Abstract
We present the first ever Single Well Chemical Tracer (SWCT) model that incorporates temperature gradients and pH-driven ethyl acetate hydrolysis rate changes when buffering by either calcite in the oil-bearing formation or by chemical species in the injected brine are accounted for. The model is applied to four generic cases with SWCT test, rock formation and brine composition data based on published values. An analytical model is used to calculate how the brine injection temperature decreases with time and an axially symmetric numerical model is used to simulate the cooling of the oil-bearing formation, chemical reactions and transport of species in the reservoir. The primary oil-water partitioning tracer ethyl acetate hydrolyses into the secondary water tracer ethanol and acetic acid that lowers the pH and changes the hydrolysis rate. The acid driven pH decrease may be significantly reduced by pH buffering mechanisms. We investigate four different buffer models. Model 1 may represent a sandstone with calcite cement. In Model 2, also with calcite cement, the brine has less calcium but more bicarbonate and initially lower pH than Model 1. Model 3 is a clean sandstone without calcite cement and the brine contains no calcium but much bicarbonate. Model 4 has no buffer capacity neither in the brine nor in the target formation. Although we discuss only four models, they are quite different and represent an important subset of the realistic SWCT test parameter space. A temperature gradient develops across the primary tracer bank in all four models and this causes the secondary tracer to be displaced away from the wellbore relative to the primary tracer. We find that most of the ethyl acetate hydrolysis takes place during shut-in. The residual oil saturation (Sor) is estimated from the synthetic tracer production curves using the simple chromatographic separation equation as well as with the mean residence time correction. The two methods underestimate the true Sor value (22%) by 3–5 and 3–11% (saturation points), respectively. These results demonstrate that the ‘handicap’ created by the temperature gradient together with the pH-driven hydrolysis rate changes should be taken into account in Sor estimates. We suggest that SWCT tests may be used to evaluate the effective wettability of the target formation at a modest extra cost. Using water tracers with very low quantification limits as primary tracers should reduce or even eliminate the problems caused by temperature gradients and pH-driven hydrolysis rate changes since they will travel well ahead of the temperature gradient and the small amount of acid generated will produce only an insignificant reduction in pH.
Highlights
Single Well Chemical Tracer (SWCT) tests are performed in oil pro ducers to estimate the residual oil saturation (Sor), i.e., the fraction of pore volume occupied by immobile oil after a displacement process
Numerical models that include realistic temperature calculations and pH-driven primary tracer hydrolysis rate changes are important in the design of SWCT tests and for interpretation of the results
Simple isothermal vial models provide useful semi-quantitative insight into the chemistry of SWCT tests, but simulation of tempera ture and chemical species transport in porous media are required for a deeper understanding
Summary
Single Well Chemical Tracer (SWCT) tests are performed in oil pro ducers to estimate the residual oil saturation (Sor), i.e., the fraction of pore volume occupied by immobile oil after a displacement process. One of the most important applications of SWCT tests is to gauge the performance of Enhanced Oil Recovery (EOR) methods. The reduction in Sor between SWCT tests conducted before and after an EOR operation has been used to measure the efficiency of hydrocarbon miscible gas floods (Cockin et al, 2000), low salinity water floods (Skrettingland et al, 2010; Khaledialidusti et al, 2015; Al-Shalabi et al, 2017; Kazemi et al, 2019), alkaline-surfactant-polymer (ASP) floods (Carlisle et al, 2014; Fortenberry et al, 2016) just to mention some examples. We estimate that close to a thousand SWCT tests have been
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