The first step in setting up a hydraulic fracture job is knowing the expected treatment rates and pressures. One of the most influential factors in determining these parameters is the bottom-hole treating pressure (BHTP). (Read Bottom Hole Treatment Pressure: What It Means to Your Fracking Project.)
During the early stages of hydraulic fracturing, small explosive charges from a perforating gun are set off in the horizontal bore, creating small, shallow holes in the underground shale formation. (Read The Hydraulic Fracturing Technique: Things You Should Know.)
High-pressure fluid is then pumped into the perforations, causing the shale layers to crack horizontally and vertically. In addition to expanding fractures into the shale, the fluid pressure must be able to keep the fractures open to allow oil and natural gas to flow into the wellbore for extraction. (Read The Essentials to Drilling and Boring Wells.)
The BHTP is defined as the amount of pressure required to fracture the rock during hydraulic fracture operations. It is essentially the pressure along the face of the fracture responsible for keeping the fracture open. At a minimum, the BHTP must be more than the fracture’s closing pressure, which is the minimum pressure needed for the fracture to remain open.
The BHTP needs to be continuously monitored as it is a crucial parameter used to determine the surface-treating pressure (STP). Correct estimation and maintenance of the BHTP is, therefore, a significant factor in the success of a hydraulic fracking project.
By knowing the STP, engineers can determine the real-time pressure required by the surface pressure transducer on the mainline. (Read Carrying Out In-Situ Stress Measurements: Hydraulic Fracturing Vs. Overcoring Methods.)
Additionally, once the STP is estimated, operators can ensure that the amount of hydraulic horsepower (HHP) available on-site during the hydraulic fracture job is adequate.
BHTP can be calculated by:
BHTP = FG x TVD
Where:
FG = Fracture gradient (the pressure gradient at which the formation breaks).
TVD = True vertical depth.
With the BHTP known, the STP can then be determined using the following formula:
STP = BHTP + Pf + Pnet – Ph
Where:
Pf = Total friction pressure.
Pnet = Net pressure (excess pressure required to extend the fractures).
Ph = Hydrostatic pressure.