Bottom hole treating pressure is specifically related to the process of hydraulic fracturing - or fracking as it is commonly referred to. The process is primarily used to liberate oil and gas from tiny pores where it is trapped in layers of shale rock.

Fracking has become a significant source of oil and gas as new reserves are found and as the technology has developed and improved over time. Two thirds of natural gas produced by the United States comes from fracking.

How Fracking Works

Exploration for new source of oil and gas have uncovered large reserves buried deep beneath the earth’s surface. These reserves were formed by the decay of organic material trapped in layers of mud and clay, which has built up over millions of years. (Read Soil Types and How They Affect Trenchless Construction.)

The process of fracking involves drilling into the shale layers containing oil and gas, pumping liquid at high pressure into the rock to cause fractures, and then allowing the liberated oil and gas to flow up the well for collection and distribution.

Drilling into shale reserves is a very particular application. The first stage requires drilling straight down from the surface to the depth where the gas and oil deposits are trapped. The second stage requires a 90-degree bend in the drilling direction so that the drill string can extend horizontally into the deposit.

Once the well is fully extended, the fracturing process can begin.

Fracking involves pumping large volumes of water with additives and proppants into the rock all along the horizontal section of the well. This process causes fissures in the rock and allows the oil and gas to flow through the cracks to the well.

Proppants are materials like sand, which have the effect of keeping the cracks open after fracking is complete.

The Meaning of Bottom Hole Treating Pressure

The question engineers must answer when designing and operating a fracking project, is what pressure of liquid is sufficient to fracture the rock. This pressure, as measured in the horizontal section of the well or at the bottom hole, is called the bottom hole treating pressure (BHTP).

BHTP can be calculated based on the conditions of the rock, the depth below the surface and the properties of the fracking liquid. The ease with which the rock will fracture is described by a fracking gradient (fg). Once the fg of the rock is identified, it can be multiplied by the depth to calculate the pressure requirement to fracture the rock.

An excess pressure must be added which represents the force required to extend a fracture into the rock once it has started cracking. A formula for BHTP can then be described as follows:

BHTP = fg * depth + excess pressure

The Importance of Design and Planning

Obtaining the BHTP is a vital part of design and planning for a fracking project. Once the BHTP is known, further calculations must be done to obtain the pressure of fluid required at the surface of the well to achieve the BHTP.

This is known as the wellhead treating pressure (WHTP). All of the specifications in terms of fracking pumps and output pressure for a well are based on these figures. In addition the maximum pressure requirements of the well itself are set based on the WHTP and BHTP. (Read The Essentials to Drilling and Boring Wells.)

This shows that calculating the BHTP accurately is a critical factor for the success of a fracking project.

Once a fracking process starts, engineers keep close watch over the actual pressures and flow rates relative to the design. When the deviation becomes too large, the process is stopped to determine the reasons. Applying a higher WHTP and therefore BHTP than the design, may eventually cause the well to fail.

This has serious consequences as it results in an uncontrolled release of fracking liquids into the ground at multiple levels below the surface. It could contaminate water sources and damage the environment.

The Difficulties of BHTP

There are complexities associated with fracking calculations. Because the liquid contains additives and solids like proppants, the fluid properties themselves are not always easy to predict. This is especially true in the case of a turbulent flow.

In addition the rock characteristics may vary from the design basis. All of these factors contribute to a level of uncertainty in fracking performance versus design, which makes it essential to have good control over the fracking operation and to make adjustments based on actual conditions on site.

It is also important to note that measuring bottom hole pressure (BHP) is difficult and expensive. A common approach is therefore to measure the well head pressure (WHP) and calculate BHP based on depth and losses.

All of these methods introduce some uncertainty into the results and must be considered when evaluating the fracking performance.

What We've Learned

Hydraulic fracturing is a well accepted method of extraction for oil and gas reserves trapped in layers of shale. Developments in the technology associated with this method have resulted in large increase in production from fracking.

BHTP is one of the key parameters for designing and implementing fracking projects. It helps engineers specify the fracking pump requirements in terms of volume and pressure to facilitate the creation of a successful well.