Laboratory Evaluation of a Low pH and Low Polymer Concentration Zirconium-CMHPG Gel System for Hydraulic Fracturing

Abstract

Low pH fracturing fluids, such as the transition metal cross-linked guar derivative, can reduce the formation damage caused by clay swelling and fines migration in hydraulic fracturing treatments. However, a major disadvantage of the fluids is the uncontrollable rapid viscosity development, especially cross-linking under a pH below 3.0. In this paper, a novel delayed organic zirconium cross-linker with lactate and ethylene glycol as ligands was designed to overcome the shortcoming. Carboxymethyl hydroxypropyl guar (CMHPG), which results in a low gel residue after breaking, was used as the polymer at low concentration. The impact of gelation parameters on the properties of zirconium-CMHPG gel system was systematically investigated using a Sydansk bottle-testing method combined with a controlled stress rheometer. Desirable gelation time delay with agreeable rheological performance could be obtained by optimizing the polymer and cross-linker concentrations even at a pH as low as 2.0. Uniformly and compactly generated 3D network microstructures could firmly lock the free water within the gel, which further improved the thermostability and acid-resistance of the system. Meanwhile, the structures could also maintain the viscoelasticity of the fluid at a high level to ensure the efficient transportation of the proppants. Proppant settling and gel breaking tests demonstrated that the low pH and low polymer concentration gel system generated fewer insoluble residues with an uncompromised suspension performance. Moreover, the formation damage evaluation revealed that the low pH fracturing fluid system produced a better return permeability than the high pH system.

Development and Evaluation of an Acidic Shear-Tolerant Nanosheet-Enhanced Cross-Linked Gel System for Hydrofracturing

“…These results can be explained based on the following mechanisms: (i) the expansion rates of montmorillonite and mixed-layer illite/montmorillonite decrease with the decreasing pH value of their environment; (ii) the swelling of expansive clay minerals exacerbates the dispersion of nonexpansive clay minerals; and (iii) clay particles and sandstone grains have lower surface charges under acidic conditions than under alkaline conditions. The repulsions between the clay particles and sandstone grains were reduced under acidic conditions, thus inhibiting clay dispersion and preventing pore blocking. ,, …”

“…The novel organic zirconium complex was developed and optimized based on our previous work . We designed several orthogonal experiments to improve the cross-linker performance, and the one used in this work was synthesized as follows.…”

“…Exploration and development possibilities for most of the conventional reservoirs in China have gradually progressed to mature or late stages. , The use of unconventional oil and gas resources has attracted increasing attention in recent years due to enormous domestic and global energy needs. − However, unconventional resources have the characteristics of strong heterogeneities and low or ultralow permeabilities, which lead to low productivities. − Hydraulic fracturing is an effective development method for enhancing oil and gas production in reservoirs. − Briefly, the hydrofracturing process is used to generate a high-conductivity proppant channel connecting the formation to the well bore. The success of this operation depends on whether the fracturing fluid has satisfactory rheological properties and a suitable proppant carrying capacity . Many novel fracturing fluids, such as viscoelastic surfactants (VESs), slick water, supramolecular polymers, energized fluids, and supercritical carbon dioxide (CO 2 ), have been designed and developed through oilfield chemistry research. − Nevertheless, traditional guar-based linear or cross-linked fracturing fluids are still the most commonly used fluids in oilfields due to their desirable performances and low costs. − …”

“…Zirconium cross-linked CMHPG, the other common guar-based fracturing fluid system that has been applied in the acidic range, can be used to address this problem. The acidic fracturing fluid not only adapts to a formation treated with CO 2 but also prevents formation damage arising from the swelling and migration of clay particles . Moreover, the acidic fluid can promote polymer hydrolysis, which may assist in gel breaking during the flowback process in hydraulic fracturing .…”

Viscosity-Reducing Agents (Breakers) for Viscoelastic Surfactant Gels for Well Stimulation

“…Another criterion for their value is their ability not to block the acidized or fractured wells. These gels need to be removed at the end of the fracturing or acidizing job because viscous gels do not easily flow out of the reservoir. ,, As a result, it is necessary to use viscosity-reducing agents (also known as breakers) in well stimulation jobs in conjunction with polymers or VES gels. ,,− …”