Study and Field Trials on Dissolvable Frac Plugs for Slightly Deformed Casing Horizontal Well Volume Fracturing.

In view of the need for slightly deformed casing horizontal well fracturing, a new dissolvable frac plug has been designed using mechanical and material methods. The results of simulation analysis, laboratory research, and field test verification show that: (1) the integral half-split inlaid tooth slip and implicit-shaped single sealing element are adopted. Under the same pressure resistance index, the outer diameter and the total length of the frac plug are small, the inner diameter is big, and the pressure resistance downhole passing performance are better. (2) Using dissolvable magnesium-based powder, hydrogenated butadiene-acrylonitrile rubber, fluororubber polyglycolic acid, and the cellulose fiber and coating process, the solubility of the frac plug is observed to be more reliable. (3) Using the structural modular design, the frac plug has outstanding advantages in terms of on-site installation, preventing early seat sealing, self-separation after release, less dissolvable matter, and so on. (4) The finite element simulation and physical simulation experiments show that 121.36 mm ID casing with the frac plug can tolerate deformation in the range of 15 mm, which meets the needs for the casing usage. The temperature increases from 60 to 98 °C, and the maximum sustainable pressure differential decreases from 7.6 to 11.2 h at 70 MPa without any leakage. The frac plug can be completely dissolved within 300 h, and the final size of the residual solid is less than 0.3 mm. (5) In field trials of eight wells, through the minimum set of change inner diameter of 107 mm, the maximum wellhead fracturing pressure is 83.9 MPa in the formation, with the temperature ranging from 36 to 110 °C, and the plug can be completely self-dissolved in 16 days (the shortest time that has been found). After the fracturing operation with this dissolvable frac plug, the average daily oil production is maintained as high as 10.45 t with 22,000 m3 gas production. A new dissolvable frac plug tool solves the problem of continuous fracturing in slightly casing variable wells, restores the full bore of the wellbore after staged fracturing, and realizes the production of oil and gas wells without drilling plug and the smooth implementation of follow-up measures. It has a wider application range, higher efficiency, safety, and economy. It is of great significance for industrialized staged fracturing of unconventional oil and gas horizontal wells and platform wells.

Experimental Study on the Stability of a Novel Nanocomposite-Enhanced Viscoelastic Surfactant Solution as a Fracturing Fluid under Unconventional Reservoir Stimulation.

Fe3O4@ZnO nanocomposites (NCs) were synthesized to improve the stability of the wormlike micelle (WLM) network structure of viscoelastic surfactant (VES) fracturing fluid and were characterized by Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray diffraction (XRD) and vibrating sample magnetometry (VSM). Then, an NC-enhanced viscoelastic surfactant solution as a fracturing fluid (NC-VES) was prepared, and its properties, including settlement stability, interactions between NCs and WLMs, proppant-transporting performance and gel-breaking properties, were systematically studied. More importantly, the influences of the NC concentration, shear rate, temperature and pH level on the stability of NC-VES were systematically investigated. The experimental results show that the NC-VES with a suitable content of NCs (0.1 wt.%) shows superior stability at 95 °C or at a high shear rate. Meanwhile, the NC-VES has an acceptable wide pH stability range of 6-9. In addition, the NC-VES possesses good sand-carrying performance and gel-breaking properties, while the NCs can be easily separated and recycled by applying a magnetic field. The temperature-resistant, stable and environmentally friendly fracturing fluid opens an opportunity for the future hydraulic fracturing of unconventional reservoirs.

Systematic Evaluation of a Novel Self-Healing Poly(acrylamide-co-vinyl acetate)/Alginate Polymer Gel for Fluid Flow Control in High Temperature and High Salinity Reservoirs.

Preferential fluid flow often occurs when water and CO2 is injected into mature oilfields, significantly reducing their injection efficiency. Particle gels have been evaluated and applied to control the short circulation problems. This study systematically investigated a novel poly(acrylamide-co-vinyl acetate)/alginate-based interpenetrated gel system (Alg-IPNG) which is designed to control the preferential fluid flow problems in high-temperature reservoirs. Chromium acetate was incorporated into the gel system to provide the delayed crosslinking feature of the particle gels. The alginate polymer system can also take advantage of the Ca2+ ions in the formation water, which exist in most reservoirs, to reinforce its strength by capturing the Ca2+ to form Ca-alginate bonds. In this paper, various characterizations for the Alg-IPNGs before and after the self-healing process were introduced: (1) the elastic modulus is set at up to 1890 Pa, and (2) the water uptake ratio is set at up to 20. In addition, we also discuss their possible self-healing and reinforcement mechanisms. In particular, the self-healing starting time of the Alg-IPNG particles are modified between 38 to 60 h, which is related to the water uptake ratio, Ca2+ concentration, and temperature. The reinforced Alg-IPNG gel has an enhanced thermal stability (180 days) at the temperature up to 110 °C.

Experimental and mechanism study: Partially hydrolyzed polyacrylamide gel degradation and deplugging via ultrasonic waves and chemical agents.

Partially hydrolyzed Polyacrylamide (PHPAM) crosslinked by Cr+3 is frequently applied to plug thief zone for the better water management in matured oil reservoir. However, PHPAM gel may certainly cause inevitable formation damage nearby the wellbore. Although various kinds of chemical agents, such as hydrogen peroxide (H2O2), sodium hypochlorite (NaOCl), and chlorine dioxide (ClO2) were employed to mitigate the nearby wellbore damage. But, huge financial investment, poor degelation efficiency, environmentally insecure, corrosion problem, and long time span requirement persuade researchers to look for other effective technique. In this connection, ultrasonic waves is characterized by reliable, environment friendly, and cost effective technology. Current work involves comparative study of PHPAM gel degradation by the individual means of chemical agent and ultrasonic waves. Subsequently, the best-performed ultrasonic parameters and well-performed chemical agent were used independently and then simultaneously to deplug (PHPAM gel) the core sample. Results showed that 20 KHz frequency (1000 W) effectively reduced gel viscosity from original (2495 mPa.s) to 1.37 mPa.s after 10 min irradiation. This degradation is attributed to cavitation, heat energy, and hydroxyl radical (HO∙). However, after 2 min further exposure, the viscosity grew back to 3.29 mPa.s (18 KHz), 1.42 mPa.s (20 KHz), and 3.74 mPa.s (25 KHz). This adverse behavior is owing to hydroxyl radical (HO∙) annihilation. In chemical treatment, H2O2 among other chemicals efficiently degelled the PHPAM gel's original viscosity to 2.64 mPa.s after 24 h reaction. Similarly, NaOCl and ClO2 brought down original viscosity to 6.5 mPa.s and 159 mPa.s respectively. SEM of the samples before and after treatment was performed for the better understanding of PHPAM gel morphology. Considering dynamic experiment, maximum 23.5% and 19.80% damaged permeability recovery (30 × 10-3 µm2 gas permeability) were obtained by applying ultrasonic waves (20 KHz, 1000 W, and 100 min irradiation) and chemical agent (H2O2) respectively. Permeability recovery was further increased to 40.90% by the simultaneous application of ultrasonic waves and chemical agent.