Water-Fracs: We Do Need Proppant After All

Abstract

Abstract The key objective of hydraulic fracturing in tight formation gas reservoirs is the creation of "effective" fracture length. The creation of effective fracture length requires that sufficient fracture conductivity be developed to allow effective fracture fluid cleanup. It is also fairly well understood that occasionally conventional cross-linked gel fracture stimulations do not create the desired fracture dimensions. The potential reasons for the shorter than desired effective fracture lengths are numerous with the most likely being excessive fracture height growth and poor fracture fluid cleanup. In the context of the Cotton Valley Formation bounding beds necessary to contain a large hydraulic fracture are non-existent except for the Taylor sand. Studies have been conducted of fracture fluid clean-up which indicate that fluid clean-up or more importantly the lack of fluid clean-up is a primary cause of ineffective or less than desired fracture length. This ineffective clean-up is believed to result from (1) the effects of time and temperature on proppant1, (2) gel residue and its damage to the proppant pack2, (3) viscous fingering through the proppant pack3, (4) the effects of unbroken gel on proppant pack permeability4, (5) non-Darcy and multi-phase fluid flow effects5-7, and (6) capillary pressure8. More recent studies9-15 have shown that for effective cleanup of fracturing fluid and length, a Dimensionless Fracture Capacity, FCD, in excess of 10 is required to overcome yield power-law effects. Dimensionless conductivities of this magnitude are not being generated with many cross-linked gel fracs. Elimination of polymer by fracture stimulating with treated water is cheaper and may provide more effective fractures. However, the use of treated water, results in poorer proppant transport due to the low fluid viscosity. Though more of the created fracture would be effective (no polymer damage) less fracture will likely be created (poor transport). Performance comparisons of Cotton Valley wells fracture stimulated with water and cross-linked gel indicate that water fracs in addition to being cheaper also perform similarly or nearly so to cross-linked gel fracs (and in some cases better). This paper details the application of treated water fracs to the East Texas Cotton Valley Formation and documents an evaluation of well performance and the cause and effects of hydraulic fracturing with treated water on productivity. Through developing an understanding of this well performance behavior, guidelines and/or success criteria are developed for the design and execution of successful water fracs in the Cotton Valley Formation or any tight formation gas reservoir. These guidelines consider all aspects of the fracturing process including reservoir, geomechanical, and design considerations for successful application of treated water as a fracturing fluid. These guidelines, in conjunction with an in depth review of the Cotton Valley Formation, were utilized to develop a modified "hybrid" water frac treatment that mitigates the associated risks with the use of treated water while maintaining the water frac treatment cost and clean-up advantages.