Matrix well stimulation by dissolving part of acid soluble materials from the matrix is a proven technique to improve production from carbonate formations. However, acid placement and diversion remains a challenge to the operators, when dealing with heterogeneous formations with zonal permeability contrast. Inefficient acid placement leads to an unsuccessful treatment as most of the injected acid will flow through the high permeability zone, whereas the main target is usually the low permeability layer. Foam-acid diversion is a common practice for acid placement. The success of any foam acid diversion depends highly on the careful selection of suitable surfactant(s) and testing its performance at relevant reservoir conditions. In this paper, we present a systematic study on surfactant screening, foam-bulk stability, and foam behavior inside the porous medium. Glutamic acid diacetic acid (GLDA) was used as acid. The surfactant screening was performed with an initial list of 29 surfactants at 25°C and 80°C with and without GLDA present in the surfactant formulations. The addition of GLDA resulted in foam collapse for most of the surfactants. However, some bulk-foam stability tests showed improved foamability at 80°C for selected surfactant formulations containing GLDA. Surfactant formulations passing the initial screening were further tested in a 76cm-long high permeability glass beads packed bed. The performance of the selected formulations was next studied in a series of high pressure foam-flooding experiments at 130°C. Foam-quality scan curves were developed to examine the influence of foam quality on foam strength. The mobility reduction factor (MRF) was considered as direct measure of foam strength. Foam coreflood experiments revealed that for most of the formulations comprising cationic surfactants, the foam was most viscous and collapsed at 85% quality.

Stimulation systems have improved over past decades, yet challenges prevail in corrosion, unwanted precipitation and handling hazardous chemicals. The role of chelating agents in coping with such concerns, is undeniably positive: their limited corrosivity, effective metal control and outstanding HSE profile, make them effective acidizing alternatives. Particularly when seeking delayed reaction at high temperature or removing insoluble material like Barite, chelating agents like GLDA and DTPA respectively have been reported effective both at laboratory and field scale. Formulations based on abovementioned chelating agents were evaluated experimentally to assess potential stimulation of Kazakhstan formations. Core-plug samples used in this evaluation are predominantly carbonate rock originating from different wells. The coreflooding experiments were performed at HPHT conditions to assess performance of treatment fluids to a) create new flow-channels (wormholes) thus improving rock permeability, and b) remove BaSO4-based solids suspected to be affecting productivity in the field. In this work, five reservoir core plugs were stimulated by GLDA based formulation to assess wormholing mechanism, while two core-plugs were treated by DTPA based fluid to study the impact of matrix cleaning. The matrix cleaning properties of DTPA based fluid were investigated on the damaged core plugs which were artificially damaged by in-situ precipitation of BaSO4 scale. The coreflood study included injection of the preflush, the treatment fluid and the post-flush system at reservoir temperature of 270 °F and low injection rates to accommodate the low permeability of the formation. It was shown that GLDA based fluid can effectively stimulate the reservoir core samples. The effective mechanism was observed to be wormholing thus increasing rock permeability by over a thousand times. No signs of face dissolution were observed despite slow injection rate at such high temperature; something that was not possible when a fast reacting acid (i.e. HCl) was used under the same conditions. In addition, it was shown that the DTPA based fluid can efficiently improve the rock permeability through matrix cleaning by both Barium and Calcium chelation. In the first treatment test by this fluid system, around 45% of the damaged permeability was recovered. While in the second test, not only BaSO4 scale was dissolved but also the CaCO3 minerals were partly dissolved and the core permeability was significantly increased (Kf/Ki >200). Experimental results bring promising prognosis for field implementation despite expected low injectivity at high downhole temperature. GLDA treatments avoid premature acid spending and face dissolution - common outcomes of HCl- which translate into deeper extent of stimulation. Additionally, in barite damaged wells, DTPA treatment represents an attractive solution for damage reduction and by-passing. Finally, intrinsic properties of chelating agents reduce asset integrity risks, improve operation HSE and simplify flow-back handling.