New Insights Into the Propagation of Emulsified Acids in Vuggy Dolomitic Rocks

Abstract

Summary Carbonate formations are very complex in their pore structure and exhibit a wide variety of pore classes, such as interparticle porosity, moldic porosity, vuggy porosity, intercrystalline porosity, and microporosity. Understanding the role of pore class in the performance of emulsified-acid treatments and characterizing the physics of the flow in the acid propagation are the objectives of our study. The study was performed by use of vuggy-dolomite cores that represent mainly the vuggy-porosity-dominated structure, whereas the homogeneous cores used represent the intercrystalline pore structure. Coreflood experiments were conducted on 6 × 1.5-in. cores by use of emulsified acid formulated at 1 vol% emulsifier and 0.7 acid volume fraction. The objective of this set of experiments is to determine the acid pore volume (PV) to breakthrough for each carbonate pore class at different injection flow rates. In this paper, we examine whether the heterogeneities observable at the thin-section scale have a significant influence on the results of the emulsified-acid coreflood experiments. The heterogeneities were characterized by use of thin-section observations, tracer experiments, scanning electron microscopy (SEM), and resistivity measurements. Thin-section observations provide means to study the size of vugs and their distribution and connectivity, and to explain the contribution of the pore class in the acid propagation. Also, the rotating-disk experiments of emulsified acid with dolomite were related to our coreflood experimental results. The acid PV to breakthrough for vuggy-porosity-dominated rocks, ranging from 0.1 to 0.3, was observed to be low when compared with homogeneous carbonates (intercrystalline pore structure) with PVbt ranging from 2.5 to 3.5. Also, the wormhole dissolution pattern was found to be significantly different in vuggy rocks than that in homogeneous ones. Thin-section observations, tracer results, and the coreflood experiments indicate that the vugs are distributed in a manner that creates a preferential flow path, which can cause a rapid acid breakthrough and effective wormholing rather than those with a uniform pore structure. The rotating-disk results also showed that the reaction kinetics played a role in determining the wormhole pattern.