Hydraulic fracturing is the main stimulation method used to economically produce from shale formations. The method requires the injection of a fracturing fluid at a pressure which is high enough to fracture the formation, and as a result, improves the well productivity. A proppant is pumped with the fracturing fluid to prevent the closure of the induced fractures after the treatment. The proppant inside the fracture is subjected to a high earth closure stress, which causes proppant crushing, embedment, and compaction mechanisms. The subsequent reduction of the fracture width and proppant porosity reduces the fracture conductivity and could be crucial to the success of the fracturing treatments. The objective of this study is to experimentally evaluate the reduction of the propped fracture width and proppant porosity between two Eagle Ford shale samples under stress conditions.
An experimental model of propped fracture in Eagle Ford shale was prepared using outcrop samples. Sand proppant of the size 20/40, 40/70, and 100-mesh were tested at the concentrations of 0.2, 0.4, and 0.6 lb/ft2. A high-pressure core holder with modified fittings was used to subject the fracture model to different closure stress values up to 8,000 psia. A new method was used to evaluate the change in the propped fracture width and proppant porosity as a function of closure stress. The fracture width was measured by the consecutive imaging of the fracture under stress using a digital borescope and an image analysis software. The change in the proppant porosity was calculated at each stress and post-experiment sieve analysis was done to quantify the crushed proppant due to the applied stress.
The fracture width and the equivalent proppant porosity under stress were found to be a function of the proppant size and concentration. The proppant porosity under stress was found to be directly proportional to the proppant concentration and inversely proportional to the proppant size. The reduction in fracture width and proppant porosity due to stress ranged from 3.66 to 22.03% and from 5.29 to 39.85% respectively. The crushing of sand proppant was found to be as high as 28.03% at 8,000 psia and 0.2 lb/ft2 proppant concentration, and reduced by increasing its concentration or decreasing its size.
The propped fracture width and proppant porosity under stress can be used as inputs to well production models, reservoir simulation models, and fracture design calculations. The results can also be used in the proppant selection process to improve the fracture conductivity and maximize the well productivity of Eagle Ford shale formations.