Successful Pilot Tests and Numerical Analysis of Acid Stimulation for Dissolving Carbonate Veins in Tight Reservoirs

Acid stimulation was applied successfully to the tight formations in Japan aiming at dissolving carbonate veins for production enhancement. The target two fields are low permeability reservoirs including natural fracture networks filled with carbonate minerals of veins: (1) the Ayukawa oil/gas field, Monterey-like biosiliceous shale reservoir including dolomite, and (2) the Katakai gas field, volcanic rock reservoir including calcite and ankerite. The key to success is to estimate the distribution of carbonate veins in the heterogeneous formations, because our concept of “vein dissolution” is different from conventional acidizing.

The pilot tests were designed to improve the reservoir permeability near wellbore using (1) hydrochloric acid (HO) and mud acid (HC1 + hydrofluoric acid) for the Ayukawa field and (2) chelating agent and HC1 for the Katakai field. The chelating agent of Hydroxyethyl Ethylene Diamine Triacetic Acid was selected for a high-temperature tolerance and low corrosivity. For each of the pilots, we conducted step rate injection test, acid injection in three stages, flowback, and oil/gas production flow with extensive data gathering and analysis.

The increases from pre- to post-acidizing production rates observed in the pilots were approximately (1) ten times in the Ayukawa oil production and (2) two times in the Katakai gas production. For the Ayukawa field, we also confirmed the increases of Injectivity Index during the acid injection and Productivity Index during the post flow. It was estimated that HC1 dissolved dolomite and mud acid dissolved not only dolomite but also siliceous rock. For the Katakai field, Productivity Index increased about two times. The observed ion concentration indicated the dissolution of calcite and ankerite minerals. The acid worked stably without decomposition under the reservoir condition of high temperature of 180°C.

Additionally, a new workflow of history-matching was developed for the Ayukawa pilot by coupling acid reaction and oil/gas production from the stimulated area. The acid dissolution is modelled in the software of TOUGHREACT™ (LBNL) incorporating the batch reaction data measured at our laboratory. The parameters of acid reaction are calibrated throughout the history-matching. The simulation of acid dissolution is followed by the post-acidizing flow simulation of EclipselOO™ (SLB), when the porosity and permeability of the dissolved area are increased. Also, a dual porosity model of the post-acidizing simulation was tested for geological representation to improve the matching quality, especially for bottomhole pressure.

This workflow can be utilized to evaluate the production enhancement in the future acidizing operations.