Factor analysis and mechanism disclosure of supercritical CO2 filtration behavior in tight shale reservoirs

Influence of Supercritical CO2 Fracturing Fluid on the Permeability of Shale Reservoir and Mechanism Analysis

Reduction of the reservoir permeability during supercritical CO2 fracturing caused significant reservoir damage, which directly affects the crude oil recovery rate. This investigation explored a novel thickener for supercritical CO2 fracturing fluid, this CO2 thickener not only effectively improves the viscosity and rheological properties of CO2 fracturing fluid but also contributes to reduce reservoir damage and improve permeability. The research results indicated that the synthesized CO2 thickener (3 wt %) can increase the apparent viscosity of supercritical CO2 fracturing fluid to 7 mPa·s, and a 9% matrix permeability damage rate and a 0.5 mD permeability decrease value are shown in a 3% CO2 fracturing fluid. However, 3 wt % of commercial CO2 thickener only increases the apparent viscosity of supercritical CO2 fracturing fluid to 3 mPa·s, while the reservoir damage rate increases to 13%. Two thickeners exhibit completely different damage capabilities to the reservoir, and the synthesized CO2 thickener shows excellent characteristics of reducing reservoir permeability and is also conducive to protecting shale reservoirs. Furthermore, supercritical CO2 fracturing fluid containing synthetic thickeners has better temperature and shear resistance compared with commercial thickeners. This may be because the synthesized thickener and the micro grid formed by supercritical CO2 reduce the adsorption in shale crevices, but a large amount of commercial thickeners can adsorb on the surface of shale.

Effect of extrusion-spheronization granulation and manganese loading on catalytic ozonation of petrochemical wastewater

The petrochemical secondary effluent (PSE) is typical refractory wastewater derived from the petrochemical industries, which requires advanced treatment due to the strict environmental protection policies. Catalytic ozonation is one of the most widely used advanced oxidation technologies in wastewater treatment because of its high mineralization rate, in which the alumina-based catalyst usually plays an important role. Extrusion-spheronization is a promising technique for the preparation of alumina spheres because the synthesized alumina particles have high sphericity, high specific surface aera and narrow particle size distribution. In this paper, two kinds of alumina-based catalysts (catalyst A: manganese nitrate added after alumina granulation and catalyst B: manganese nitrate added into alumina powder before granulation) were prepared by the extrusion-spheronization method and used for PSE treatment by catalytic ozonation. The prepared alumina samples were characterized by Brunauer-Emmett-Teller (BET) method, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the wastewater samples were analyzed for Total organic carbon (TOC), UV₂₅₄ and fluorescence spectroscopy. Results showed that manganese was uniformly distributed in both catalysts, and the specific surface area of two catalysts was 318.36 m²/g and 354.95 m²/g, respectively. Catalytic ozonation experiments were repeated nine times with each catalyst under the same conditions. The TOC removal rates for catalysts A and B in the first run were 48.88% and 49.06%, respectively, then it dropped to 28.05% for catalyst A but remained 47.81% for catalyst B after using for nine times. This implied that the long-term performance of catalyst B would be more stable than catalyst A. Similar result were found in three-dimensional fluorescence analysis. UV₂₅₄ results indicated that the removal efficiency of aromatic and unsaturated substances by catalyst B was higher than catalyst A. A possible explanation is that the active component manganese oxide formed a catalyst skeleton in catalyst B, which makes it hard to dissolve. Effect of extrusion-spheronization granulation and manganese loading on advanced oxidant treatment of petrochemical wastewater.

