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Early Hydration Process of Well Cement Slurries for Cementing CO2-Contained Gas Reservoirs and Factors Affecting the Performance of the Cement Slurries
DENG Zhuoran, LIU Wenchao, BI Yi, LUO Yangli, CHENG Xiaowei, MEI Kaiyuan
, Available online  
Abstract(40) HTML (26) PDF (3728KB)(2)
Abstract:
In cementing a gas well with CO2 contained in the reservoir, it is inevitable for the cement slurries to contact with the CO2. The length of the contact time will possibly affect the early hydration characteristics and the development of the microstructure of the cement slurries, and worse still is the negative effects of this contact on the quality of well cementing. In this study, the change patterns of the early hydration characteristics, the mechanical properties and the permeability of a cement slurry were systematically investigated by controlling the ventilation time of CO2. Using XRD, TG and SEM etc., the early phase composition and the evolution pattern of the cement slurry were analyzed. The study showed that the fluidity of the cement slurry was improved in the early stage and was then deteriorating with the time of CO2 ventilation. The setting time of the slurry, on the other hand, was shortened continuously. The hydration heat release curve showed that CO2 treatment significantly accelerated the cement hydration process, and increased the early cumulative released heat. However, as the time of CO2 ventilation increased, these effects became weakening. Compressive strength and permeability test results showed that CO2 treatment significantly enhanced the early strength of the cement slurry, and the best result can be obtained at ventilation time of less than 3 minutes. However, as the ventilation time of CO2 increased, the strength and permeability of the set cement both gradually decreased. XRD, TG and SEM analyses showed that CO2 treatment increased the formation of CaCO3, disrupted the coating structures formed by Ca(OH)2 and the C—S—H gel, and hence accelerated the hydration process of the cement particles.
Lost Circulation Prediction Based on Long Short-Term Memory Network and Random Forest Algorithm
CAI Aiting, SU Junlin, DAI Kun, ZHAO Han, WANG Jiayi
, Available online  
Abstract(370) HTML (296) PDF (5349KB)(8)
Abstract:
Lost circulation is one of the key factors restricting drilling safety and efficiency. To realize accurate prediction of lost circulation, a hybrid model for the prediction of lost circulation is presented based on long short-term memory (LSTM) and random forest (RF) algorithm. The LSTM model, the RF model and the LSTM-RF hybrid model are constructed based on algorithm principle. Fourteen lost circulation characteristic parameters are selected using correlation analysis method, and are input into three lost circulation prediction models for training. The performance and lost circulation prediction accuracy of the three models are then analyzed and compared. The experimental results show that the root mean square error (RMSE) of the hybrid model on the test dataset is 0.11, the mean absolute error (MAE) is 0.22, the coefficient of determination (R2) is 0.95, and the overall accuracy reaches 84.2%, each indicator is better than that of the single model. Furthermore, hybrid model has successfully predicted 5 times of lost circulation in field application. The results of this study show that LSTM-RF hybrid model is a model with optimal comprehensive performance in lost circulation prediction, it can predict lost circulation more precisely, and can provide reference for the prevention of lost circulation and for the decision making in drilling operation.
Flow Characteristics of Dual-Increasing Stimulation Slurry in Unconsolidated Silty Sandstone
LIU Xilong, SUN Qian, ZHANG Guobiao, LI Bing, ZHANG Kewei
, Available online  
Abstract(418) HTML (311) PDF (7827KB)(11)
Abstract:
The dual-increasing stimulation slurry is a novel stimulation fluid developed for weakly cemented reservoirs, such as submarine methane-hydrate-bearing silty sandstones. After injection into the formation, it consolidates to form porous-media slurry veins that enhance permeability. This study employed a slurry fracture flow visualization apparatus to investigate the flow characteristics of the slurry within muddy silty sediments. The experiments revealed the influence of geological parameters, slurry formulation, and operational parameters on slurry flow, fluid loss, and slurry-vein porosity. The results indicate that the slurry flows uniformly and exhibits a convex fracture flow profile, flowing to the end of main fracture and branch fracture, effectively filling fractures. Lower fluid loss increases the proportion of medium-to-large pores within the slurry veins. Adjusting the slurry formulation can reduce fluid loss in formations of varying permeability, whereas a high injection rate expands the fluid loss zone. The effective porosity ranges from 50% to 60% with a uniformly distributed pore space, forming a structure dominated by large pores (pore diameter > 50 nm) and densely distributed micro- to mesopores (pore diameter < 50 nm). This pore network can serve as high-conductivity channels for gas and water migration, while the dense distribution of small and medium pores is conducive to sand control.
