Synthesis and Characterization of Reversible Emulsions: Application to Completion Fluids

by
Kassim Mohamed Al-Riyamy, Ph.D.

University of Texas at Austin, 2000
Supervisors: Mukul M. Sharma

A new class of emulsifying agents that form reversible oil-in-water emulsions are synthesized. The emulsifying agent can be triggered by changes in the pH. The new polymeric emulsifier consists of hydrophilic and hydrophobic blocks. The hydrophobic blocks are formed spontaneously and reversibly by the complexation of two hydrophilic segments. Changing the pH of the solution breaks these complexes allowing the emulsifier to partition into the oil phase. The synthesis procedure and the effect of varying the hydrophobic monomer concentration, chain transfer agent concentration and initiator type are investigated. The emulsifier is characterized by drop coalescence, emulsion stability and interfacial tension measurements. The emulsification properties of this new emulsifying agent were found to be excellent and very pH sensitive at pH values below 3.

The objectives of this research are to synthesize and characterize a new pH sensitive emulsifier and to investigate the viability of using it as a temporary blocking agent during well completion. This blocking agent would reduce fluid loss to the formation, which reduces the extent of formation damage.

The experimental program consisted of injecting well-characterized oil-in-water emulsions into cores containing residual water saturation. Experiments were conducted on short cores in a modified HPHT API cell where filtration properties and return permeabilities are measured. Long core experiments were conducted to investigate the extent of invasion of emulsion droplets into the formation by measuring the return permeabilities of different sections of the core. Texas limestone and Berea sandstone cores were used as porous media.

Experimental results indicated that the emulsified completion fluids reduced fluid loss to the formation compared to fluids with no emulsified oil. The filtrate volume for the emulsion fluids tested compares favorably with other completion fluids but is large compared to conventional drilling muds because of the absence of other filtration control agents, such as, bio-polymers and starch. Dimensionless return permeabilities to n-decane varied between 65 to 95 %. However, flow initiation pressures were higher than sized CaCO3 drill-in fluids reported in the literature. This is attributed to invasion of emulsion droplets into the formation and the absence of starch as a filtration control agent. The permeability of the core, injection pressure, temperature, percent oil, viscosity of the continuous phase, solid particles present, and droplet size all play important roles in determining the filtration properties of the completion fluids. It is observed that high overbalance pressures, high temperature, low percent oil, low viscosity of continuous phase, absence of CaCO3 and small droplet sizes contribute to higher cumulative filtrate volume. Overbalance pressure has a major effect on the filtration properties of the emulsion. Higher overbalance pressures result in deeper invasion of emulsion droplets. The deformability of the emulsion droplets makes this behavior possible. The larger extent of invasion results in increased flow initiation pressures and lower return permeabilities. In some instances emulsion droplets were found to flow through the entire 1 inch length of the core. CaCO3 particles played an important role in controlling the fluid leakoff for high permeability cores and to limit the invasion of emulsion droplets into the formation.

Long core experiments (6 inch cores) showed deep invasion of the emulsion droplets into the core in the absence of CaCO3 particles in the completion fluid. Excellent return permeabilities (~100 %) were obtained with fluids containing CaCO3 particles. Long core experiments showed that the completion fluid is acid degradable if the invasion of the completion fluid is shallow.

Outmans model for static filtration of compressible filter-cakes was used to calculate the filtrate volume versus time. The experimental results were not in agreement with this classical filtration theory. This indicates that emulsion droplets and solids are invading the formation while the external filter-cake is mainly composed of the solid particles. A simplified mathematical model has been adapted from the literature to compute the pressure required to squeeze an emulsion droplet through a pore constriction. These calculations show that the maximum pressure needed to squeeze a droplet through a pore constriction is dependent on droplet size, interfacial tension and geometry of the porous medium. The values were low compared to the high filtration pressures used, which implies that emulsion droplets do invade the formation.

Models provided by Sherwood and Zydney et al. were used to calculate the permeability of a bed of deformable droplets. These models suggest ultra-low permeability at high filtration pressures due to deformation of the droplets. This was not observed in our experiments. In fact measured filter-cake permeabilities were estimated to be in the range of 0.1 md which is several orders of magnitude larger than those estimated by using models for emulsion filter cakes. This also clearly points to the fact that emulsion droplets do not form filter cakes but rather are pushed into the porous medium.

The study presented in this dissertation is the first systematic investigation of the application of reversible emulsifiers in completion fluids. The results presented and their interpretation provide new insights into the mechanisms responsible for filtration of emulsions containing solids and will form the basis for comparing other completion fluids tested under the same conditions. While the reversible emulsion system developed appears promising, problems with high fluid leakoff and invasion of emulsion droplets need to be addressed in the future.

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