The fluid pressure exerted downhole during a bore can be the driller's best friend when it comes to borepath stability and cuttings transport or can become an expensive and time-consuming problem. Rarely do we plan or quote on bores with the thought that we will be spending extra time on a bore, or spending money on repairs for damaged roads, footpaths, and driveways. In the directional drilling industry, frac-out (creating inadvertent returns) is a common problem.

In most cases, this situation arises from a combination of poor fluid selection for the geology encountered and poor drilling practices. One or both of these factors can lead to increased downhole pressure. If the pressure exerted by the overburden above the bore is less than the pressure exerted inside the bore, the fluid will seek to balance this pressure imbalance by taking the path of least resistance. This path is almost always going to be the ground surface.

The purpose of this paper is to demystify downhole pressure and help minimise the potential for frac-out situations. Simply by understanding some fundamental hydraulics and answering some simple questions prior to beginning the pilot shot, we can avoid unnecessary extra rig and personnel time in the field. Requiring less rig and personnel time per shot means greater profitability. Encountering fewer problems helps earn client respect, which leads to more work.

It's no secret that success in directional drilling is closely linked to making sure that reamer and fluid selection fits the ground conditions. However what is sometimes overlooked is the fact that we should also link good drilling practices with the reamer and fluid selection, or downhole problems such as excessive pressure build-up and frac-outs can occur. Understanding hole volumes, hydrolock, and downhole pressure can help minimise the risks and unnecessary costs associated with frac-outs.

Hole Volumes

A legitimate concern often voiced in the trenchless industry is the need to minimise the amount of fluid used per metre of hole drilled, thereby saving the operator money on slurry disposal costs. This makes sense when the drilling fluid is not being recycled (no shale shaker or hydrocyclones) and the spent slurry is simply being vacuumed off for disposal. Unfortunately this effort to cut costs can be taken to the extreme, where there is too little fluid used per metre to generate a flowable slurry and operators waste valuable time on jobs making multiple reaming passes or pigging out cuttings. More often than not the extra time taken on a job (capital equipment costs, rental equipment, labour and wages) can easily exceed the fluid and waste disposal costs. If other costs are also factored in, such as repairing ground heaving, frac-outs, road, footpath, driveway, and/or yard damage, the extra fluid and slurry disposal costs seem inexpensive by comparison.

These downhole problems are not all due to operators simply trying to save on disposal costs. Part of the problem can be attributed to a poor understanding of hole volumes. The average bore diameter has increased steadily over the past few years and large diameter multiple conduit pull-backs are becoming the industry norm. Unlike smaller bores that can be forgiving to variable drilling practices, larger bores require adherence to basic drilling principles and practices to get on and off the job in a timely manner and avoid costly problems. Hole volumes are used not only to estimate the amount of soil that needs to be removed from the bore path, but to estimate the fluid volumes required to create a flowable slurry.

There is a simple formula used to calculate the soil volumes in the borepath:

(Bit or reamer size in mm)2 ÷ 1273 = litres per metre (L/m)

For example:

100mm pilot
100mm2 ÷ 1273 = litres soil per metre
10,0002 ÷ 1273 = 7.9 (8) litres of soil per metre

150mm reamer
150mm2 ÷ 1273 = litres soil per metre
22,5002 ÷ 1273 = 18 litres of soil per metre of bored hole

250mm reamer
250mm2 ÷ 1273 = litres soil per metre
62,500 2 ÷ 1273 = 49 litre of soil per metre of reamed hole

Table 1 illustrates the relationship between the size of the hole and the volume of material (soil/rock) that needs to be removed to make way for the product pipe(s).

Table 1. Relationship between Bore Diameter and Soil Volume.

  Soil Volume to be Removed (L)
Bore Diameter (mm) per metre per 3 metre rod per 4.5 metre rod
100 8 24 35
200 32 94 141
300 71 212 318
400 126 377 566
500 196 589 884
600 283 848 1273
1000 786 2357 3535

There is a common misconception that when we double the size of the bore we double the amount of soil that is removed. Table 1 illustrates that when we double the size of the bore we actually quadruple the volume of soil that needs to be removed. For example, a 100mm bore has a soil volume of 8 L/m while a 200mm bore has a soil volume of 32 L/m. When the reamer size is doubled, the amount of soil to remove becomes four times greater. Since the pump output or the rate at which fluid is introduced to the bore is often fixed, the bigger the bore, the more pronounced this effect.

