Adequate determination of the propped fracture geometry is critical for Frac Pack hydraulic fracture designs and analyses. Unfortunately the fracture geometry is difficult to determine in complex reservoirs, primarily because of the difficulty to determine the post tip screenout (TSO) fracture geometry. Two-dimensional analysis or even more sophisticated computer simulations typically only model the fracture process up to the onset of TSO, and do not calculate the fracture geometry beyond the onset of TSO. This paper presents results of three-dimensional hydraulic fracturing simulator calculations of the fracture geometry beyond the onset of TSO, and as TSO followed by fracture re-growth occurs. Comparative analyses of the three-dimensional simulations indicate that fracture lengths can be substantially underestimated if the three-dimensional analysis is not considered. This leads to an incorrect and non-conservative estimate of the propped fracture geometry-both in length and in width.
During the past decade, hydraulic fracturing that creates the onset of fracture tip screenout (referred to as "TSO") has been recognized as an effective technology to enhance formation conductivity in high permeability reservoirs. Modeling the TSO mechanism is complex, and generally two-dimensional and even pseudo three-dimensional models do not represent these fracturing features adequately. These analyses calculate fracturing only up to the onset of TSO and then assume that the fracture length is fixed; i.e. no further fracture growth occurs. They assume that proppant packing occurs (the fracture width increases) without further fracture growth, until all proppant is injected and pumping stops. In many cases this is likely not an adequate assumption.
For example, Bai et. al. (2003) using a three-dimensional fracturing simulator (Clifton, 1989) presented numerical results showing that fracture propagation can continue after the initiation of the first TSO. This understanding is very important, and allows improving the assessment of Frac Pack propped fracture efficiency.
As background, it is noted that before the Frac Pack treatment is initiated, a fracture calibration test is usually conducted by injecting a quantity of proppant-free fracturing fluid. This is done primarily to estimate the formation leakoff coefficient. Based on the calibration fracture results (and using an estimate of other required properties), the initial pad (without proppant) volume and slurry schedule to be used are then generated, usually using a two-dimensional or a pseudo three-dimensional computer simulator. The time between the calibration fracture and the Frac Pack treatment is usually hours to a day, and no time is allowed between the clear fluid pad injection and the fluid-proppant slurry injection.
Since the two-dimensional or pseudo three-dimensional modeling used in the Frac Pack design does not generally estimate beyond the onset of TSO, actual fracturing mechanics may not be adequately represented. This paper uses a fully three-dimensional simulator to calculate fracture growth beyond the onset of TSO and continuing as TSO occurs, and then as re-growth occurs which is often followed by another TSO and again fracture re-growth (Bai, et. al. 2003). The paper shows that the fracture geometry is often significantly underestimated without considering the complete fracture mechanics.