DOE OSTI2018
Although sand proppant has been used since 1949 to increase efficacy of hydraulically fractured treatments by preventing fractures from closing after well depressurization, many questions remain regarding the behavior and distribution of proppant in the subsurface. To this end, we quantified the amount of recovered proppant in open fractures within a slant core drilled through hydraulically fractured Wolfcamp Shale. To improve understanding of hydraulic-fracture growth and patterns, we then addressed basic questions such as the following regarding proppant behavior in the subsurface: Is there variation along the length of the examined core? Is the proppant mainly found in hydraulic fractures, in reactivated natural fractures, or is it transported into drilling-induced fractures during recovery? Do the majority of proppant grains remain whole or tend to get crushed? To answer these questions, we collected all material from every open fracture (407 samples total) within ~600 ft of core. Samples consisted of drilling mud, aluminum shavings from the core barrel, shale and lithic fragments, calcite, and, potentially, sand proppant (95% plain and 5% colored resin-coated grains) pumped during the hydraulic fracturing of adjacent wells. We then washed, sieved into four size fractions, weighed, and imaged all samples for quantitative image analysis. Retrieved samples were labeled with respect to a master fracture database, where fractures were identified in the core and on CT scans of each 3-ft interval, allowing us to code them as either hydraulic, natural reactivated, or drilling/recovery-induced fractures. From this coding, we quantified the amount of proppant within each fracture, analyzed the distribution of grains among different fracture types, and traced base versus colored grains. Proppant packs 0.5–1 cm thick were found in two locations where there was complex fracture geometry, but fracture faces showed no evidence of proppant embedment. In situ sand patches were found on a few fracture faces, mostly in fractures near the proppant packs. Pumped proppant was 100 and 40/70 mesh (149–420 μm), and even though widespread grain crushing is not observed, the majority of recovered proppant ranges from 63 to 250 μm. The distribution of retrieved proppant, especially the larger retrieved sizes (250– >500 μm), mirrors the proximity of the slant core to the hydraulic fracture treatment wells. Hydraulic fracture samples contained variable amounts of proppant, and natural reactivated fractures sometimes contained more proppant than nearby hydraulic fractures, suggesting transference between fracture types in the subsurface. Regarding colored proppant, some colored sands occurred in discreet fractures, whereas other colors appeared to be mixed and widespread. Ultimately, all fractures contain at least some proppant, especially in the smallest size fractions; however, the extent to which the proppant was introduced during the core-drilling process is still unknown. Our study is one of the first attempts at physically recovering and quantifying injected proppant from parted fracture breaks within a stimulated reservoir, and the results are significant in terms of understanding propped fracture networks and predicting future proppant distribution during hydraulic fracturing.