For example, for a 500mm bore there is almost 200L of soil that needs to be removed per metre of bore. While most operators would choose to step cut a 500mm hole using several different sized reamers, what these numbers illustrate is that there is a lot more soil to remove in a larger hole than is often considered. If we fail to consider these soil volumes relative to the output of our pumps, we can understand why, in some bores, cuttings tend to chop up easily but then reconstitute behind the reamer rather than forming flowable slurry. In this situation, the operator who is probably using the rotational torque and pullback pressure as a guide could be completely unaware that the downhole pressures may be reaching unmanageable levels. So how much fluid do we need to cut these large bores?

It is not possible to make a universal statement about how much fluid should be used. The final fluid volume required will depend on the soil type. Sand, for example, is inert. It doesn't swell and it doesn't get sticky. A slurried flow can be created in sand by using little more than one litre of fluid for each litre of soil. The caveat here is that longer bores (>70m) do require a higher percentage of fluid in order to maintain the flow ability of the slurry. As the clay content increases and sand content decreases so does the ratio of fluid required. Clay is more reactive and tends to swell and become sticky. In clay, three to five litres of fluid may be required for each litre of clay to be removed.

The fluid volumes needed to make soil/fluid slurry is that soil conditions are variable. Very often no two bores will be alike even when drilled in the same street. We can generate some guidelines related to the basic ground types, but ultimately soil conditions on the individual bore will dictate the necessary fluid volumes.

What happens if we use too little fluid and don't produce a flowable slurry? With a 200mm reamer, there are 32 litres of soil per metre to remove. If we're pulling 140mm product line, we're pulling 15.5 litres of product line per metre. In sand, at least one litre of fluid per litre of soil is required. If we pump only 12.5 litres of fluid per metre instead of the necessary minimum of 32 litres, we create unnecessary downhole pressure and increase the risk of placing an undesirable speed bump or frac-out across our highway. Failure to produce a flowable slurry means that 60 litres of material (32 L soil, 15.5 L product line and 12.5 L fluid) must be forced into a space that originally accommodated only 32 litres.

Enough fluid should be pumped. Enough annular space should be created to allow for flow when backreaming with and without the product line(s). Unless the fluid volume pumped matches the size of the bore, it will be difficult to create a flowable slurry. Without a flowable slurry, flow is likely to be lost and the pressure in the bore will increase. This increase in downhole pressure can manifest itself in hydrolock of the product line(s) and ultimately lifts the street or footpath, resulting in a frac-out.


Hydrolock occurs when you lose flow and create a hydraulic cylinder in front of the reamer and/or compactor and/or product line that can exert more pressure than your rig has thrust. For example, if we put 500 psi (3,447 kPa) of fluid pressure against a 250-mm hydraulic cylinder, that creates 34,250 pounds (152 kN) of thrust. The same situation can be created in the borehole where a 250-mm reamer has 500 psi of pump pressure on it and there is no flow. If the machine can only generate 25,000 pound (111kN) of pull-back thrust, then the product pipe(s) is not likely to go far.

Hydrolock is not limited to those times when the product line(s) are being pulled. This same effect can be brought about during any backreaming pass, where pullback pressure begins to rise at the same time the flow begins to wane. We are often asked "How do we know ahead of time if we are going to get locked up and/or frac-out?" The simple answer comes back to flow. If a change in flow is observed in either the exit or entry pits, then either the ground conditions have changed and/or the operator has changed the reaming speed.

In line and grade work, the best possible scenario is when the slurry is flowing such that an air-gap is visible between the slurry surface and the top of the bore path. This gap makes up part of the annular space (the space between product and formation or the drillpipe and the formation). This annular space acts as a pressure relief pathway and helps maintain near atmospheric pressure from the point of discharge back along the bore toward the cutting head. If near atmospheric pressure can be maintained in all or part of the bore then it is not possible to form a hydrolock situation or frac-out.

In non-line and non-grade bores where the invert of the bore is well below either the entry or exit point, it is not possible to maintain anything other than a borepath full of slurry. In this situation it becomes critical to maintain a constant even flow through the annular space. This annular flow serves as a pressure relief pathway helping to constantly move slurry toward the exit point. When flow abates or stops altogether, the fluid weight combined with cuttings loading in the borepath can quickly build up downhole pressures which can exceed the overburden pressure and result in frac-outs.

Fluid Weight - Downhole Pressure

The fluid or mud weight is the weight of the drilling fluid or resulting slurry per a given volume. Typically we refer to mud weight in terms of specific gravity (SG) in units of g/cm3 or pounds per gallon (lb/gal). Every cubic centimetre (cm3) of water weighs 1 gram (SG of water is 1.0).

If we add solids like bentonite or soil cuttings to water, the mud weight is going to increase such that SG of a drilling fluid will always be greater than SG of 1.0.

