The use of a polymer gel is an effective method for water shutoff in mature oilfield development. As for fractured reservoirs, in order to mitigate the filtration of gelant (fluid solution of cross-linker and polymer that exists before gelation) to matrix and increase the enduring erosion ability of mature gel, chromium(III) acetate, and phenol-formaldehyde cross-linking, the HPAM gel system of a secondary cross-linking method is used more often. Chromium(III) salt is often used as the first cross-linker. However, the crosslinking mechanism is achieved by an ion bond, which is less stable than a covalent bond when used as an organic cross-linker. Resorcinol and phenol-formaldehyde used as the first and secondary cross-linker, respectively, are discussed in this paper. Results showed that resorcinol can quickly cross-link with HPAM at room temperature. Gelant formulated with a combination of 0.3 wt % HPAM added to 10-30 mg/L resorcinol can increase its viscosity from 10.2 to 150 mPa 3 s within 2 h. SEM results show that the microstructure of the first cross-linking gel appears in typical dendritic shape, with branched chains diffused in arbitrary directions. The high shearing tolerant ability of the first cross-linking gel can be achieved by these branched chains. However, a tight 3-D network structure is formed in the microstructure of the secondary cross-linking gel. This is the benefit of the stability of the skeleton structure of gel enhancing. The main factors, including temperature and total dissolved solids (TDS), to affect the gelation performance of this secondary cross-linking gel are also discussed. Results show that gelation time decreased and gel strength increased with increasing temperature and TDS. Especially for TDS, the adverse law of the gelation performance with PEI/PAtBA or PEI/HPAM gel systems is shown. The gelation performance of a resorcinol/phenol-formaldehyde/HPAM gel system of a first cross-linking state after flowing through porous media is studied. Atomic force microscopy (AFM) scanning results show that in comparison to the original gel, the structure of the weak cross-linking (code B) gels has a certain degree of damage after flowing through porous media. However, the final gel strength of both gels do not show an apparent difference. This demonstrated that the first cross-linking achieved by resorcinol can guarantee the effectiveness of secondary cross-linking. This study suggests that a resorcinol/phenolformaldehyde/HPAM secondary cross-linking gel system can be used for water shutoff in fractured reservoirs.

Using polymer gel is still the mainstream technique of the chemical method for water shut-off in mature oilfield development. The gelation performance of polyethyleneimine (PEI) cross-linking partially hydrolyzed polyacrylamide (HPAM) gel was systematically investigated in this paper by using three types of molecular weight (M w ) HPAM. Results show that the gelant solution (the fluid solution of cross-linker and polymer that exists before gelation) can slightly gel even at room temperature and form a rudimentary 3D network structure for protecting the subsequent gelling. The main factors, including HPAM, PEI concentrations, HPAM M w , and total dissolved solids (TDS) to affect the gelation performance are also discussed. The gelation time is around 18−72 h at 65 °C, which is much longer than the commonly used chromium(III)-acetate crosslinking HPAM gel system. The gelation performance of the PEI/HPAM gel system is improved after flowing through porous media and shows the adverse law for a porous media sheared chromium(III)-acetate or phenol-formaldehyde cross-linking HPAM gel system. Atomic force microscopy (AFM) scanning results show that the microstructure of PEI/HPAM gel distributes a lot of cavities. Compared to the original premature gel (code B), the cavities are more uniform and regular after flowing through porous media. In addition, this study revealed that the presence of free oxygen in the uniform cavities of the weak crosslinked gel after flowing through porous media can accelerate the oxidation reaction to produce a darker brown gel. It also discusses how these new findings will affect the application of the gel system in the oil field.

