Enhanced oil recovery in fractured low-permeability reservoirs by a novel gel system prepared by sustained-release crosslinker and water-soluble thixotropic polymer

Highlights

• A sustained-release crosslinker (SRC) was prepared by coating polyethyleneimine into the W/O/W multiple emulsions.

• The SRC/WTP gel system has long gelation time and high gelation strength.

• The SRC/WTP gel system exhibits a good plugging performance in fractured low-permeability cores.

• The EOR performance of the SRC/WTP gel system has been verified by the field tests.

Abstract

Polymer gels have been widely used in improving oil recovery and decreasing excessive water production in heterogeneous reservoirs after the long-term water flooding process. However, the high initial viscosity of polymer and the short gelation time still restricts the performance of polymer-based in situ cross-linked gels for in-depth conformance control. Herein, a novel gel system formed by in-situ crosslinking of water-soluble thixotropic polymer (WTP) and sustained-release crosslinker (SRC) was proposed. The WTP was synthesized by acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and N-[3-(dimethylamino)propyl] methacrylamide. It exhibits low viscosity under high shear rates and becomes thick under low shear rates, which ensures good injectability during the injection process and high gelation strength after cross-linking. The SRC was prepared by coating poly(ethyleneimine) (PEI) into the W/O/W multiple emulsions. The sustained-release mechanisms were well elaborated by confocal laser scanning microscopy (CLSM) observations. Only after the demulsification of multiple emulsions, the PEI would be released from the internal aqueous phase to the external aqueous phase. Therefore, these emulsions can be employed to deliver crosslinkers to specified targets in deep regions and prolong the release of crosslinkers to prevent undesirable crosslinking reactions near the injecting wells. According to the environmental scanning electron microscope (ESEM) observations and viscosity measurements, the low viscosity and an incompact spatial network could be observed in SRC/WTP gel system in the first 17 d, after which the PEI molecules were released to the external aqueous phase and cross-linked with WTP, forming the solid gel with small pores and yielding a high viscosity of 8793 mPa⋅s. Rheology tests have demonstrated that the elastic modules and the liner viscoelastic region of the novel gel system was nearly two times and twenty times than the traditional polymer gel, indicating higher gelation strength and better shear resistance. The core flooding tests showed that the SRC/WTP gel system had good injectability and could effectively plug the millimeter and submillimeter-sized cracks in low-permeability cores. 0.6 pore volume (PV) of SRC/WTP gel system with 3 gel slugs and a low injection rate can yield the best plugging performance in fractured core samples. And the gelation time shortens with the decrease of fracture apertures. Most importantly, the field tests in Ordos Basin verified the desirable EOR performance of the SRC/WTP gel system in fractured low-permeability reservoirs.

Introduction

By introducing crosslinking agents into the macromolecule polymer solution, polymer gels with steric network structure can be formed (Yin et al., 2022). Some of them exhibit good compressive deformation and strength due to the large amounts of water locked in their three-dimensional network structures, which make them have wide application in many fields (Yu et al., 2022; Song et al., 2022a; Wu et al., 2022a). Among them, the application of in situ cross-linked weak gels (ISCWG) in conformance control of heterogeneous reservoirs is particularly successful (Luo et al., 2022), (Wu et al., 2022b). The purpose of the ISCWG in this application is to improve the sweep of the reservoir by plugging the high permeability region and leading the flooding fluid into the formation with low permeability (Zhu et al., 2021), (Wu et al., 2021). With the increasing demand for fuel and energy and the decreasing of easily recoverable oil from conventional reservoirs, fractured low-permeability reservoirs play a growing important role in the global energy supply (Li et al., 2022a; Ding et al., 2018; Zhang et al., 2018; Bai et al., 2021). While the distinct characteristics of fractured low-permeability reservoirs put forward new requirements for some parameters of ISCWG in conformance control applications (Li et al., 2022a). Compared with conventional medium/high permeability reservoirs, ISCWG is difficult to be injected into fractured low permeability reservoirs (Zhang et al., 2016), (Liu and Seright, 2001). The ISCWG is expected to plug large fractures in in-depth target regions far from the wellbore of injecting wells (Yin et al., 2022), (Zhao et al., 2015), (Bai et al., 2015a). Therefore, an ideal ISCWG used in fractured low-permeability reservoirs should have good injectability and long gelation time (Reddy et al., 2012), (Bryant et al., 1997).