How does mud weight relate to downhole pressure? If we know the weight of a fluid we can calculate the hydrostatic head in Pascals (KPa) or pounds per square inch (psi). Hydrostatic head is the pressure exerted by a fluid and any given depth. Hydrostatic head can be calculated from the following equation:

Hydrostatic Head (KPa) = Density (kg/m3) x depth (m) x 0.00981 (m/s2)
= SG x 1000 kg/m3 x depth (m) x 0.00981 (m/s2)
For example: Calculate the hydrostatic head at 3m and 10m in a fresh water pool. SG of water = 1.0
(At 3m) Hydrostatic Head (KPa) = 1.0 x1000 kg/m3 x 3 m x 0.00981 m/s2 = 29.4 KPa
(At 10m) Hydrostatic Head (KPa) = 1.0 x1000 kg/m3 x 10 m x 0.00981 m/s2 = 98.1 KPa

If we now repeat those calculations for a drilling fluid slurry (fluid and soil) with a mud weight (SG) of 1.2 and look at the hydrostatic head around the drill string at 3m and around the reamer at 10m.

(At 3m) Hydrostatic Head (KPa) = 1.2 x1000 kg/m3 x 3 m x 0.00981 m/s2 = 35.3 KPa
(At 10m) Hydrostatic Head (KPa) = 1.2 x1000 kg/m3 x 10 m x 0.00981 m/s2 = 117.7 KPa

At 3m depth and a mud weight of 1.2 we are looking at 35.3 KPa of pressure exerted on the formation. The hydrostatic head is the pressure exerted when the fluid is static, meaning that the pump is not running and no fluid or slurry is circulating. The circulating density or circulating pressure is going to be considerably higher than the static density. The circulating density is the hydrostatic head added to the annular pressure loss inside the bore. This is dependant on the size of the annulus, solids content in the slurry and pump output. The thicker the slurry the higher annular pressure loss will be, resulting in a higher circulating density. In our example above, a static mud weight of 1.2 results in a hydrostatic head of 35.3KPa. However, with the annular pressure loss added the circulating density could have an SG of 2.0 or more. Applying a circulating density of 2.0 and recalculating yields a circulating pressure of 59 KPa at 3m.

Having a circulating density of 59 KPa is fine if we assume the 3m of earth above this point can resist this pressure. Just as water and drilling fluids have density, so do the soil and rock above and below the bore path. If we are drilling in typical clay with an SG of 1.7, we calculate an effective stress of 49 KPa: the pressure the 3m of earth is exerting on the borepath. Now there is cause for alarm, because in the 3m example above, we calculated a static pressure of 35.3 KPa and a circulating pressure of 59 KPa. Unfortunately the 3m of clay can only exert 49 KPa of pressure. Since the circulating pressure of the slurry is greater than pressure exerted by the clay at 3m, the slurry is likely to migrate vertically and frac-out because there is less resistance from above than from back along the bore path.

What this example illustrates is that slurry weight is only part of the story. If the slurry is too thick and will not flow, the circulating pressure can quickly build to the point where it will exceed the pressure exerted by the ground above, resulting in ground heave and/or frac-out. To further demonstrate the point, perform the same set of calculations using medium sand with an SG of 1.9. Then try the calculations at 10m for both clay and sand.

While our hydrostatic pressure is dependant on the initial mud weight of the fluid (i.e., fluid additives plus drilled solids), the circulating density is really the telling factor and will bly depend on the drilling practices employed. In particular these drilling practices include relating borehole volumes to the ground conditions (sand, silt, clay, cobbles) to come up with an appropriate fluid-to-soil ratio to create a flowable slurry. Good drilling practices also include being able to estimate the appropriate pullback speed when reaming and/or pulling product line(s). The better able we are to generate flowable slurry in the bore, the lower the circulating pressure is going to be and the less likely we are to heave the formation or fracout.


The three main contributing factors to leading to downhole pressure problems (frac-outs) are:

  1. hole volumes (underestimating how much soil needs to be removed from the bore path);
  2. hydrolock or losing flow; and
  3. balancing mud weight and circulating pressures.

Once the hole volume issue is understood and ground type taken into account, it is relatively easy to mix the appropriate amount of drilling fluid (based on pump output and pullback speed) to create a flowable slurry. Enough annular space should be created to allow for flow when backreaming with and without the product line(s). Failing to consider hole volumes can result in insufficient fluid pumped, leading to loss of flow and increased borehole pressure. Once the borehole pressure exceeds the overburden pressure, it is easier for the slurry to migrate vertically than it is to flow 50 plus metres laterally along the intended bore path.

The bigger these HDD projects become, the bigger the risk and the hopefully the bigger the reward. By following the principles presented here, you can help decrease the risks and increase the reward. For specific questions regarding this paper or help with bore path planning please contact Robert Petrie at  (+1 801-803-8341).


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