Effective Matrix Acidizing in High-Temperature Environments

Abstract

Abstract World demand for energy is substantial and continues to grow. By 2020, it is expected that the world will need approximately 40% more energy than today, for a total of 300 million barrels of oil-equivalent energy every day. Meeting higher energy demands will require a portfolio of energy-generation options including but not limited to oil, natural gas, coal, nuclear, steam, hydro, biomass, solar and wind. New horizons are being explored. Wells are drilled in greater water depths. Drilling units are continually upgraded to target deeper hydrocarbon-bearing zones. Wellbore tubular metallurgy is continually upgraded. Drilling, completion and stimulation fluids are being developed for extreme temperature and pressure environments. As the preferred technology to enhance "oilfield" energy production, well stimulation has and will continue to have an important role in fulfilling the world's future energy needs. Well stimulation generally uses fluids to create or enlarge formation flow channels, thereby overcoming low permeability, as in "tight" formations, and formation damage, which can occur in any formation type. A common and very successful stimulation option, matrix acidizing, utilizes acids that react to remove mineral phases restricting flow. Depending on the formation and acid type, flow is increased by removing pore-plugging material; or by creating new or enlarged flow paths through the natural pore system of the rock. However, higher-temperature environments present a challenge to matrix acidizing effectiveness. High temperatures can negatively affect stimulation fluid properties and certain acid reactions. Thus, careful fluid choice and treatment designs are critical to successful high-temperature matrix acidizing. With proper fluid selection, design, and execution, matrix acidizing can be applied successfully to stimulate high-temperature oil & gas wells and geothermal wells. These types of wells have some common features, but they also have significant differences (e.g., completions, mineralogy, formation fluids and formation flow) that influence stimulation designs and fluid choices. This paper summarizes best practices for designing matrix acidizing treatments and choosing stimulation fluids for high-temperature oil & gas wells and geothermal wells. Included are case histories from Central America. Lessons learned about differences and commonalities between stimulation practices in these well types are also discussed. Introduction As today's rate of finding new reserves is lower than in previous decades, exploration has turned more to deeper basins. Deeper wells are typically hot (greater than 250º F, for example). Permeabilities are also often lower and occasionally are the result of a network of natural fissures. Offshore wells in the Gulf of Mexico are now reported to reach bottomhole temperatures of 500º F. Recently discovered gas fields offshore Brazil have bottomhole temperatures ranging from 350 to 400º F. Over the past years, great improvements in matrix acidizing have taken place, parallelling the developments in hydraulic fracturing. Provided that the forecasted production/injection results make economic sense, matrix acidizing is still simpler, often less risky, and more economic to implement than hydraulic fracturing. Sophisticated laboratory equipment, expertise, and well testing software can help the engineer diagnose production or injection damage effects and mechanisms - making it easier to select proper well candidates and optimize job design. Treatment placement is better ensured through the use of chemical or mechanical diversion methods and technologies, and placement tools (coiled tubing, straddle packers, etc.). On-site quality control is enabled by modern sensors, monitors and software, enabling the engineer to determine the evolution of skin with time, and radius of formation treated. Modern blending and pumping equipment have provided the means to mix acid continuously without the need for pre-blending fluids. This eliminates the need for mixing tanks on location, and enhancing safety on location 10.