A New High-temperature Tackifier for Solid-free Drilling Fluids
ZHOU Guowei, ZHANG Xin, YAN Weijun, HUA Guiyou, ZHUANG Zhenhua, QIU Zhengsong
, Available online  
Abstract(389) HTML (347) PDF (2972KB)(12)
Abstract:
The Ordovician buried-hill reservoir in Liaohe Oilfield exhibits a challenging high-temperature (200℃ at reservoir center) and low-pressure (pressure coefficient 1.01～1.06) environment characteristic of typical high-temperature, low-pressure oil/gas reservoirs. To achieve formation protection, a solids-free water based drilling fluid was prioritized, with tackifier selection being critical. Through molecular structure optimization, a novel high-temperature/salt-resistant tackifier was developed using four monomers: N-vinylpyrrolidone (NVP), 2-acrylamido-2-methylpropane sulfonic acid (AMPS), N'N-diethylacrylamide (DEAA), and 1-(3-sulfopropyl)-2-vinylpyridinium hydroxide inner salt. The synthesis employed N'N-methylene bisacrylamide as crosslinker with potassium persulfate and sodium bisulfite as redox initiators. FTIR and TGA analysis confirmed successful polymerization, demonstrating superior thermal stability with 296.66℃ initial decomposition temperature and only 45.96% mass loss during degradation phase, outperforming commercial HE300. The fluid achieved remarkable rheological performance with 722 consistency coefficient (K) at 0.5% concentration. Laboratory evaluations verified exceptional thermal stability up to 220℃ and saturated salt tolerance. Field applications demonstrated excellent viscosity-enhancing performance and robust durability of this novel tackifier, providing vital technical support for buried-hill reservoir development and high-temperature formation drilling operations.
Anti-ultra-high Temperature Suspension Stabilizer HPAS for Oil-based Drilling and Completion Fluid and Its Mechanism of Action
KOU Yahao, NI Xiaoxiao, WANG Jianhua, ZHANG Jiaqi, YIN Da, CHI Jun
, Available online  
Abstract(402) HTML (376) PDF (4286KB)(10)
Abstract:
Aiming at the problem that the suspension stability of oil-based drilling and completion fluid was difficult to maintain at high temperature above 240℃, based on the principle of enhancing colloidal stability by space grid structure, a strong hydrophobic suspension stabilizer HPAS was developed by using sepiolite fiber and n-octyltriethoxysilane as raw materials and organic modification after hydrochloric acid treatment. The monomer was characterized by infrared spectroscopy, thermogravimetric analysis, particle size analysis and surface wettability, which proved that the modification was successful. A set of high density oil base drilling fluid based on HPAS had good performance after aging at 260℃, its AV and PV were maintained at about 33 mPa·s and 27 mPa·s, the dynamic shear force was maintained above 4 Pa, ES was higher than 800 V, the FLHTHP was controlled below 5 mL, and the mud cake thickness was less than 2 mm. Through the settlement stability evaluation, it was found that there was no hard sink at 240℃ for 7 days, and the open tank state was that the glass rod fell freely and touched the bottom easily, which met the requirements of field application. In addition, the system maintained YP above 4.5 Pa in the temperature and pressure range of 65-240℃ and normal pressure −190 MPa, which ensured the good suspension stability and cuttings carrying capacity of the system. It provided technical support for the further application of oil-based drilling and completion fluid in deep wells, ultra-deep wells and 10,000 meters deep wells.

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