ABSTRACT: The goal of hydraulic fracture stimulations in unconventional reservoirs (e.g., shales) is creating an extensive, high-density fracture network. In December, 2010 the ~3050-m horizontal section of a ~3200-m deep well in a North America shale was stimulated using two strategies over a 30-stage stimulation. Stages 1-22 used ball-actuated, sliding-sleeve technology to open pre-existing wellbore casing slots, giving the stimulation fluid access to the surrounding rock; Stages 23-30 used pump-down-perforation-guns-with-plugs technology to deploy the plugs between stage sections and shoot casing perforations, again giving access to the surrounding rock. The whole stimulation was microseismically monitored using one, vertical, 45-level, 2,164-ft long, 3-component geophone array with its deepest geophone package deployed at the approximate depth of the stimulated well. The microseismic data showed (a) the sliding sleeve stages generated sparse, narrow, somewhat-isolated microseismic event clouds, indicating a less-developed, but deeper-penetrating fracture network and (b) the plug and perforation stages showed dense, highly-overlapping microseismic event clouds, indicating a more-extensive, less-penetrating fracture network. In support of these findings, a data set from an unconventional reservoir comparing oil production using the two stimulation strategies from different wells, showed plug-and-perforation stimulated wells substantially out produced sliding sleeve wells.
1. PERSPECTIVE Stimulating oil and gas wells dates back to the 1860’s and included nitroglycerin, used both legally and illegally, to fracture shallow, hard formations in the northeastern US [1]. By 1949, the year of the first commercial “hydraulic fracture stimulation,” the fluid had evolved to high-pressure water, sometimes with thickening agents, carrying sand (“proppant”). The proppant was so named because it propped the fractures open once the injected fluid had dispersed or flowed back to the surface. The goal of stimulation was and continues to be: break rock in a target zone to increase “pay fluid” flow up the well.