Redesigned Ester Single-Well Tracer Test That Incorporates pH-Driven Hydrolysis Rate Changes
- S.L. Wellington (Shell Development Co.) | E.A. Richardson (Shell Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1994
- Document Type
- Journal Paper
- 233 - 239
- 1994. Society of Petroleum Engineers
- 1.14 Casing and Cementing, 5.2.1 Phase Behavior and PVT Measurements, 4.2.3 Materials and Corrosion, 3.1.6 Gas Lift, 3 Production and Well Operations, 4.3.4 Scale, 5.3.4 Reduction of Residual Oil Saturation, 5.6.5 Tracers, 4.1.2 Separation and Treating, 5.3.1 Flow in Porous Media, 5.3.2 Multiphase Flow, 4.1.5 Processing Equipment, 1.6 Drilling Operations
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The rate of alkyl ester hydrolysis was found to be sensitive to the pH changes that occurred during application of the single-well tracer test (SWTT) in both sandstone and turbidite reservoirs. Detection methods, an improved test sequence, and the detrimental consequences that this variable can have on oil saturation determination are discussed.
Field tests in carbonate-cemented California turbidite and shaly gulf coast sandstone reservoirs, laboratory studies, literature review, and computer model calculations confirm that alkyl ester hydrolysis rate is sensitive to pH changes observed under field conditions in some oil reservoirs. pH dependence is a consequence of (1) formation of acid, a byproduct of ester hydrolysis, and (2) the partial oil solubility of esters, which causes the ester bank to lag behind the brine front. During injection, generated acid is carried away by the overflush brine as it passes through and runs ahead of the slower ester bank. Therefore, pH and hydrolysis rate remain relatively high in the ester bank. When injection stops, the produced acid remains in the ester bank, lowers the pH, and retards hydrolysis. As production begins, the hydrolysis rate increases because the brine overflushes the ester bank again, carries away the acid, and raises the pH.
When pH-driven rate effects are not included in the test design, implementation, and analysis, interpretation is uncertain because one may be curve matching the shape of transit hydrolysis reaction products that are not linked to the oil saturation through the governing equations. The paper includes field-tested methods to detect pH-driven rate effects and a test sequence that measures and accounts for transit reactions that can invalidate the results. We hope that the detection methods and new test sequence can be used to minimize the problem and improve the reliability of the ester-based SWTT.
Ester SWTT Process Fundamentals
The process involves injecting a tracer "slug" or "bank," then overflushing it into the reservoir to the desired distance beyond the wellbore. Then, injection is halted to allow time for the reacting tracer to hydrolyze (the soak time or period). Next, the well is put on production and the returning tracer concentrations are monitored. Tomich et al.1 showed that injected ethyl acetate (CH3COOC2H5) solutions partially hydrolyze at reservoir conditions to form ethyl alcohol (C2H5OH). The generated ethyl alcohol, which is essentially a nonpartitioning tracer, separates from the unreacted partitioning tracer, ethyl acetate, during production and allows determination of oil saturation.
Other chemically distinct nonreacting water-soluble-only tracers are usually added at various stages of the injection sequence to address process- and reservoir-specific questions and to improve interpretation. Most commonly, a water-soluble-only cover tracer is added to the reactive tracer slug and a second chemically distinct water-soluble-only material-balance tracer at constant concentration is added to both the slug and overflush brine.
The relationship between the ratio of partitioning and nonpartitioning tracer arrival times, called the retardation factor, FR; the remaining oil saturation (ROS), Sor; and the oil/water partition coefficient, Ko, is
For example, if Sor = 0.20 and Ko = 8, the nonpartitioning water tracer arrives three times faster than the partitioning tracer.
The governing chromatographic equation requires the unreacted ester and product alcohol to begin the return trip to the wellbore from the same position. Therefore, a significant amount of the hydrolysis must occur during the soak period or a definitive oil saturation cannot be determined from the tracer response.
For good sensitivity to ROS, Deans and Majoros2 proposed that, when possible, the soak time should be at least twice the injection plus production time. Assuming a constant reaction rate, this timing allows hydrolysis of a minimum of twice as much ester during soak as during transit. In practice, the "two-times rule," (i.e., soak time equals approximately twice the transit time) is compromised if a high fluid-drift velocity is encountered. More recently, Deans and Carlisle3 reported that the soak period appears to be reduced to about the same duration as transit time. The California turbidite tests described later are an example of a test with a relatively short soak period.
Methyl, ethyl, and propyl acetates and formates are commonly used in commercial SWTT's. The reaction of acetic acid (CH3COOH) and ethyl alcohol (to form ethyl acetate and water) is
In the presence of water only, this is an equilibrium reaction with a constant keq˜4 given by
Addition of acid or base does not affect the equilibrium position, but the rate with which equilibrium is achieved increases significantly.4
Ester Hydrolysis vs. pH
Fig. 1 shows an example of the extreme sensitivity of ester hydrolysis rate to pH. The family of curves was calculated from the following analytical equation derived from kinetic data for methyl formate5:
Equilibrium and rate constants can be found for other acids and alcohols, but hydrolysis rate sensitivity to pH is similar for all esters.5 Reaction rates for methyl and ethyl acetate (Fig. 2) were found to vary inversely proportional to H+ concentration at pH of more than ˜4. The measured points are plotted over a portion of the methyl formate data (the solid line) taken from Fig. 1. As expected, the other esters show the same 10-fold change in rate per pH-unit change.
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