Tracer Fluid Flow through Porous Media: Theory Applied to Acid Stimulation Treatments in Carbonate Rocks

Most carbonate rocks are heterogeneous at multiple length scales. These heterogeneities strongly influence the outcome of the acid stimulation treatments which are routinely performed to improve well productivity. At the pore scale, carbonate rocks are very complex and exhibit a wide variety of pore classes. However, most of the previous studies reported in the literature have focused on the injection rate, temperature, and fluid properties. This study focused primarily on the effect of carbonate pore structure on the acid stimulation treatments.

The objective of this study were to: 1) understand the effect of carbonate pore structure on the acid fluid flow through porous media by the use of thin section analysis, electrical measurements, nuclear magnetic resonance (NMR) and mercury injection capillary pressure (MICP) measurements, and tracer tests; 2) correlate the parameters that govern the tracer fluid flow through porous media to the acid fluid flow through the porous media of the carbonate rocks; 3) quantify the heterogeneity of the carbonate rocks at the pore scale so that the response of the carbonate rock to acid treatments in terms of acid volume needed to propagate the wormhole beyond the damaged zone, the wormhole fractal dimension, and the skin factor evolution during the treatment, can be correlated to the magnitude of pore scale heterogeneity; 4) find correlations between the flowing fraction obtained from tracer tests and the petrophysical parameters obtained from MICP and NMR measurements so that the response of the carbonate rocks to acid treatments can be connected to the basic petrophysical parameters; and 5) build a new methodology to predict the performance of acid stimulation treatments in carbonate rocks using non-destructive tracer tests.

In order to achieve the objectives of the study, a carbonate characterization study was conducted first on different carbonate rock types using thin section analysis, electrical measurements, NMR, and MICP measurements. Then, tracer tests were conducted on these carbonates using two different tracer fluids. Following the tracer tests, coreflood experiments using two different acid systems (15 wt% HCl, and emulsified acid formulated at 1 vol% emulsifier and 0.7 acid volume fraction) were conducted at field conditions.

The results reveal that the pore scale heterogeneity has a significant effect on the acid stimulation treatments. It was also found that the response of carbonate rocks to acid treatments is correlated to the basic petrophysical parameters and the tracer fluid flow through porous media. A tracer fluid can be injected into a core, or on a field scale, between two wells, or injected into a single well configuration and then flow it back. The analysis of the tracer flowback samples can be then used to predict the acid stimulation treatments in carbonate rocks. This approach helps to optimize the acidizing job to get the ultimate benefit of the treatment.