Filtration Properties of Oil-in-Water Emulsions Containing Solids
- Kassim Al-Riyamy (U. of Texas at Austin) | Mukul M. Sharma (U. of Texas at Austin)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2004
- Document Type
- Journal Paper
- 164 - 172
- 2004. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.6.9 Coring, Fishing, 2.2.2 Perforating, 3 Production and Well Operations, 4.6 Natural Gas, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant), 2.2.3 Fluid Loss Control, 2.7.1 Completion Fluids, 1.6 Drilling Operations, 5.3.2 Multiphase Flow
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This paper describes a laboratory study of the factors controlling the filtration and fluid-leakoff properties of emulsions containing solid particles. Such fluids are sometimes used as low-leakoff completion fluids.
High-pressure/high-temperature (HP/HT) filtration tests clearly demonstrate that emulsions have the ability to reduce filtrate loss to the formation. However, the emulsion droplets tend to invade the formation and form an internal filter cake. This is evident from the liftoff pressures needed to initiate flowback and deep invasion of emulsion droplets in long-core experiments. Emulsions containing solids (CaCO3, in our case) had lower filtrate volume and higher return permeabilities than solids-free fluids. The effects of percent oil, filtration pressure, core permeability, temperature, and viscosity of the continuous phase were investigated. Filtration pressure and core permeability had major influences on the filtration properties of the emulsified completion fluids. Higher injection pressures increased internal damage and lowered the return permeability. Higher-permeability cores had higher filtrate loss. Emulsion droplets were observed in the effluent, and solid particles are needed to form a stable external filter cake.
Long-core experiments showed that using emulsions containing acid-soluble solid particles have 100 percent return permeability after an acid squeeze, showing that the emulsion-droplet invasion depth is less than 1 in. It is shown that solids-free fluids had the highest formation damage and higher liftoff pressures.
Analysis of the fluid-leakoff data indicates that emulsions containing solids do not behave as classical filtration theory predicts because of the invasion of emulsion droplets into the formation.
Sizing the solid particles and emulsion droplets in completion fluids and muds according to the permeability of the rock is important in forming stable external filter cakes. This increases the return permeability and results in easier cake removal (low lift-off pressures).
A number of studies have shown that oil-in-water emulsions can be used as perforating/completion fluids.1-3 Such fluids form stable external filter cakes with minimal invasion. Other studies show that emulsions can cause permeability reduction.4-7 Oil-in-water emulsions have been used as plugging agents and as diverting agents.8 This use of emulsions as both fluid-loss-control materials and plugging agents suggests that they should be used with care. This paper addresses factors influencing the filtration properties of oil-in-water emulsions.
Priest and Morgan1 developed a nonplugging perforating fluid; the principal components of the oil-in-water emulsion were an aqueous phase consisting of either sodium chloride or calcium chloride and an oil phase consisting of diesel oil, tetrachloroethylene (not environmentally acceptable today), or a combination of both. They demonstrated by laboratory tests, perforating tests, and experimental field tests that certain emulsions serve effectively as nonplugging perforating fluids. An emulsion prepared with a suitable emulsifying agent was found to have high fluid loss. To reduce the fluid loss of the emulsion, different film-strengthening agents were used to envelop each oil droplet with a tough film. The completion fluids prepared had no insoluble solids. The filtration properties of the emulsions prepared with different emulsifying agents and film-strengthening agents were tested in an API filter cell. The API fluid loss of the emulsions varied from 0.5 to 9 mL at room temperature. Simulated perforation tests conducted by Priest and Morgan1 showed that the perforations had almost no debris; the emulsion was easily removed (flow-initiation pressure <3 psi) from the perforations, and the return permeability of the cores was between 66 and 105 percent of the original permeability (160 to 240 md). They noted that the flow rate into the wellbore was highest with an oil/water emulsion containing calcium chloride as a weighting material.
Priest and Allen2 developed a solids-free emulsion specifically for gun perforating. It consisted primarily of 40 percent oil emulsified in saturated calcium chloride brine. The oil phase was either diesel or carbon tetrachloride. The maximum density achieved was 12.5 ppg. A lignosulfonate was used as an emulsifier and filtration-control agent. The emulsion was stable for 24 hours only. Results from the field (160 perforating jobs) showed that the emulsion caused no impairment either when perforating or during workover jobs (˜61 wells) with exposed perforations.
Darley3 developed a process of preparing a formation-plugging material, which may be unplugged readily without causing damage to the formation. The oil phase was mixed with fine, oil-wet, acid-soluble, solid particles that help in forming an emulsion, which is stable at the temperature of the formation. In the emulsion, the acid-soluble particles were selectively oil-wetted so that they can be concentrated at the oil/water interface. The order of mixing the components is important. The acid-soluble particles such as chalk (CaCO3) and the oil-wetting surfactants such as tall oil (Acintol A1) were added to the oil phase. An interfacial-tension-lowering surfactant was added to the water phase, and the water and oil phases were mixed together and agitated to form a stable emulsion.
The experimental part of this paper presents the results of experiments run in a modified HP/HT filtration cell (modified to accommodate 1-in.-long cores) and in a coreholder. The objectives of these experiments were to evaluate the effectiveness of the emulsified fluids tested in forming a stable external filter cake with minimal damage (i.e., low initiation pressures) and to analyze the filtration properties of emulsified fluids containing solids. Constant-pressure tests were used to evaluate the filtration properties of concentrated emulsions, permeability reduction, initiation pressures, and return permeability by flowback with n-decane.
The formation of filter cakes from fluids containing emulsion droplets and solid particulates is a complex phenomenon. To the best of our knowledge, there are no mechanistic published studies in this area. The model we present here is an attempt to understand the filtration (fluid-leakoff) behavior observed in our filtration tests.
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