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Water-Based Drilling Fluid Using Nanoparticles Proves Effective in Unconventional Shales

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Traditionally, oil-based drilling fluids are preferable in drilling shale plays because of their negligible chemical interactions, but strict environmental regulations have motived the industry to design water-based muds (WBMs) capable of controlling shale/water interactions. Still, conventional additives in general are too large to plug shale microfractures and nanopores. Thus, nanoparticles, because of their size, shape, and other properties, can provide a solution for WBMs. This study focuses on the design and evaluation of a customized water-based mud (NP-WBM) using silica oxide nanoparticles (SiO2-NPs) and graphene oxide nanoplatelets (GNPs).

Experimental Description

Experimental Materials. Conventional Drilling-Fluid Additives. The micro­size additives used in this work included bentonite to provide primary viscosity and xanthan gum to adjust the final rheological properties of the base fluid. Polyanionic cellulose low-­viscosity and pregelled starch were included as conventional filtrate-loss additives. Graphite was used as conventional lost-­circulation-material (LCM) additive. Potassium hydroxide was included as an alkalinity-control agent.

Nanomaterials. SiO2-NPs were supplied in a white powder form. The SiO2-NPs are near-spherical with an approximate diameter ranging between 15 and 20 nm. The NPs were unmodified (bare surface) and nonporous with a density of 0.1 g/cm3, a specific surface area of 170–200 m2/g, and purity greater than 99.5 wt%. GNPs used in this study had a density of 0.13 g/cm3. The GNPs have a 2D structure with an average particle size ranging from 1.3 to 2.3 µm and a thickness of less than 3 nm.

Shale Rock Properties. Woodford shale was used to test the effect of using NPs to enhance the inhibition capabilities of WBM. This rock is described as a late Devonian-Early Mississippian marine shale. In this study, shale samples of 1 in. in diameter and lengths between 0.5 to 2 in. were cored and stored in high-purity mineral oil to avoid property alterations. The rock contains 31.5% clay minerals (22.8% illite, 5.4% chlorite, and 3.3% kaolinite), 67% quartz, and 1.5% pyrite, indicating a high degree of brittleness as well as low water sensitivity. Fig. 1 shows a sequence of scanning electron microscope (SEM) images in which Image A is equivalent to 7,000x magnification and Image B shows a 30,000x magnification with respect to the original sample size. The SEM images suggested a pore-size distribution between mesopores (2–50 nm) and macropores (greater than 50 nm) with an estimated median pore size of 112.84 nm. Therefore, the nanomaterials in this study have a scale matching that of the Woodford shale and can be used as theoretical bridging agents.

Fig. 1—SEM images of the Woodford shale.

 

NP-WBM Preparation and Screening Criteria. Different nanofluids were prepared by mixing Drilling Fluid A with the desired NP dispersion at a high shear rate (22,000 rev/min) for 20 minutes while adjusting the pH to 9.5. This study established a concentration of 1% by weight of NPs as an upper limit. Their extremely high surface-area/volume ratio suggested that small concentrations might enhance overall WBM properties. Also, low concentration aims to reduce the costs associated with the technique as well as aggregation tendencies. The initial step of the screening criteria included the addition of different concentrations of SiO2-NPs or GNPs to the basic Drilling Fluid A.

The second step in the design of the NP-WBM included the mix of the best two concentrations of each nanomaterial into a single NP dispersion. This stage included the analysis of four different combinations of both nanomaterials. The drilling fluid that showed desirable properties by means of API filtrate, high-pressure/high-temperature (HP/HT) filtrate, and rheology at fresh conditions in comparison with the other fluids was designated as the optimal NP concentration. Finally, graphite was included as a conventional shale stabilizer. The experimental methodology is presented in the complete paper.

Results and Analysis

Selection and Characterization of Nanomaterials. The selection of SiO2-NPs was made primarily on the basis of their size, which can theoretically fit the unconventional pore size of the Woodford shale. Also, their low cost, owing to well-known preparation methods, makes them economically attractive. GNPs were included in the belief that, at downhole temperatures, their malleable nature might seal the shale counters, especially in high-illitic shales. The authors of this paper believe that both nanomaterials together might help to reduce fluid invasion and its consequences.

Effect of SiO2-NPs and GNPs in Drilling Fluids. In general, the addition of NPs indicated that these additives might be added to WBMs as long as stable NPs are used. However, under static conditions, the GNPs generate higher gel strength values compared with the ­fluids containing SiO2-NPs. No progressive gels were observed.

