An Experimental Study to Demonstrate that Nanoparticles can Filter into Shale Formations and Improve Wellbore Stability

Abstract For fields with hard and brittle shale sections that have microfractures (either preexisting in the formation or induced by drilling operations), we experienced troublesome drilling with a relatively low rate of penetration (ROP). A systematic experimental work has been implemented to discover whether nanoparticles added to drilling fluids penetrate shale formations and improve wellbore stability. Starting with quantitative analysis of the shale rocks, we found the mineralogy and clay composition of these rocks by using X-ray diffraction (XRD). The XRD analysis shows that the shale rocks consist of mixed-layer clays and are very heterogeneous. The shale samples were then characterized by X-ray computed tomography (XCT) and scanning electron microscopy (SEM) to gain an understanding of preexisting microfractures. The XCT results show that multiple cracks are irregularly distributed inside the shale with widths between 10 to 25 μm. Smaller cracks with widths ranging from 100 to 250 nm were also observed via imaging on the shale sample surface using SEM. To answer whether adding the nanoparticles improves wellbore stability, we first designed the nanoparticle infiltration test by flowing nanoparticle dispersions through the shale assembly at an inflow pressure of around 800 kPa. After the shale samples were subjected to infiltration tests with durations between 9 to 27 days, the injected depths of nanoparticle along cracks was determined to range from 190 to 420 μm, as seen by optical microscope images. Higher resolution SEM images show that the maximum infiltration depths of the nanoparticles range from 2 to 5 μm in different crack-free locations. Nanoindentation tests were then conducted under the continuous stiffness measurement (CSM) mode to probe the mechanical properties of the pre-treated and post-treated shale samples. At two indentation depths, the averaged Young's modulus E for the shale sample before and after nanoparticle infiltration significantly increased from 29.5 to 34.0 GPa (at a depth of 500 nm) and from 29.0 to 33.3 GPa (at a depth 3000 nm), respectively. The nanoparticle infiltration and nanoindentation test results demonstrate that nanoparticles can filter into and further strengthen the shale formation. We also formulated nanoparticles in our drilling fluid and performed the pressure transmission test (PTT) to see their sealing effect on the field cores. A highly favorable plugging result was obtained from the PTT using base mud with nanoparticles. Therefore, it is concluded that our systematic experimental work has delivered results, which support the practice of using nanoparticles to improve wellbore stability for shale formations.