A Case Study of a Successful Matrix Acid Stimulation Treatment in Horizontal Wells Using a New Diversion Surfactant in Saudi Arabia

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMatrix acidizing of horizontal and multi-lateral wells is a challenging task for several reasons. First, oil reservoirs are heterogonous in nature and the presence of high permeability streaks is common. Proper diversion technique is always required to better distribute the injected acid across the target zone. Secondly, stimulation of horizontal wells requires injection of large volumes of acids. This will require using an effective corrosion inhibitor package that can protect down hole tubulars and coiled tubing, especially in sour environments. Thirdly, recovering large volumes of spent acid is a concern, especially in tight formations. Acid treatments should be designed to address these issues in a cost effective way.Currently, there are a limited number of chemicals that can divert the acid into the formation. This paper introduces a new viscoelastic surfactant which was used to form stable foam during matrix stimulation of a seawater injector. The length of the target zone was 1,500 ft, and the average permeability of the carbonate was 700 mD. A 1.75-inch coiled tubing was used to better distribute the acid in the open hole section (6.125-inch in diameter).As with previous viscoelastic surfactant systems, the surfactant molecules form structures in solution in the presence of salts. These structures enhance the viscosity of solution significantly and can be broken by dilution with injection water or by adding mutual solvent (3-5 vol%) to the preflush/postflush stages.Unlike previous viscoelastic surfactant-based systems, the new surfactant builds viscosity faster and its gel breaks down in a shorter period of time.This paper examines the results of extensive lab testing that led to the development of this improved self diverting system. It will also give details of the first field application of this system. Following the acid treatment, the well injection rate improved by five folds, from 16,000 to 80,000 BPD. Benefits and improvements introduced by the new diversion technique will be discussed in detail.

Optimization of Surfactant-Based Fluids for Acid Diversion

“…It has been found that the rheological properties are affected by numerous factors such as surfactant concentration, temperature, acid concentration, acid reaction products, solvents, salt concentrations, and other additives such as corrosion inhibitors. 16,17 Surfactants gels can be prepared using anionic, cationic, or amphoteric surfactants. 13 Amphoteric surfactants have been the subject of several lab 16 and fields studies.…”

Investigation and Field Evaluation of Foamed Viscoelastic Surfactant Diversion Fluid Applied During Coiled-Tubing Matrix-Acid Treatment

“…[1][2][3][4][5] Most of the production comes from heterogeneous carbonate reservoirs and long open hole productive sections are generally characterized by wide contrasting permeability. The majority of the damage occurs during the drilling process and the primary challenge for matrix acidizing is effective placement to maximize production potential.…”

“…The surfactant has a low viscosity in concentrated acid and shows increased viscosity as the acid spends. 5 The shear thinning nature of this material acts as a friction reducer to decrease surface treating pressure while pumping through small ID tubulars and allows it to regain maximum viscosity for diversion once in the wellbore. 8 Extensive laboratory testing has been done to better understand how viscoelastic surfactants work in the field.…”

“…6 Rheological properties differ tremendously at various shear rates and are affected by other parameters such as surfactant concentration, temperature, base acid concentration, acid reaction product, in-situ formation fluids, and corrosion inhibitors . 5 Petroleum-based solvents and mutual solvents act as breakers when they come in contact with viscoelastic surfactants. 8 These surfactants have a very high viscosity at low shear rates, which can impede cleanup if too much radial zone is invaded by diversion stages.…”

Nonpolymer Surfactant Enhances High-Strength Hydrofluoric Acid Treatments

“…Another paper reported on the successful use of this same viscoelastic surfactant to form a stable foam to successfully restore the injectivity of a seawater injector well in a carbonate reservoir in Saudi Arabia. 2 These recent successes show that certain viscoelastic surfactants have unique properties that make them particularly attractive for use as in-situ acid gelling agents for diversion. These include a more dependable diversion and better cleanup of the formation.…”