A new method to measure propped fracture width and proppant porosity in shale fractures

Abstract

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 available methods to evaluate such reduction do not take into account downhole stress or the compression of rock under stress. The objective of this study is to experimentally evaluate the reduction of the propped fracture width and proppant porosity between two shale samples using a new method that allows the direct viewing of the fracture at downhole conditions.A new experimental model was designed to measure fracture width and porosity of proppant under downhole conditions. Outcrop shale samples were used. Varies proppant types and sizes were used including: sand, resin-coated sand, and ceramic proppants of the sizes 20/40, 40/70, and 100-mesh. The proppants were tested at 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 stresses up to 8000 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 viewing fracture under stress using a digital borescope. The change in the proppant porosity was calculated at each stress and a sieve analysis was conducted to quantify the crushed proppant because of the applied stress.A set of tables were provided for the values of fracture width and proppant porosity under stress for each proppant type, 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 5.68 to 25.88%, and from 7.88 to 44.16%, respectively. The crushing of sand proppant was found to be as high as 28.03% at 8000 psia and 0.2 lb/ft2 proppant concentration, and reduced by increasing its concentration or decreasing its size. The embedment of ceramic proppant at 8000 psia reduced the fracture width by 29.2 and 32.5% for the Eagle Ford and Marcellus shale, respectively.The propped fracture width and 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 shale formations.