Pressure Variations Inside the Hydraulic Fracture and Its Impact on Fracture Propagation, Conductivity, and Screen-out

Abstract

Abstract. Field measurement of fluid pressure inside hydraulic fractures have shownrapid pressure decline along fracture length. The consequence of this pressureprofile is rapidly tapering fracture width. This means that a disproportionatevolume of fluid and proppant injected inside hydraulic fractures remain nearthe wellbore, thus creating excessive near wellbore and substantially less farfield fracture conductivity. This explains why history matching of oil wellproductions yields much lower effective fracture lengths than gas wells, sinceoil wells because of their higher permeability require higher fracture flowcapacity. The rapid tapering of the fracture width also restricts the movement of theproppant inside the fracture, causing its accumulation near the wellbore. Asthe treatment progresses, and if sufficient proppant volume has been injectedinside the fracture, the near wellbore segment of the fracture can begin tofill with proppant, thus reducing the open width available for further movementof the fluid. Essentially, accumulation of proppant near the wellbore reducesthe fracture width available for fluid flow which then results in higherfrictional pressure losses inside the fracture, further skewing the pressuredistribution, and eventually screen-out. Introduction Over the last two decades the industry is gradually recognizing thecomplexity of theory and practice of hydraulic fracturing. Simple theories oftensile fracturing[1] evolved successively into concepts of near wellboretortuosity[2], presence of branches and shear fractures[3], and off-balancegrowth[4]. The hydraulic fracture is now viewed as extending under a mixture oftensile and shear forces and containing numerous branches, with the extensionoccurring randomly around the fracture tip and dominated by localinhomogeneiety and planes of weakness. Randomly distributed proppant packsformed behind narrow shear fractures are offered as alternatives to simpleviscosity dominated proppant transport models[5]. The natural consequence ofthese developments is the recognition that our simple models of pressuredistribution inside the fracture also need revision and re-evaluation. Thispaper extends the new fracturing concepts into analysis of fluid pressurevariations inside the fracture and reviews its impact on fracture shape, itsconductivity, analysis of post-frac treatment pressure analysis, causes ofscreen-out, as well as the other major application of hydraulic fracturing;namely in-situ stress measurement.