Over the last several decades, the industry has generated a large amount of proppant performance data using designated API methods. However, recent studies validate that correlations and conclusions generated from standard API testing do not accurately predict ceramic proppant performance in unconventional reservoirs. This overestimate of proppant performance can have negative effects on initial well production, in addition to steepening the production decline curve. The same study suggests the modified API RP 19-C procedure for testing of proppant performance under thin layer/monolayer conditions yields results which better correlate with well production data trends. Also demonstrated in the study, the proppant pack failure mode and mechanism can be vastly different in a monolayer pack versus a multilayer pack and suggests that in order to improve well performance, operators would benefit by choosing proppants based on their ability retain fracture width under low proppant concentrations; such as 0.5 and 0.25 lb/ft2.
The presented study expands previously mentioned work to evaluate the influence of stress cycling on the performance of various proppants in thin pack/monolayer conditions. A cycle test involves a 2,000 psi/min ramp-up to desired pressure, 2 minute hold at pressure and ramp down to 25 psi. The process then repeats to obtain the desired numbers of cycles on the proppant. The results compare the performance of proppants under a single stress cycle to samples subjected to 3 or 5 cycles. Testing is performed with sample concentrations of 4 lb/ft2 (API standard) and 0.5 lb/ft2 (thin layer pack) at 6,000psi.
The results reveal a decrease in mechanical performance for all tested samples under 3 and 5 cycles as compared to standard 1 cycle test. Clay-based ceramic proppants and sand proppants show a significant increase in crush with an increase in number of cycles. Clay based samples appear to be especially sensitive to stress cycling under thin layer/monolayer conditions. For example, 3 cycles at 0.5 lb/ft2 loading, the crush resistance of clay based proppants increases by close to 8%, while sand crush increases by only 6%. This high sensitivity is not present when testing is performed at 4 lb/ft2. Intermediate strength ceramic (ISC) proppants show the highest resistance to cyclic stress under both thick proppant pack (4 lb/ft2) and thin proppant pack (0.5 lb/ft2) conditions.
Paper quantifies the effect of reduced fracture thickness and stress cycling on performance of different proppant types. This study further discusses advantages and limitations of designed of used method as compared to common API procedures.