Acid Selection for Volcanic Tuffaceous Sandstone with High Analcime Contents: A Laboratory Study in Kita-Akita Oil Field, Northern Japan

Abstract

Summary Investigation of the effectiveness of matrix-stimulation treatments for removing drilling-induced damage in the Akita region of northern Japan is of interest because of the presence of large quantities of acid-sensitive minerals, such as analcime. A feasibility study of the subcommercial field redevelopment in the Kita-Akita Oil Field, one of the satellite fields of the main Yabase Oil Fields that produced from 1957 to 1973 and was plugged and abandoned, was conducted. As a part of the studies, matrix-acidizing laboratory experiments were performed. Conventional mud acids and formic-acid-based organic-mud-acid systems cause significant permeability damage because of the instability of analcime when exposed to these acids. This study focuses on the development of a treatment fluid that removes drilling-induced damage and is also compatible with the formation. Petrology studies and core flow tests were used in conjunction with geochemical modeling to achieve this objective. A petrographic analysis on the untreated cores showed abundant tuffaceous pore-filling mineral phases, ranging from 12 to 20% in volume. Smectite clay and microcrystalline quartz are the major constituents present as alteration products of volcanic glass. Analcime was present in significant quantities in all samples tested. Six core flow tests were performed on formation cores to optimize the acid preflush and main acid stage. Permeability change resulting from treatment fluids was recorded for the tests. Chemical analysis of the effluent was performed on three core flow tests. Core samples before and after acidization were characterized on the basis of thin section, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mineral mapping. Core flow tests with conventional retarded organic mud acid (ROMA) resulted in only 75% retained permeability. The permeability damage by the ROMA was surprising, because it usually performs well in acid-sensitive formations. A chelant-based retarded mud acid was tested next and resulted in minor formation damage. It can be potentially used in a field treatment, because its high dissolving power is expected to more than compensate for the damage. The highest retained permeability was obtained with an acetic-hydrofluoric (HF) acid system. It was successfully able to remove drilling-induced damage and was also compatible with the native mineralogy. Core flow tests were used to calibrate the permeability/porosity relationship used in the geochemical simulator. The geochemical simulator was then used to predict the field-level acid response. The analytic methods presented are general enough to be of interest to sandstone-acidizing studies, where detailed analysis is needed for damage identification and removal. The fluids developed for this formation are good candidates for other formations where conventional acid systems have not performed well. This study also highlights a close collaboration between the operator and the service company to find a workable solution to a challenging stimulation requirement.