typical reservoir engineering approach is based on mobility reduction by reducing oil viscosity through effective heating, and by producing oil through viscous and gravity displacement. In carbonate systems, which are fractured in general, introduce rock complexity at different scales, i.e., faults, fissures, micro fractures, vugs, poorly interconnected matrix pore structure, and unfavorable wettability are combined with high oil viscosity. Thus oil recovery from this type of reservoir becomes a real challenge and classic thermal application theories fail to define the process. Main drive mechanisms in fractured reservoirs are shown in Figure 1 (Taber et al. 1997, Farouq and Meldau 1983).
Thermal methods (steam injection or in-situ combustion) and non-thermal methods (VAPEX) may be cited as examples of such processes.
The barrier property and self healing behavior of the coating systems containg organosilane and epoxy was investigated. The organosiolane componenet of coatings was doped with cerium nitrate and applied on cold rolled carbon steel substrates. Electrochemical impedance spectroscopy (EIS) and immersion tests were used for assessment of corrosion resistrance behaviour of coatings. Also SEM and EDS techniques were employed to investigate the self healing ability of cerium nitrate. Addition of 2 wt. % of cerium nitrate into the coating solution led to superiour self-healing property for scribed coating systems. Furthermore healing mechanism of cerium nitrate pigments was explained in detail.
Keywords: cerium nitrate; organosilane; self healing; leaching
Colloidal gas aphron (CGA) based drilling fluids, because of their non-coalescing nature, excellent capability in minimizing deep invasion, and also behaving like a flexible bridging material, are indicated for drilling permeable and fractured formations. Their unique feature is to form a solid free, tough, and elastic internal bridge in pore networks or fractures to minimize deep invasion by means of air microbubbles, which can be removed easily during the initial stage of production.
CGA based fluids combine certain surfactants and polymers to create the system of microbubbles. Surfactant is used to produce the surface tension to contain the aphron as it is formed, build the multilayer bubble wall, and create interfacial tension to form a non-bonding network capable of bridging openings in permeable and fractured formations. Polymer is used as viscosifier and aphron stabilizer. The surface activity and aggregation behavior of the surfactant affects the stability and also other physico-chemical properties of generated microbubbles. Therefore, selection of a suitable surfactant is important for the generation of microbubbles with the desired rheological and filtration properties.
The goal of this paper is to investigate the potential use of a new plant-derived surfactant as an aphronizer surfactant in preparation of CGA based drilling fluids for accomplishing desirable rheological and filtration properties. For this purpose, natural surfactant obtained from leaves of special tree namely Zizyphus Spina-Christi and used for preparation of aphron-based fluids. To evaluate the potential use of new plant-derived surfactant as an aphronizer, various physico-chemical properties of aphron-laden muds were investigated. To achieve the research objectives, laboratory tests of suspension generation, microscopic visualization, initial yield, filtration loss, and rheological behavior with varying concentrations of surfactant and polymer were performed.
Effect of base fluid viscosity and surfactant concentration on size/size distribution of microbubbles, rheology, and yield of CGA based drilling fluids will be presented. Three rheological models, namely, Bingham Plastic, Power Law and Casson models were used for characterizing rheological properties of the muds studied.