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.

Preformed particle gels (PPG) have been successfully applied to control conformance for mature oilfields due to its advantages over conventional in-situ gels. However, field applications have demonstrated that current particle gels cannot efficiently plug opening fractures, fractures-like channels, or conduits which exist in many mature oilfields. The objective of this study is to systematically evaluate a new re-crosslinkable preformed particle gel (RPPG) product which can be used to efficiently control the conformance for the abnormal features. The novel particle gels can re-crosslink to form a rubber-like bulky material in the large opening features after placement to significantly enhance the plugging efficiency. We systematically evaluated the effect of temperature, brine concentration and RPPG swelling ratio on the re-crosslinking time, the gel strength after crosslinking, and their thermos-stability. Core flooding tests were run to test whether RPPG can significantly improve the fracture plugging efficiency comparing to a traditional PPG which cannot re-crosslink after pumping. The RPPG can be customized for the mature reservoirs with the temperature from 23 to 80°C with controllable size from tens of nanometer to a few millimeters. The RPPG swelling ratio can be controlled from 5 to 40 times. Its re-crosslinking time can be controlled from 2 to 80 h, depending on absorbed water amount, brine concentration, and temperature. The gel elastic modulus after re-crosslinking can achieve from 300 to 10,800 Pa, depending on swelling ratio. Core flooding tests showed that the breakthrough pressure of the re-crosslinked RPPG can reach up to 300 psi/ft for the fracture with the width of 5 cm and 0.2 cm aperture, which is more than 5 times higher than traditional PPGs. In addition, the plugging efficiency of the RPPG is 20 times higher than 40 K.

Cretaceous Bashijiqike ultra-deep tight sandstone, the main pay zone of Keshen gas field in Tarim Basin, has characteristics such as huge buried depth (6500 m ~ 8000 m), ultra-low matrix permeability and well-developed natural fractures. Due to lacking of a thorough research on the formation damage mechanism, there is no corresponding formation damage control method. And that's why this reservoir is suffering from severe formation damage. In this paper, the multi-scale characteristic of the reservoir space and seepage channel was described firstly. Then, a series of experiments were carried out to determine the multi-scale damage mechanisms, including the fluid sensitivity damage of matrix and fracture, the phase trapping damage of matrix and fracture, the loading capacity and the dynamic damage of fracture induced by drilling fluids. Then, the multi-scale formation damage mechanisms were summarized. Results showed the gas reservoir are characterized by typical multi-scale structures, i.e. micro-nano pore-throat and multiscale natural fractures. Severe salt sensitivity damage, alkali sensitivity damage and water phase trapping damage were the main damage mechanism of micro-nano pore-throat. For micro-fracture (aperture ≤ 100 μm), the dynamic damage degree induced by drilling fluids can reach up to 60.01 %. For Mesoscale fracture (aperture > 100 μm), lost circulation induced by inadequate loading capacity of drilling fluids was the main damage mechanism. Then, a complete multi-scale approach for damage control was proposed: ① Using oil-based drilling fluids to inhibit the fluids sensitivity damage and phase trapping damage of micro-nano pore-throat and natural microfracture; ②Optimizing the solid particle size distribution of drill-in fluid to reduce the dynamic damage degree of micro-fracture induced by drilling fluids; ③Adding acid soluble temporary plugging materials while drilling to prevent lost circulation. According to the proposed approach, the total production of the test well was 94 × 104 m3, which is much higher than that of non-test wells. This research provides a detailed case of forming the multi-scale approach for damage control based on the multi-scale formation damage mechanisms. This method is practical and useful, and it has important guiding significance to develop the ultra-deep fractured tight gas reservoirs efficiently.