Each natural gas and oil zone is unique and requires a hydraulic fracturing design tailored to the particular conditions of the formation. While the process remains essentially the same, the sequence may alter depending upon differing local conditions. It is key to note not all additives are used in every hydraulically fractured well.
The exact mixture and proportions of additives vary based on the site-specific depth, thickness and other characteristics of the target formation. The sequence noted below from the Marcellus Shale in Pennsylvania is just one example.
Stages are as follows
Acid stage: consists of several thousand gallons of water mixed with a dilute acid such as hydrochloric or muriatic acid. This clears cement debris in the wellbore, providing an open conduit for other frac fluids by dissolving carbonate minerals and opening fractures near the wellbore.
Pad stage: consists of approximately 100,000 gallons of slickwater without proppant material. The slickwater pad stage fills the wellbore with the slickwater solution (described below), opens the formation and helps to facilitate the flow and placement of proppant material.
Prop sequence stage: may consist of several substages of water combined with proppant material (consisting of a fine mesh sand or ceramic material, intended to keep open, or “prop” the fractures created and/or enhanced during the fracturing operation after the pressure is reduced). This stage may collectively use several hundred thousand gallons of water. Proppant material may vary from a finer particle size to a coarser particle size throughout this sequence.
Flushing stage: consists of a volume of fresh water sufficient to flush the excess proppant from the wellbore.