Hydrate as a by-product in CO2 leakage during the long-term sub-seabed sequestration and its role in preventing further leakage

Subsurface sequestration of CO₂ produced by industrial production is an effective way to control the excessive emission of greenhouse gases and alleviate the potential global warming. However, CO₂ leakage is likely to occur in its long-term sub-seabed sequestration, posing a great threat to the marine environment and the marine ecology. Previous investigations mainly focused on the implementation of CO₂ sequestration project, ignoring the subsequent potential environmental hazards such as CO₂ leakage, let alone the post-treatment of these accidents. In the present work, secondary sequestration mechanism of hydrate-bearing sediment for leaked CO₂ was explored, and the effect of two important factors (hydrate saturation and sediment thickness) on it was then analyzed. It is expected to provide reference for exploring engineering measures to secondary sequestrate the leaked CO₂ and avoid the catastrophic environmental accidents. The experimental results demonstrate that the role of hydrate in secondary sequestration for leaked CO₂ is mainly due to its filling and occupation of the migration channels in sediment. In addition, due to the secondary sequestration of CO₂ hydrate, change in seawater pH value caused by dissolution of leaked CO₂ in water can be significantly weakened. Besides, with the increasing hydrate saturation and sediment thickness, CO₂ hydrate plays a progressively obvious role in secondary sequestration of CO₂ and avoiding great change in the marine environment. In this way, the leaked CO₂ can be secondary sequestrated by designing/optimizing the characteristics of hydrate-bearing sediment. The investigation demonstrates that most of the leaked CO₂ can be secondary sequestrated only when the hydrate saturation exceeds 0.30 and/or thickness of hydrate-bearing sediment exceeds 3.0 cm. The experimental investigation herein can provide technical support for avoiding environmental disasters caused by the leakage of long-term sequestrated CO₂.

Influence of organoboron cross-linker and reservoir characteristics on filtration and reservoir residual of guar gum fracturing fluid in low-permeability shale gas reservoirs

To effectively reduce the filtration rate of water-based fracturing fluid and promote the pressure holding effect of fracturing fluid in underground unconventional reservoirs, an efficient and clean organic-boron cross-linker was synthesized with boric acid and low alcohols. The results obtained that the synthesized organoboron cross-linker exhibits better fluid loss performance to water-based fracturing fluid than the commercially available cross-linker. This organoboron cross-linker allowed decreasing filtration coefficient more than 0.74 × 10^-2 m³·min^1/2 as a result of the network structure formed by the organoboron cross-linker and guar gum molecule. However, commercially available cross-linker exhibits a relatively large filtered mass of water more than 1.33 × 10^-2 m³·min^1/2 at the same condition. Meanwhile, the cross-linked guar gum fracturing fluid can significantly improve the fluid loss property with the increase of cross-linker content and pressure, and an increased fluid filtration gradually was revealed with increasing the reservoir temperature and current speed. Moreover, the damage of shale reservoir caused by the prepared boron cross-linker was only 11%, which was lower than 18% of the commercial boron cross-linker under the same conditions.

Experimental investigation on the high-pressure sand suspension and adsorption capacity of guar gum fracturing fluid in low-permeability shale reservoirs: factor analysis and mechanism disclosure

Guar fracturing technology has been considered as a kind of popular EOR technology, but the weak static suspension capacity becomes a challenge due to the poor temperature resistance and stability of guar fracturing fluid. The main goal of this investigation is to explore the effect of different factors on the high-pressure static sand suspension of guar gum fracturing fluid by a synthetic efficient nano-ZrO₂ cross-linker. In particular, a mechanism of static suspended sand of nano-ZrO₂ cross-linker is analyzed by microscopic simulation. The adsorption performance of guar fracturing fluid on the shale surface is also studied for analyzing the environmental pollution and damage of guar gum fracturing fluid to shale reservoirs after cross-linking in this investigation. The results obtained that the inclusion of a small content of nano-ZrO₂ cross-linker (0.4%) leads to an apparent increase of fracturing fluid viscosity and decrease in the falling quality of gravel (104 mPa·s and 0.3 g) compared to the classical cross-linker (63 mPa·s and 3.5 g). The lower adsorption capacity of guar fracturing fluid containing nano-ZrO₂ cross-linker on the shale surface means that it has a weaker pollution ability to the shale reservoir than the commercially available cross-linker. Meanwhile, the grid structure density formed by nano-cross-linker and guar gum is considered to be the key factor to significantly change the suspended sand capacity. The investigation of nano-cross-linker cannot only provide necessary theoretical technology and data support for the stability of water-based fracturing fluid, efficient sand carrying, and the development of water-based fracturing technology, but also effectively protect the underground shale reservoir.