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High-pressure air injection (HPAI) in light oil reservoirs has proven to be a valuable improved oil recovery (IOR) process and aroused more attention worldwide. In this research, laboratory experiments were performed to study the potential of HPAI application in Keke Ya oilfield (Tarim Basin, China). Test oil and actual reservoir core samples were taken from Keke Ya oilfield, and rock composition was prior analyzed through X-ray method. The thermal behaviors of oil-only, cuttings, and oilcuttings were studied by thermogravimetry/derivative thermogravimetry (TG/DTG), differential thermal analysis (DTA) tests, and also, rock composition and clay mineral type/relative content effects on light crude oil oxidation behavior were systematically investigated. The results show that Keke Ya oil has a favorable exothermic behavior, exhibiting apparent low-temperature oxidation (LTO), fuel deposition, and high-temperature oxidation (HTO) stages. An extended fuel deposition stage was observed by addition of cuttings. Smectite present in illite/smectite plays a positive role of catalyzing effects for oil oxidation in different reaction regions. In this study, it revealed that smectite is ranked first, illite or chlorite is ranked second, followed by kaolinite, in the aspect of catalytic ability for crude oil oxidation. The Arrhenius model was introduced for kinetics analysis. How these findings will provide guidelines to achieve high-performance HPAI projects is also discussed.

In previous study, we have discussed the gelation performance of the polyethyleneimine (PEI) cross-linking partially hydrolyzed polyacrylamide (HPAM) gel system. The major goals of this paper are to investigate the conventional application performance of the PEI cross-linking HPAM gel system, including injectivity and sealing charactersic through core flowing experiments. Results show that polymer gels formualted with a combination of 2.0 wt % HPAM (M w = 8000 kDa) and 0.35 wt % PEI can achieve the critical pressure gradient above 500 psi/ft on average and with a maximum at 1136.38 psi/ft through the core flowing experiments with apertures ranging from 0.080 to 0.200 cm. The pressure gradient and brine permeability reduction factors (F rrw) are also attractive. The gradient pressure is inline with expectations of the gradient pressure predicting curve obtained by Seright for the chromium(III) acetate cross-linking HPAM gel. A strong adhesive force of stretching 2 cm width rather than breakage in fracture profile is observed after gel washout, showing that PEI/HPAM gels have strong and durable mechanical performance to withstand brine flowing. Atomic force microscope (AFM) scanning results exhibit that disorderly cavities are distributed in the microstructure of dehydrated gels, seem like a honeycomb. Free water existing in the gel network can be easily removed through these cavities for gel dehydrating. The evaluation of the basic application performance demonstrates that the PEI cross-linking HPAM gel is a promising sealing agent for use in fractured reservoirs.

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Thermogravimetry and differential scanning calorimetry were performed on a tight oil that was collected from the Wolfcamp shale reservoir, USA. The results indicate that although the oil has similar properties to those of a light oil, it shows similar thermal-oxidative behaviors to those of a heavy oil. Kinetic data of the tight oil as activation energy and frequency factor were estimated by the Arrhenius method. This kinetic analysis could provide us insight to characterize the reactivity of the tight oil, and establish parameter values for kinetic models used in the numerical simulation of the air injection process.

Microgel is a novel conformance control technology that can be injected into high permeability zones and transport into deep area through pore throats. It can efficiently plug large pores to force injection water diversion to achieve conformance control. In this paper, we synthesized the microgel with a double-cross-linking structure of non-labile and labile characteristics. Then, a polarizing microscope, laser particle analyzer, Brookfield DV-III, and atomic force microscopy (AFM) were used to investigate the swelling mechanism of the microgel in high-saline water (total dissolved solids is from 30 to 100 g/L) at 65 °C. Results show that the swelling ability of the double-cross-linking structure microgel is not sensitive to salinity and the solution viscosity can increase to a certain degree with the maximum value of 18 mPa s. The decomposition of labile cross-linker results in a loose and explosive structure, which seems like a “mushroom cloud”. Because of the high-density cross-linking sites provided by the non-labile cross-linker, the integrity of the spherical feature of the microgel was reserved after aging for 70 days. On the basis of the viscosity increase of microgel solution during the swelling process, a concept to develop a novel water shutoff agent is proposed. That is, the conventional cross-linkers, such as formaldehyde, methenamine, or phenolic or chromic salt, can be added to microgel solution during the injecting process. Thus, the cross-linker can have the potential to cross-link with the groups that released during the swelling process to form a bulk gel system, which can further improve the water shutoff performance.