Observations From Tight Gas Reservoir Stimulations in the Rocky Mountain Region

Abstract

Abstract Introduction Unconventional tight gas reservoirs are made economical through effective stimulation techniques. Hydraulic fracture mapping combined with an in-depth knowledge of reservoir geology and geomechanics can give a better understanding to the effectiveness of reservoir stimulation. Massive hydraulic fractures from two wells in the Rocky Mountain region were mapped in real time with a 3-D stimulation viewer software package. One well employed techniques standard to the area –while some experimental fracture techniques were tested on the other. A general east-west orientation of planar fracture geometry was found with a maximum fracture event length of 800 feet. The planer fracturing scheme is consistent with low amounts of acoustic anisotropy recorded. Increasing treating and bottom hole pressures with time observed in this study indicate fracture length growth for each stage. 11 and 12 stages were chosen for the two well completion program based on data from an open hole logging and geomechanics. Some experimental fracturing techniques were tested including longer pump times, larger sand volumes, high viscosity fracture fluids, re-fracturing for re-orientation, using temporary, degradable fiber plugs between stages, and plugging of stages with ball sealers. Perforation entry by stage was effective and some stages could possibly be eliminated to save cost. There is not a direct correlation between pump time, sand volume, and stage height. Ball sealers were not an optimum method for closing off a stage for fracturing, but there is evidence that degradable fiber plugs could be a cost saving option between stages. This paper outlines in more detail the observations from hydraulic fracture mapping in this area. Regional Background This area of study includes 3000 vertical feet of fluvial and marine sands located in the Rocky Mountain region of the United States. (Figure 1) The formation contains natural fractures, laterally restricted lenticular sandstones, and tight brittle sands. The main natural gas reservoir interval is from 5000 to 8500 feet. The productive sands have an average porosity of about 7% and permeability is in the micro-Darcy range. The first well was drilled in this region of the Rocky Mountains in 1955. In the late 1990's the discovery of increased production by fracturing lenticular sands has made this a profitable area. Due to the nature of these tight sands – wells can be located in closer proximity without lowering the production rates of neighboring wells. Wells are currently drilled on 10 acre density. Much of the area uses directional drilling from pad wells to reduce surface damage.