Synthesis and Characterization of Surfactant for Retarding Acid-Rock Reaction Rate in Acid Fracturing.

Acid fracturing is an effective method to develop ultra-low permeability reservoirs. However, the fast reaction rate reduces the effect of the acid fracturing and increases the near-well collapse risk. Therefore, it is necessary to retard the acid-rock reaction rate. In this work, we synthesized an acid-resistant Gemini zwitterionic viscoelastic surfactant (named VES-c), which has good performances such as temperature resistance, salt resistance, and shear resistance. Besides, a low concentration of VES-c increases the viscosity of the acid solution. The CO2 drainage method was used to measure the reaction rate between the dibasic acid and dolomite/broken core. We find that the dibasic acid containing 0.3% VES-c retards the dolomite reaction rate of 3.22 times compared with only dibasic acid. Furthermore, the dibasic acid containing 0.3% VES-c exhibits uniform distribution and is not easy to adhere to the solid surface. The VES-c also is favorable to reduce the formation of amorphous calcium carbonate. Retarding the rate of acid-rock reaction and enhancing the acidification are mainly attributed to VES-c's salt-tolerance, anti-adsorption, and the property of increasing the viscosity of the solution. Hopefully, this kind of surfactant retarding reaction rate is applied to other acid-rock reactions.

Study on the Performance Degradation of Sandstone under Acidification.

In most oilfields, acid fracturing is widely used for oil production. Understanding the relationship between the individual factors (i.e., carbonate rock types, acid rock reaction kinetics, and deterioration of rock mechanical properties) can provide practical guidelines that can be used for the design and optimization of acid fracturing operation. This paper takes hydrochloric acid, acetic acid, and citric acid as the main research objects and carries out acidification experiments on sandstone in Changqing Oilfield, China. In addition, the effects of tribasic, dibasic, and monobasic acids on the mechanical properties of sandstone were studied. Results show that in this study area, the most obvious effect was seen with the use of dibasic acids (hydrochloric acid + acetic acid), which effectively reduced the sample quality, uniaxial compressive strength, and elastic modulus. Citric acid and Mg promote the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. Furthermore, it is found by scanning electron microscopy analysis that experimental group 5 (hydrochloric acid + acetic acid) has the most ideal rock erosion effect. Inductively coupled plasma emission spectrometry analysis shows that the acid rock is present in the solution. X-ray diffraction qualitative analysis of the composition and concentration of ions shows that the formation of white precipitates is citric acid and Mg promotes the conversion of amorphous calcium carbonate to high-crystallinity calcite, forming a white precipitate. The findings of this study can help to better understand the erosion, failure state, and failure mechanism of different acid types on sandstone, which may provide certain references and guidelines for sandstone acid fracturing oil production.

High-Strength, Waterproof, Corrosion-Resistant Nano-Silica Carbon Nanotube Cementitious Composites.

This study aims to prepare a nano-silica-carbon nanotube (NS-CNT) elastic composite using NS (nano-silica), CNTs (carbon nanotube), and (D3F) trifluoropropyltrimethoxysilane. The results show that the activated NS could promote the hydrolysis of D3F. Polymerization products of nano-silica and D3F are uniformly adhered onto the surfaces of CNTs, thereby forming a NS-CNT composite. The composite is composed of irregular ellipsoids of 3-12 µm in length and 2-10 µm in diameter. The activated NS-CNT composite material effectively promotes the further hydration of (CaOH)2 in the cement to form hydrated calcium silicate, and further dehydration-condensation between the surface hydroxyl group of the composite material and the inherent hydroxyl group of (CaOH)2. The cementitious composite-based composites containing the activated NS-CNT exhibit high mechanical strengths, high water resistances, and good durability and corrosion resistance. The chemical characterizations reveal the morphology, nucleation mode of the composite, and its influence on the hydration structure and products of cementitious composite.

Data Processing Methods for 3D Seismic Imaging of Subsurface Volcanoes: Applications to the Tarim Flood Basalt.

The morphology and structure of plumbing systems can provide key information on the eruption rate and style of basalt lava fields. The most powerful way to study subsurface geo-bodies is to use industrial 3D reflection seismological imaging. However, strategies to image subsurface volcanoes are very different from that of oil and gas reservoirs. In this study, we process seismic data cubes from the Northern Tarim Basin, China, to illustrate how to visualize sills through opacity rendering techniques and how to image the conduits by time-slicing. In the first case, we isolated probes by the seismic horizons marking the contacts between sills and encasing strata, applying opacity rendering techniques to extract sills from the seismic cube. The resulting detailed sill morphology shows that the flow direction is from the dome center to the rim. In the second seismic cube, we use time-slices to image the conduits, which corresponds to marked discontinuities within the encasing rocks. A set of time-slices obtained at different depths show that the Tarim flood basalts erupted from central volcanoes, fed by separate pipe-like conduits.