The Effect of Pore-Scale Heterogeneities on Carbonate Stimulation Treatments

Abstract

Abstract Most carbonate reservoirs are heterogeneous at multiple-length scales. These heterogeneities strongly influence the outcome of acid stimulation treatments which are routinely performed to improve well productivity. However, most previous studies reported in the literature have focused on investigating the effects of injection rate, temperature, and fluid properties and few have focused on the influence of rock properties on stimulation treatments. This study primarily explores the influence of pore scale heterogeneities on stimulation treatments. Some results on the influence of core scale heterogeneities are also presented. Core samples from eight different carbonate rocks were selected for the study. Samples were characterized for mineralogy, texture, fabric, porosity and density distribution using Nuclear Magnetic Resonance (NMR), Computed Tomography (CT) scanning, Scanning Electron Microscopy (SEM), mercury injection as well as resistivity measurements, chemical testing, etc. Each sample was then classified into a Reservoir Rock Type (RRT) which is primarily based on the porosity spatial distribution (PSD) in the carbonate. Each RRT represents a group of carbonate rocks with similar porosity spatial distribution and is expected to exhibit similar behavior to fluid flow and, therefore, to acid dissolutions. The 8 carbonate rocks investigated in this study represent 6 different RRTs. Each carbonate type was treated with reactive fluids. Two different sets of coreflow tests were performed on 6 x 1.5 inch plugs. The first set of experiments involved determination of the evolution of permeability with porosity for the RRT. In these experiments the core was treated at high injection rates with a fluid of low reactivity. Injection at a high flow rate ensured uniform dissolution of the medium. Porosity was monitored by analysis of the effluent and permeability was monitored by the pressure drop across the cores. The second set involved determining the acid breakthrough curves for each of the carbonate types using HCl (hydrochloric acid) as the reacting fluid. The characteristic pore-volume to breakthroughs (PVBT) and the wormhole velocities were obtained for each carbonate by injecting acid at different flow rates. The results of the tests confirm the original hypothesis that the response of the carbonate rock to acid depends on the RRT it belongs to. In other words, rock samples with similar PSD exhibit similar trends in PVBT. The significance of the results to the design of matrix treatments in carbonate reservoirs is discussed.