PH and Reaction Rate Changes During Single-Well Chemical Tracer Tests

Abstract

ABSTRACT The single-well chemical tracer (SWCT) method has been used to measure residual oil saturation to water flooding, (Sor)w, in more than 200 reservoirs since 1968. An essential step in the procedure is the hydrolysis of an ester, the primary tracer, in-situ. At the previous SPE/DOE Symposium4, the variation of hydrolysis rate with pH was claimed to cause serious errors in interpreted (Sor)w in certain cases. This effect has not been accounted for in SWCT test simulator programs, which are used to obtain (SOR)w by matching field measured tracer profiles. The present work investigates the space-time variation of pH in the formation during a SWCT test. The method employed is simulation, using the chemical flood simulator program UTCHEM. The program was modified to account for the effects of ester hydrolysis reaction products on pH. The hydrolysis rate of ester, already present in the program was modified to depend on pH according to known reaction mechanisms. UTCHEM was then used to simulate the SWCT test previously reported4 to be effected by pH variations. The shape of the predicted product tracer profiles is indeed changed by the variation of hydrolysis rate with pH. The new tracer profiles come closer to fitting the field-measured results, using a single-layer reversible-flow model. However, the interpreted value of (Sor)w is not significantly changed. It is recommended that the necessary improvements be added to existing SWCT test simulator programs to account for pH changes. Although this will increase the computing cost of matching field data, the effects may be important enough in some cases to warrant the added complication.