Abstract
The productivity and economics of horizontal wells are governed by the ability of the transverse fractures to communicate efficiently with the wellbore, which is
strongly controlled by the conductivity of the proppant bed and the effectiveness of the fluid additives. These impact the relative permeability, the capillary pressure
and the effective conductivity in the proppant bed. When time at temperature, stress cycling, embedment, multiphase flow and non-Darcy effects are considered, the effective conductivity can be reduced 100-fold. Another investigated parameter is the impact of the wellbore location relative to the propagated fracture. If the
wellbore is high in the fracture, gravity segregation will cause liquid removal from the lower portion of the fracture to be very difficult. In low conductivity proppant
beds, capillary pressure will tend to retain high water saturations, thus lower conductivity even for the portions of the fracture above the wellbore.
Laboratory experiments have addressed these issues for proppants with a range of permeabilities from 10 to 100 Darcies; 100 mesh to 20/40 mesh and ceramics. The relative permeability to gas can be as low as 0.01, with as much as a 70-fold improvement when suitable proppants and additives are employed. Evaluation of the production performance of 240 wells, 98 with effective additives and 142 without, and covering a similar range of proppant types and sizes, shows a similar benefit to the wells' normalized 30 day recovery and gross value. These results clearly demonstrate that economic expediency can be detrimental to a well's ultimate value and hydrocarbon recovery.
Introduction
The propped fracture conductivity in a shale frac is one of the most important factors in determining post frac well productivity. Previous authors have placed a premium on Stimulated Reservoir Volume1 and creating fracture network conductivity by pumping large volumes of water with low concentrations of 100 mesh. A larger proppant is then used as a tail in to offer conductivity in the near wellbore area2. The practice of pumping 100 mesh is often carried to extremes where several wells have nothing but 100 mesh and several have more 100 mesh than 30/50 or 20/40 tail-ins. These wells have very little conductivity and decline in productivity rapidly with very short effective frac lengths. We have all heard that we need very little conductivity in low permeability shales.
There is however a great deal of evidence that higher conductivity does in fact lead to higher productivity in low permeability reservoirs. It has been reported by
Crafton that higher conductivity results in more favorable liquid distributions and greater productivity 3, 4. Vincent has also reported increased productivity with higher conductivity proppants5, 6.
