Key Factors in MHF Design

Summary The important factors to consider when designing a massive hydraulic fracturing (MHF) treatment are presented. The factors considered and the basis for presented. The factors considered and the basis for selection are discussed for zone interval; injection rate; fracture geometry; fluid requirements as a function of temperature, time, and proppant transport; and proppant size and type. Analysis of field data indicates that the effect of sand on pipe friction and slurry viscosity is much less than generally used, based on Newtonian theory. The basic design steps are outlined with rule-of-thumb estimates for job volumes and job cost. Finally, an operator's comparison of service company computer programs shows that the fracture length varies only programs shows that the fracture length varies only +/- 6.5 %, which is reasonable engineering accuracy. Introduction Major new gas reserves have been discovered in the Cotton Valley formation in many areas of Arkansas, Louisiana, and east Texas. The Cotton Valley reservoirs consist of Cotton Valley lime in east Texas, blanket sands in north Louisiana, and massive sands in north Louisiana and east Texas.Papers on Cotton Valley sands have been published that cover geology, early history, published that cover geology, early history, fracturing techniques, prefracturing analysis, laboratory core studies, and mechanical rock properties. Collins provides a review of the properties. Collins provides a review of the geology, areal distribution, and producing characteristics.Papers on Cotton Valley limestone have been limited. The topics of published papers on Cotton Valley limestone are geology, MHF case histories, and laboratory studies on cores. Many related papers also have been published on tight gas sands. papers also have been published on tight gas sands. The tight Cotton Valley gas reservoirs must be stimulated by MHF treatments to be commercial. Accurate knowledge on reservoir properties are necessary for effective MHF design. The design of MHF treatments is far more crucial than the design of conventional treatments. With high costs and rising prices, the optimal cost-effective MHF design will continue to have high priority.The approach taken in this paper is to (1) provide a basic understanding of key parameters in MHF design, (2) examine current practice in Cotton Valley design, and (3) discuss areas where improvement of design is probable. Zone Selection Zone selection for completion of Cotton Valley wells is a multifaceted decision-making process that involves such major factors as porosity, water saturation, proximity to water-bearing zones, and barrier competency between zones. Permeability generally is not used in zone selection because of the relatively low values (on the order of 0.1 to 0.001 md) found throughout Cotton Valley sandstones. The low permeability of this formation makes massive hydraulic fracturing necessary in order for these wells to be commercially productive. It further has been realized by Ahmed et al. that permeability damage from hydraulic fracturing fluids is generally greatest in rocks that have higher initial permeabilities. This consequently produces a leveling effect on tight sands that have a variance in permeability. Thus, damage could lead to misleading fracture test results. Although some operators conduct extensive prefracture flow testing of selected zones (both prefracture flow testing of selected zones (both acidized and untreated), primary selection methods are based on the evaluation of openhole logs and offset well experience.Prefracture testing results require careful analysis to be conclusive. JPT P. 1501