2 Oct, 2017
Reduce Non-productive Time by Understanding Differential Sticking
This post contains material from Stuck Pipe Prevention – Train Wreck Avoidance™ (SPP).
Stuck pipe is often a major contributor to non-productive time (NPT) events while drilling. It is commonly thought of as the inability to remove the pipe from the wellbore, as a result of downhole forces, which causes a suspension of normal operations. Depending on the scenario encountered, it may be possible to move the pipe, but the pipe cannot be removed from the wellbore when planned. This also causes a suspension of normal operations. This article focuses on differential sticking and the activities to best prevent it from causing stuck pipe.
Differential sticking was first described in industry literature in the 1950s. These incidents drew attention because they differed markedly from incidents related to hole cleaning. It is very common for circulation to be restricted or impossible in a packoff event, but with differential sticking it is typical to maintain full circulation. Based on these early observations, it became apparent that five events occur for there to be a significant risk of differential sticking.
1. Permeable Formation in an Open Hole
Permeability is a characteristic of rock that allows liquids and gases to flow through the pore spaces between the rock grains. As illustrated in Figure 1-1, porosity is the fraction of total rock volume that is made up of pore spaces between rock grains.
Figure 1-1 Porosity
Permeability is a measure of the “interconnectedness” of the pore spaces between rock grains, as shown in Figure 1-2. Rocks with relatively coarse grains, such as sandstones, may have both a high percentage of porosity and significant permeability. Coarse grains means potentially larger flow channels for fluids to flow between the grains. However, finer-grained rocks such as shale, while retaining significant porosity, have very little permeability. This is because the pore spaces in shale are so small, due to the small grain size and the mineral structure, that fluids cannot flow freely.
Figure 1-2 Permeability
2. Wellbore is Overbalanced to the Permeable Formation
With an overbalanced condition at the permeable formation, higher wellbore pressure tends to force wellbore fluids to flow into the pore spaces of the permeable formation at a lower pressure. This is not related to fracturing behavior where the rock breaks under tension hoop stress allowing whole mud, both liquid and solid phases, to enter the rock.
3. Filtration and Filter Cake Formation
Filtration occurs when the liquid phase, called filtrate, of mud, flows into the pore spaces and displaces the pore fluids that already exist in the rock. Most of the solids in the mud are deposited on the face of the permeable formation as a filter cake, as shown in Figure 1-3. The volume of filtrate is typically very small because filtration will stop as soon as the filter cake becomes impermeable. This requires a range of particle sizes to block the pore throats and some very fine particles, typically clay, to seal off the formation and block further flow into the pore spaces.
Figure 1-3 Filter Cake formation on the face of the permeable formation.
The more effective the mud solids are at creating an impermeable filter cake, the thinner the deposit of solids in the filter cake that is required, and the smaller the volume of filtrate that enters the formation. In addition, while circulating, the mud flowing in the annulus prevents any additional mud solids from building up on the filter cake. This thinner deposit is called a dynamic filter cake and is shown in Figure 1-4. A thin, impermeable filter cake that stays in place on the formation to block the flow of filtrate into the formation is the desired filter cake behavior.
Figure 1-4 Dynamic filter cake is thinner due to flowing mud.
When the mud is static, such as on trips and connections, extra mud solids may adhere to the filter cake. This creates a thicker deposit called a static filter cake (shown in Figure 1-5). Also, as shown in Figure 1-6, if coarse solids such as sand accumulate in the mud, the filtration properties of the mud typically deteriorate. A thicker deposit of solids usually accumulates and the filter cake may never become completely impermeable.
Figure 1-5 Static filter cake is thicker due to static mud.
Figure 1-6 Coarse solids contribute to the thicker filter cake.
4. Pipe in Contact with Filter Cake
The pipe contacts the filter cake across the permeable zone because of inclination or curvature in the hole. This can be either a drill string or a casing string. BHA components such as drill collars are more susceptible to differential sticking because of the contact area. Drill collars typically have a longer segment of larger diameter than other components. Stabilizers in the drill collar string and spiral grooves milled into the outer surface of the drill collars reduce this contact area. The drill pipe and heavyweight drill pipe (HWDP) are supported on the wall of the hole by tool joints, resulting in a smaller contact area and a lower likelihood of differential sticking.
The pipe becomes stationary and in contact with the filter cake when making connections or taking directional surveys. NPT events such as rig repair or well control, when the pipe is not moved for a period of time, can also result in higher differential sticking tendency.
When the pipe is positioned across an impermeable formation, such as the shale shown in Figure 1-7, there is a low likelihood of differential sticking. Because the shale is impermeable, filtrate does not flow into the shale and no filter cake forms, even though the wellbore is overbalanced to the shale. Instead, overbalance generates a force that provides mechanical support directly to the formation face. When no filter cake forms, the drill string comes in contact with the shale, and the hydrostatic pressure very nearly balances around the drill string because the contact area is so small. As a result, the differential sticking force is small and the string usually will not get stuck.
