Stimulation of Tight Formation in Kazakhstan by Chelate Based Treatment Fluids
Authors: Nazari Rasoul , Zhulomanov Nurlan , van Doorn Marcellinus , Dos Santos Auribel , Medeuov Nurbek 
Published: October 2021

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 &gt;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.