An expression for irregularly distributed hydrocarbon volume using exponential decay method

Abstract

In both the oil and gas and the environmental remediation industries, the ability to locate and quantify hydrocarbons in the subsurface is of value. Partitioning interwell tracer testing has been shown effective at detecting and measuring uniformly distributed hydrocarbon mass. Difficulty arises when the hydrocarbon is irregularly distributed – that is, when the overall hydrocarbon saturation within the swept pore volume is low but is high locally. The method of moments underestimates the saturation when most of the tracer bypasses the hydrocarbon. The small zone containing the hydrocarbon that is contacted by the tracers produces a long ‘tail’. An alternative method was developed to compute the irregularly distributed hydrocarbon saturation, and the complete mathematical derivation is presented that utilises the exponential decay of each tracer. The method was verified using experimental data from a two-dimensional physical model where the hydrocarbon saturation was high on a local scale but low overall. The proposed analytical approached yielded results that were significantly better than those using the traditional, first-moment method for computing the hydrocarbon volume. It is concluded that the exponential decay method yielded estimates of the irregularly distributed hydrocarbon mass that were both reasonable and significantly more accurate than those from the standard method of moments.