The currently used polymers for preparing ISCWG mainly include partially hydrolyzed polyacrylamide (HPAM), acrylamide-based copolymers, guar gum and xanthan gum (Jia et al., 2010; Zhang et al., 2022a; Kolb, 1971; Gales et al., 1994). The HPAM is regarded as the ideal polymer for preparing ISCWG in conventional reservoirs, the high molecular weight and the electrostatic repulsion between polymer coils contribute to the thickening capability of HPAM as well as the high gelation strength after crosslinking (Ren et al., 2014), (Jia et al., 2012). The HPAM with high molecular weight exhibits high viscosity of the initial solution, which would seriously affect the injectivity of HPAM-based ISCWG in fractured low-permeability reservoirs (Zhang et al., 2016). Some acrylamide-based copolymers with functional monomers have good salt and temperature tolerance and shear resistance (Chen et al., 2019). But the functional monomers, such as long-chain alkanes, may lead to poor water solubility, high cost and complex synthesis process (Wever et al., 2011). Those issues would pose challenges to the transformation from laboratory research to industrial application.

The environmental-friendly materials of guar gum and xanthan gum have received increasing attention due to the current environmental requirements, and there are many successful field applications in medium/high permeability reservoirs (Kolb, 1971), (Gales et al., 1994). The high gelation strength of guar gum and xanthan gum-based ISCWG lies in the polymer's concentration, while the high polymer concentration would lead to the high viscosity of the initial injecting fluid as well as the poor injectivity. Therefore, synthesizing a polymer with high strength after crosslinking and good injectivity has become a major challenge (Yin et al., 2022).

Chromium acetate (Ac₃Cr), phenol-formaldehyde resin (PFR) and poly(ethyleneimine) (PEI) are three commonly used crosslinkers for preparing ISCWG (Zhang et al., 2015a, 2019, 2020). Although the chromium acetate (Ac₃Cr) cross-linking partially hydrolyzed polyacrylamide (HPAM) is the most widely used method to get the weak gel, the short gelation time (almost 1–5 h) makes it unsuitable for the fractured low permeability reservoirs (Ghriga et al., 2019). And the salt concentrations, especially bivalent cations concentrations, seriously affect the degree of the cross-linking reaction of HPAM/Ac₃Cr (Zhang et al., 2015b). Besides, the enormous pressures brought by the increasing need for environmental controls also restrict its application in the future (Guo et al., 2022). Jia et al. investigated the gelation mechanism and kinetics of HPAM with PFR and PEI (Jia et al., 2012). They found that the gelation time of PFR/HPAM system can be regulated between 2 h and 2 d at 25 °C. The PEI/HPAM system showed a longer gelation time, which was tunable between 15 h and 9 d by altering the concentration of HPAM, PEI and the salt. Despite great efforts have been made to prolong the gelation time of ISCWG, it is still difficult to meet the requirements of in-depth water shutoff and conformance control for fractured low-permeability oil reservoirs (Ghriga et al., 2019). In order to avoid the undesirable increase of injection pressure of injecting wells due to the gel plugging near the wellbore, and to enhance the conformance control performance of ISCWG, the required time duration in fractured low-permeability oil reservoirs should be several weeks.

Multiple water-in-oil-in-water (W/O/W) emulsions are promising systems for encapsulating and releasing water-soluble substances (Shi et al., 2021; Huang et al., 2021a; Yuan et al., 2022). Multiple emulsions consist of an inner aqueous phase (W₁), the larger oil droplets (O) and the continuous aqueous phase (W₂) (Oppermann et al., 2015). The water-soluble substances were preserved in the inner aqueous phase by the oil layer, which acts as a barrier against the external aqueous medium (Huang et al., 2021b). Many examples of pharmaceutical compounds solubilized in W/O/W emulsions are reported in recent research, such as delaying the degradation of drugs, avoiding undesirable reactions between drugs and the compounds in the body, or masking the unpleasant taste of drugs (Schmidts et al., 2010a, 2010b; Beer et al., 2013; Bodin-Thomazo et al., 2022; Iqbal et al., 2020; Hoppel et al., 2015). Inspired by the research works on the utilization of W/O/W emulsions in pharmaceutical, cosmetics and food technology, the multiple emulsion might be used to slow down the release of encapsulated crosslinkers and achieve sustained or targeted release to prevent undesirable crosslinking reactions near the injecting wells.

To successfully implement the in-depth water shutoff and conformance control application for fractured low-permeability reservoirs, a novel ISCWG formed by sustained-release crosslinker (SRC) and water-soluble thixotropic polymer (WTP) was proposed. The sustained-release mechanism of SRC was elaborated using confocal laser scanning microscopy. And the environmental scanning electron microscope observations and viscosity measurements were employed to study the gelation process of the novel gel system. After that, the factors affecting the gel performances were examined by a series of bottle tests and optical microscopy analyses. And the gelation strength and rheological properties were investigated through a series of strain sweep experiments. The injectability and plugging performance of SRC/WTP gel system in low-permeability cores with millimeter and submillimeter-cracks were evaluated by core flooding tests. Finally, this study would present the field tests of the novel ISCWG-EOR application in Ordos Basin, China.