Filtrate results at low-pressure/low-temperature (LP/LT) conditions showed that SiO2-NPs had a negative effect at concentrations greater than 0.5 wt%. The reduction of interparticle spaces at higher concentration might promote the aggregation of NPs, resulting in a highly permeable filter cake that allows more fluid to pass. In the case of GNPs, it was observed that concentrations greater than 0.4 wt% did not provide further reductions in the filtrate test. Further results indicated that this concentration might be the upper limit for this additive. For both nanomaterials, the best concentration was 0.25 wt%, resulting in a filtrate reduction of 11.63 and 13.95% for the SiO2-NPs and the GNPs, respectively, compared with the control fluid. Overall, NPs performance at HP/HT conditions was better compared with the behavior at LP/LT conditions. On the basis of the initial outcomes, the best two concentrations of SiO2-NPs (0.25 and 0.5 wt%) and GNPs (0.25 and 0.4 wt%) were selected to continue with the second step in the screening process.

Overall, Nanofluid C showed the best performance and was selected as the optimal concentration to be included in the final NP-WBM design. Once again it was evident that the NP has a better effect at HP/HT test conditions. Also, comparing these results to the ones obtained in the NPs’ individual tests, the combined formulation exhibited a slight improvement with respect to the nanofluids containing just GNPs. However, the positive effect of reducing the plastic viscosity was only achieved after mixing both nanomaterials in a single formulation.

Characterization of the Final NP-WBM Formulation

Rheological Performance of the Optimized NP-WBM. To further optimize Nanofluid C and evaluate a possible cooperation effect between NPs and conventional LCM additives, graphite was added to the final formulation at a concentration of 2 wt%. Rheology and filtrate tests were performed on the optimized NP-WBM for a final characterization. The results were compared with those of the base fluid and a fluid containing only graphite plus the basic formulation.

The results showed a minimal effect of graphite on the optimized NP-WBM. No extreme changes were observed in its gel strength behavior, which indicated that NPs maintained their dispersibility within the drilling fluid despite the increase of additives. Therefore, it can be concluded that all the products selected for the optimization of the NP-WBM could perform at field conditions with no need for rheological modifiers.

Filtration Performance of the Optimized NP-WBM. Experimental results showed a decrease in the filtrate volume by 12.79% for basic Drilling Fluid A plus graphite at LP/LT conditions when compared with the base fluid. The optimized NP-WBM exhibited a better performance after mixing the conventional LCM additive with the NPs, leading to a final reduction of 20.93% with respect to Drilling Fluid A. The same benefit was observed for the NP-WBM at HP/HT conditions. The cooperative effect between the NPs and the graphite yielded a 27.21% reduction, while the addition of only graphite decreases the cumulative filtrate volume by 16.91%. A synergistic effect was observed when the products were combined in a single WBM formulation. Also, the optimized NP-WBM exhibited a reduction of greater than 10% in the thickness of the filter cakes generated after both LP/LT and HP/HT tests.

Immersion Analysis Results. To obtain an insight about hydration effects that could take place when the Woodford shale is exposed to water, thin shale sections with no cracks were immersed in the fluid to be tested for a soaking period of 14 days at 150°F. Both the SiO2-NPs and GNPs tend to cover the entire surface of the shale sample. The nanoscale and the high-surface area/volume ratio of the additives allow them to provide a better bridging network between water and the grain boundaries of the different minerals present along the bedding planes, reducing the dissolution tendency and the subsequent exposure of microfractures. Also, this condition might be beneficial in controlling the dispersion of shale cuttings in WBM.

Dispersion Test Results. The dispersion results of the shale after being hot-rolled with different fluids indicated a high erosion for the Woodford shale when exposed to alkaline water without any other additive, resulting in a shale dispersion of 22.03%. Basic Drilling Fluid A still exhibited a high dispersion (17.44%). The addition of only graphite to the base fluid improved its performance by only 9.63%. However, when the optimal concentration of NPs was mixed with the appropriate concentration of the conventional LCM additive, the tests exhibited the lowest value of cutting dispersion (11.23%). In other words, the optimized NP-WBM reduced shale erosion by 35.61% compared with the base fluid. This result suggests that the addition of SiO2-NPs and GNPs might enhance the inhibition capabilities of WBM for unconventional shales.

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 19342, “Design and Evaluation of a Water-Based Drilling-Fluid Formulation Using SIO2 and Graphene Oxide Nanoparticles for Unconventional Shales,” by Jose Aramendiz, SPE, Abdulmohsin H. Imqam, SPE, and Sherif M. Fakher, SPE, Missouri University of Science and Technology, prepared for the 2019 International Petroleum Technology Conference, Beijing, 26–28 March. The paper has not been peer reviewed. Copyright 2019 International Petroleum Technology Conference. Reproduced by permission.

Water-Based Drilling Fluid Using Nanoparticles Proves Effective in Unconventional Shales

01 November 2019

Volume: 71 | Issue: 11

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