Figure 1-7 No filter cake gives small contact area across shale.
When the pipe is positioned across a permeable formation, such as the sandstone shown in Figure 1-8, there is a higher likelihood of differential sticking. An effective filter cake becomes impermeable with a thin deposit of solids and would stop further filtrate flowing into the sandstone, despite the overbalance. Instead, this impermeable barrier provides the surface against which the hydrostatic pressure from the mud can provide support to the sandstone. If the filter cake is not impermeable, the hydrostatic pressure will cause filtrate to flow into the pore spaces of the rock, and little support to the formation will be provided. As the permeable filter cake allows filtrate to enter the sandstone, a thick deposit of solids will build upon the formation face.
Figure 1-8 High filtration rate across permeable sandstone produces thick filter cake.
When the pipe is adjacent to the sandstone, it may come in contact with the filter cake deposited on the formation face, as shown in Figure 1-9. While the pipe is moving, sufficient drilling fluid passes between the pipe and the filter cake; the force from hydrostatic pressure still very nearly balances around the pipe. This causes the differential sticking force to be small, and the pipe will not usually get stuck. However, because the filter cake is still permeable, the filtrate continues to flow into the sandstone. The mud that passes between the pipe and the filter cake keeps it from collapsing by keeping it supplied with fresh filtrate. When the pipe is in contact with the filter cake, the contact area is much wider because the thick filter cake deforms around the pipe.
Figure 1-9 Pipe in contact with thick filter cake.
5. Stationary Pipe
Finally, when the pipe becomes stationary while in contact with the filter cake (shown in Figure 1-10) fresh drilling fluid is no longer passing between the pipe and the filter cake. However, because the filter cake is permeable, the filtrate continues to drain out of the filter cake that lies between the pipe and the wellbore wall, and move into the sandstone. This causes the stationary pipe to block the flow of fresh mud into this area, so the filter cake begins to collapse as the last of the filtrate passes through it. As this happens, the differential pressure that was causing the filtrate to flow into the sandstone pushes against the pipe, compressing the pipe against the filter cake and holding it in place. The contact area between the pipe and the filter cake increases as the pipe is pushed into the thick filter cake. Now, the differential sticking force is large and the string is more likely to get stuck.
Figure 1-10 Pipe Stationary in contact with thick filter cake.
The size of the sticking force can be quite large, as shown in Figure 1-11. In this example, the wellbore pressure is 3,500 psi, and the pressure in this permeable formation is 2,500 psi, creating a 1,000 psi overbalance. The drill collar contact area with the filter cake is three inches wide and extends over the entire 50 ft. thickness of the sandstone. This gives a 1,800 in2 contact area.
Figure 1-11 Differential sticking force
Assuming a coefficient of friction, μ, between the pipe and the filter cake is 0.5, the differential sticking force is:
The differential pressure in this example is far from extreme. Many depleted reservoir intervals are drilled with an overbalanced pressure of several thousand psi. Similarly, the contact area could be much larger than this example, especially if multiple permeable sand intervals exist in the same open hole section. Finally, this equation assumes that the coefficient of friction is constant, but early research suggests that it is not a constant. It increases over time. As the last of the filtrate drains out of the filter cake, the cake collapses and the friction coefficient between the cake and the pipe increases. This can cause the actual sticking force to be much higher than in this example, and worsen over time. However, even at 900,000 lbs sticking force, it is far more than a typical drill string can handle, and even more, than some rigs can.
The message from this calculation is that once a differential sticking diagnosis is determined, the first attempted pipe movement should be downward, using the weight of the pipe to help overcome the sticking force. It is unlikely that the pipe can be pulled with a high enough force to overcome the sticking force.
In addition, differential sticking can occur with wireline and coiled tubing. The mechanism is the same. While the contact area is much smaller, due to the diameter of these tools in comparison to a typical drill string, the tensile capacity to pull these tools is also correspondingly lower. Again, there is little likelihood that either coiled tubing or wireline can be pulled free from a substantial differential sticking force.
How to Avoid Differential Sticking
Maintain mud properties with low filtration rate to create thin, impermeable filter cake. Design BHA and trajectory to minimize contact area. Plan and equip crews to make safe and fast connections to minimize stationary pipe time.
Maintain mud weight and manage equivalent circulating density (ECD) to minimize excess overbalance. Consider isolating depleted zones with casing strings before raising mud weight.
To learn more about this topic, we recommend attending an upcoming session of Stuck Pipe Prevention – Train Wreck Avoidance™ (SPP) or any of our courses in Well Construction/Drilling.
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