Basic magnesium sulfate cement (BMSC) is a kind of magnesium oxide (MgO)–magnesium sulfate (MgSO 4 )–water (H 2 O) modified ternary cementitious material prepared from light burned magnesia, magnesium sulfate heptahydrate, water and additives. The effects of α-magnesium oxide/magnesium sulfate ratio (MO/MS), water/magnesium sulfate ratio (W/MS), sand-to-binder ratio (s/b) and superplasticiser dosage (SPD) on the performance of BMSC mortar were studied for the first time in this work. The results showed that for, optimal 28-day flexural strength, α-MO/MS = 10:1, W/MS = 16:1, s/b = 0.8 and SPD = 0.3%. For optimal 28-day compressive strength, α-MO/MS = 8:1, W/MS = 14:1, s/b = 0.6 and SPD = 0.4%. For best water resistance, α-MO/MS = 12.5:1, W/MS = 14:1, s/b = 0.2 and SPD = 0.1%. Based on these results, the basic mix proportion ranges of BMSC mortar should be α-MO/MS = 8:1–12.5:1, W/MS = 14:1–16:1, s/b = 0.2–0.6 and SPD = 0.1–0.4%.

Gel treatment has been widely applied to control conformance for improving oil recovery and control water production in mature oil fields. However, most of the hydrogel systems are limited when being applied in the harsh environments of high temperatures. A systematic evaluation was conducted in this study to evaluate a modified PPG product, the high temperature resistant re-crosslinkable preformed particle gel (HT-RPPG) which can re-crosslink to form a bulky material and keep thermostable in the large-opening features after placement. This material was developed to overcome the limitations of conventional PPGs in the reservoirs with large-opening features such as open fractures, void conduits, wormholes, and so on. The HT-RPPG can swell up to 18 times of its original size at room temperature (23 degrees C), and the swelling ratio is independent of brine concentration and types. We conducted a series of experiments to evaluate the effect of particle size, temperatures, swelling ratios, brine types on re-crosslinking time, as well as the gel strength, blocking performance and thermostability after re-crosslinking. Smaller particle sizes result in the HT-RPPGs swell and re-crosslink much faster. Higher temperatures increase the swelling and re-crosslinking rate, while the larger swelling ratios (more feeding brine) can slow down the re-crosslinking time. HT-RPPG re-crosslinking process can be delayed when the particles contact with Ca2+. Additionally, the re-crosslinking of HT-RPPG is a temperature-responsive reaction which can only start after reaching the target temperature of 100 degrees C or above. The HT-RPPG has kept its volume and strength stable at 100 to 130 degrees C for over 10 months so far. A blocking performance test was conducted by using the tubing model to simulate void-space conduit (VSC), and breakthrough pressure reached to 427 psi/ft.

In order to improve the toughness of reactive powder concrete doped steel fibers, a novelty method of multi-scale toughening cement-based materials was investigated. The multi-scale synergistic toughening cement-based materials (nominated basic magnesium sulfate cement based reactive powder concrete, BMSC based RPC) were designed and discussed for the first time by endogenous whiskers and doped steel fibers. Through orthogonal trials, curing regimes, sand packing and optimization of steel fibers dosage, the synergistic toughening at different microscales was realized by using the properties of endogenous whiskers (the 5.1.7 phase) and doped steel fibers. The results showed that the 28-day flexural strength and compressive strength of BMSC based RPC were 21.4 MPa and 120.5 MPa under ambient temperature curing, respectively, when the steel fibers dosage was 2% (volume fraction). The preliminary research proved that the toughness of reactive powder concrete can be further improved under the multi-scale synergistic toughening effect of endogenous whiskers and doped steel fibers.

Abstract Conformance improvement for ultra-high-temperature (130 °C) reservoirs is challenging due to the poor thermostability of conventional preformed particle gel (CPPG). To overcome the defect of thermal degradation, a novel hydrostable PPG (HT-PPG) was developed using the high-temperature tolerant crosslinker. In this work, a comparative study between the HT-PPG and CPPG has been presented in respects of their swelling behaviors, rheology properties and thermal stabilities. Particle swelling behaviors and viscoelasticities were firstly assessed in ambient. Using the swollen particles, a long-term aging at 130 °C underwent during which the physical status was monitored through high pressure vials (HPV). Furthermore, characterizations involved Scanning Electron Microscope (SEM) and Fourier Transform-Infrared Spectroscopy (FT-IR) were performed for both virgin and aged specimen. Thereby, an observation of gel microstructures and elucidation upon bonds or functional groups were provided. In addition to aging tests, we deployed the Differential Scanning Calorimetry (DSC) to investigate the inflection temperature as another indicator of particle thermostability. Attributed to the hydrostable crosslinker, the HT-PPG withstood 130 °C for at least 90 d. It was found that the HT-PPG effectively maintained its particulate shape, whereas, the CPPG completely degraded after 3-d aging. The HT-PPG maintained 28.8% of its initial storage modulus (G′). On the contrary, the normalized elasticity (G′/G0‘) of CPPG was only 0.43%. The SEM morphologies illustrated HT-PPG kept its rigid microstructure even after 90-d aging, while indicated destruction within CPPG network. According to FT-IR characterization, the decomposition of pristine crosslinker, N,N′-Methylenebisacrylamide in CPPG may account for its instability. DSC measurements furtherly demonstrated the favorability of HT-PPG in which HT-PPG exhibited a higher inflection temperature of 133.1 °C, however, CPPG only had an inflection temperature of 127.7 °C. This work turned out the novel HT-PPG could withstand ultra-high-temperature (130 °C) for more than 90 d, maintaining its particulate shape and viscoelasticity. This a durable plugging agent was notably superior to the CPPG, offering a candidate material for the conformance improvement in ultra-high-temperature reservoirs.