Surfactant Transport Through Porous Media in Steam-Foam Processes
- H.C. Lau (Shell Development Co.) | S.M. O'Brien (Shell Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1988
- Document Type
- Journal Paper
- 1,177 - 1,185
- 1988. Society of Petroleum Engineers
- 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.1.2 Separation and Treating, 2.4.3 Sand/Solids Control, 5.2.1 Phase Behavior and PVT Measurements, 2.5.2 Fracturing Materials (Fluids, Proppant), 5.2 Reservoir Fluid Dynamics, 2.7.1 Completion Fluids, 5.3.4 Reduction of Residual Oil Saturation, 5.4.1 Waterflooding, 4.1.5 Processing Equipment, 5.4.6 Thermal Methods
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Summary. Experimental and theoretical studies show that the transport of steam-foam surfactants through reservoir sands can be substantially retarded as a result of cation exchange between the surfactant solution and the formation clays. Results, however, show that a high injected salinity favors surfactant propagation by displacing divalent cations faster and by reducing partitioning when divalent cations are present.
Efficient transport of injected surfactant through the formation is important to the success of a steam-foam process because the speed at which a steam foam traverses the formation cannot exceed the speed at which the surfactant is transported. Therefore, a proper understanding of the mechanisms that govern surfactant transport is important.
In situations where the formation sands possess a high cation exchange capacity (CEC) and a high divalent-cation content, as is the case with the Kern River field in California, cation exchange between the injected surfactant solution and the formation clays can be a crucial mechanism for controlling surfactant transport. This is because the monovalent cation in the surfactant solution can exchange with the divalent cations on the clays, leading to a buildup in divalent cation in the aqueous phase. This buildup in divalent cation can cause partitioning and/or precipitation of surfactant. Combined with the effect of adsorption, partitioning and precipitation can lead to substantial retardation of surfactant propagation.
These effects are illustrated in Fig. 1. which shows the results of a steam-foam experiment performed with a sandpack containing Kern River sands, crude oil, and synthetic interstitial water. The experiment began with the sandpack at residual oil saturation (ROS) to steamdrive at 212 deg. F [100 deg. C]. Fifty-percent-quality steam with 0.5 wt% Enordet AOS 1618 steam-foam surfactant and 1 wt % NaCl in the aqueous phase and 0.6 mol % nitrogen in the vapor phase was then injected at a high flow rate. phase was then injected at a high flow rate. Fig. 1 shows that cation exchange between the injected surfactant solution and the clays resulted in a buildup in calcium ions, which retarded propagation of the surfactant. Also, if we define foam breakthrough as the time when steady state in pressure is reached throughout the sandpack, Fig. 1 shows that there was no substantial lag between the surfactant and foam breakthroughs. This implies that foam propagation through a formation may be limited by surfactant propagation.
The importance of cation exchange depends on two factors: the CEC and the fraction of formation clays in divalent-cation form. The former can be measured from core samples and the latter can be calculated from the interstitial water composition and the selectivity coefficient of the clays (cf. Eqs. 13 and A-2).
Appendix A shows that even for a relatively "soft" synthetic Kern River interstitial water (Table 1), which contains 64 ppm Ca++, as much as 87% of the Kern River formation clays are in calcium form. Under such conditions, the theory of ion exchange predicts that cation exchange between the injected surfactant predicts that cation exchange between the injected surfactant solution and the clays is controlled by the injected salinity.
Effect of Salinity on Surfactant Propagation In the Absence of Oil
In our experimental study, we first considered the effect of injected salinity on surfactant propagation in the absence of oil but including the effects of ion exchange and adsorption. To simulate conditions similar to those of the Shell steam-foam pilot at the Mecca lease, Kern River field, we conducted corefloods with Kern River reservoir sands. To simplify our studies, we conducted these hot surfactant corefloods in the absence of a vapor phase and at a constant temperature of 212 deg. F [100 deg. C]. The validity of these simplified corefloods to simulate surfactant transport in a steam-foam process was established by similar results obtained by corefloods process was established by similar results obtained by corefloods in which steam and nitrogen were coinjected with the surfactant (Fig. 1).
At the beginning of each experiment, the sandpack was saturated with synthetic Kern River interstitial water (Table 1). Table 2 gives sand data and experimental conditions. Each experiment began with injection of 0.5 wt% (active matter) Siponate A-168, an alpha olefin sulfonate (AOS) supplied by Alcolac Inc., at a pre-determined salinity.esults of corefloods conducted at two salinities, pre-determined salinity.esults of corefloods conducted at two salinities, 1 and 4 wt% NaCl. are shown in Fig. 2. In either case, a calcium peak was noted in the effluent, indicating a substantial buildup in calcium resulting from cation exchange between the injected solution and the formation clays. In the case with 4 wt% NaCl, the calcium peak was much higher and the subsequent decrease in calcium more rapid compared with the case with 1 wt% NaCl. This is in agreement with the predictions of the theory of ion exchange.
Fig. 2 also shows that the frontal propagation rate of the surfactant was retarded. Had there been no retardation, the midpoint injected concentration would have broken through at 1.0 PV. To elucidate whether this retardation was beyond that caused by adsorption alone, we compared the effluent surfactant concentrations of Fig. 2 with those of dynamic adsorption experiments using a coreflooding technique (see Adsorption Experiments). The comparison (Fig. 3) clearly shows that surfactant retardation in the presence of ion exchange (cf. curves labeled "Adsorption and presence of ion exchange (cf. curves labeled "Adsorption and Precipitation") is larger than that caused by adsorption alone. This Precipitation") is larger than that caused by adsorption alone. This increased retardation is a result of precipitation caused by a buildup in calcium-ion concentration.
Fig. 2 shows an overshoot in the effluent surfactant concentration at both salinities. This is believed to be the result of redissolu-tion of the surfactant, which had precipitated at high calcium concentrations.
From the surfactant breakthrough curves, it was estimated that the propagation rate of the surfactant front with 1 wt% NaCl was less than that with 4 wt% NaCl by 22%. This result shows that in the case where cation exchange takes place between the formation clays and the injected surfactant solution, the surfactant prop-agation rate is a sensitive function of salinity. A low salinit prop-agation rate is a sensitive function of salinity. A low salinit results in a more pronounced surfactant retardation.
Effect of Salinity on Surfactant Propagation In the Presence of Oil
In the presence of oil, surfactant retention is caused by partitioning in addition to precipitation and adsorption. partitioning in addition to precipitation and adsorption. SPERE
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