Predicting the Performance of the Acid-Stimulation Treatments in Carbonate Reservoirs With Nondestructive Tracer Tests

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, and microporosity. Geologists have defined carbonate pore classes on the basis of sedimentology, thin sections, and porosity/permeability relationships, but the question remains concerning how these pore classes govern the acid flow through porous media. Core samples from six different carbonates, mainly limestone, were selected for the study. The samples were first investigated with thin-section analysis, high-pressure mercury-injection tests, and nuclear-magnetic-resonance measurements for pore-structure characterization, and X-ray diffraction for mineralogy examination. Next, tracer experiments were conducted, and the tracer-concentration profiles were analyzed to quantify the carbonate pore-scale heterogeneity. The heterogeneity is expressed with a parameter f—the available fraction of pore structure contributing to the flow. The data were used to study the flow of acid through carbonate rocks and correlate the pore classes to the acid response. More than 30 acid-coreflood experiments were conducted at 150°F and a hydrochloric acid concentration of 15 wt% on 1.5 × 6-in. core samples at different injection rates on each carbonate rock type. The objective of these sets of experiments is to determine the acid pore volume to breakthrough for each carbonate pore class. The findings of this study help us to connect the results from different characterization methods to the acid flow through the porous media of carbonate rocks. It was also found that the response of the acid depends on the carbonate pore classes. Application to the design of matrix acid treatments in carbonate rocks is discussed.