A novel thermotransformable controlled
polymer system (tPPG) is
developed that can be injected into fractures or fracturelike features
as a millimeter-sized particle gel (100 μm to a few millimeters)
and acts as a plugging agent, then dissolves into linear polymer at
a designated period (e.g., 6 months), because of the reservoir’s
temperature. The dissolved polymer seeps into the depth of the formation
and performs as a mobility control agent with high viscosity. Working
together with permanent cross-linking the polymer, polyethylene glycol
diacrylate 200 (PEG-200) entails the role of controlling dissolution
time which has been added into the tPPG as a labile cross-linker.
The polymer’s viscosity will not be influenced by the shearing
stress during pumping or salinity in the reservoir. The time tPPG
requires for transformation is dependent primarily upon the reservoir
temperature and labile cross-linker concentration. This strategy offers
a facile and economic approach to fabricating a promising dual-functional
polymer system. In order to evaluate our proposed approach, main properties
of the tPPG polymer are probed, including the swelling ratio, mechanical
strength, and thermostability before transformation, viscosity, moving
ability, and mobility control ability after transformation.
Summary
Preformed particle gels (PPGs) have been successfully applied to control conformance for mature oil fields because of their advantages over conventional in-situ gels. However, field applications have demonstrated that current particle gels cannot efficiently plug open fractures, fracture-like channels, or conduits that exist in many mature oil fields. The objective of this study is to systematically evaluate a new recrosslinkable-PPG (RPPG) product that can be used to efficiently control the conformance for abnormal features. The RPPG can swell to 38 times its initial volume, and the equilibrium swelling ratio is independent of the brine salinity. Temperature and the particle size showed a gradient effect on the swelling rate of the gel. Additionally, the particle gels can recrosslink to form a rubber-like bulky material in the large-opening features after placement that significantly enhances the plugging efficiency. We systematically evaluated the effect of temperature and RPPG swelling ratio on the recrosslinking time, the gel strength after crosslinking, and the gel thermostability. Coreflooding tests were run to test whether RPPG can significantly improve the fracture-plugging efficiency compared with a traditional PPG that cannot recrosslink after pumping. The RPPG can be customized for mature reservoirs with a temperature from 23 to 80°C with a controllable size from tens of nanometers to a few millimeters. The recrosslinking time can be controlled from 2 to 80 hours, depending on the swelling ratio and temperature. The gel elastic modulus after recrosslinking can achieve from 300 to 10 800 Pa, depending on the swelling ratio and the temperature. Coreflooding tests showed that the breakthrough pressure of the recrosslinked RPPG can reach up to 300 psi/ft for a fracture with a 0.2-cm aperture, which is more than five times higher than that of the conventional PPG.
The influence of the molecular weight (M w ) of hydrolyzed polyacrylamide (HPAM) on the cross-linking reaction of HPAM/Cr 3+ and the transportation of HPAM/Cr 3+ in microfractures is systematically studied using viscometry, ultraviolet− visible absorption spectrophotometry, and displacement experiment with a visual microfractured model. The results show that a high-M w HPAM is advantageous to the intramolecular cross-linking reaction of the HPAM/Cr 3+ system but disadvantageous to the transportation of the HPAM/Cr 3+ system in microfractures. At the intramolecular cross-linking stage, the injection pressure of the HPAM/Cr 3+ system in microfractures is almost equal to that of the HPAM solution, which undergoes no change with the degree of the cross-linking reaction. The higher the HPAM M w , the earlier the intramolecular cross-linking ends (thus, the intermolecular cross-linking reaction of HPAM/Cr 3+ occurs earlier, which leads to an earlier increase in the injection pressure of the HPAM/Cr 3+ system). Moreover, there is a matching relationship between the fracture aperture and the HPAM/Cr 3+ system to minimize the chromatographic separation when the HPAM/Cr 3+ system transports in the microfracture. For the conformance control of a fractured tight oil reservoir, we conclude that an HPAM/Cr 3+ system with a low-M w HPAM can more easily enter the deep reservoir to expand the swept volume on a larger scale. However, the system with a high-M w HPAM can form a gel with a higher viscosity to produce a higher plugging strength.
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