The Impact of Effective Fracture Length versus Effective Fracture Height on Ultimate Recovery in Tight-Gas and Unconventional Reservoirs

Abstract

Abstract This paper evaluates the effect of different proppant pack cleanup scenarios on welltest analysis results and long-term recovery in tight gas and unconventional reservoirs. Simulated buildups are created in a detailed reservoir simulation model. The simulated buildups are analyzed in a commercial welltest analysis package to obtain an apparent effective propped fracture length, as is common practice in the industry. Two different proppant pack cleanup scenarios are compared, both of which result in a short apparent effective fracture length based on welltest analysis. The effect on long-term recovery for the different cleanup scenarios is then simulated using detailed 3D reservoir simulations, showing that there is a significant difference in recovery, in spite of the comparable welltest analysis results. In a previous publication (Shaoul et al. 2015), we showed the effect of stress sensitive reservoir permeability on reducing the fracture length estimated from welltest analysis. Now, using detailed 3D reservoir simulations, we simulate welltests with different proppant pack cleanup scenarios, from piston-like flow to complex channeling and analyze them in a welltest package. Welltest analysis estimates effective fracture area, and then calculates length based on an assumed fracture height. When only a portion of the fracture area cleans up, this gives a reduced fracture length. The maximum fracture extent (the furthest distance from the well that the proppant pack has cleaned up, relevant for drainage extent) may be close to the full propped length, while the small effective area calculated from the welltest can actually be caused by a reduced effective fracture height. The impact of this assumption on the buildup analysis and ultimate recovery is examined by simulating two different proppant pack cleanup scenarios. This work supports field observations that have been made by Barree showing a correlation between ultimate drainage area and fracture size. This work suggests that trying to achieve maximum possible propped length is still important in tight and unconventional reservoirs to maximize long-term recovery and connected gas in place, even when well testing shows a short effective fracture length. Including the effect of effective fracture height, or using more realistic assumptions about proppant pack cleanup, would help to avoid underestimating the actual effective fracture length. Welltests showing short fracture lengths have led some people to reduce treatment volume and increase proppant concentration to achieve higher fracture conductivities. They hope to increase the effective length even while reducing the created propped length. Using more realistic proppant pack cleanup scenarios, we show that this approach may actually be counterproductive in tight gas and unconventional reservoirs, because it reduces ultimate recovery due to a reduced fracture extent.