Gel Strengths for Horizontal vs. Vertical Drilling
By Heather Otell and Joshuah Hathcox, Halliburton Apr 12, 2013
A proper drilling fluid is designed based on the requirements of the application in which it will be used. A fluid’s properties directly relate to its functions, and properties can be engineered for a variety of subsurface conditions. In order to create a fluid that realizes all the necessary functions, the desired properties must be defined, understood, and routinely evaluated through testing.
A series of measurements are taken to test the properties of a drilling fluid. Crucial information can be obtained about the fluid by performing these measurements consistently while drilling. This information includes physical and chemical characteristics of the fluid such as viscosity, density, filtration control, pH, hardness, cuttings carrying capacity, hole cleaning ability, and hole stabilization potential. Some of these measurements are direct while others involve application of the data and experience. One property that involves both measurements and further evaluation to fully understand and derive maximum benefit from its use is gel strength. Gel strength is a rheological measurement taken with a viscometer after varying lengths of static conditions (generally at 10 seconds, 10 minutes and 30 minutes). These readings are correlated to the suspension capacity of a drilling fluid at rest in lbs per 100 ft2. The gel strength of a fluid is particularly important when assessing fluids used in horizontal operations and the importance of gel strength in the horizontal directional drilling industry differs compared to vertical drilling applications.
First, it should be stated that the gel strength is an important property of a fluid in all drilling industries. Many drilling fluids are thixotropic; as a drilling fluid remains static, or un-agitated, for a period of time, it begins to gel. This “gelling” aids the suspension of cuttings while fluid motion is stopped. A drilling fluid must be capable of effective cuttings transport under dynamic conditions and appropriate but not excessive suspension under static conditions. The same fluid must efficiently transport cuttings during active drilling operations and maintain those same cuttings in suspension when active circulation stops during connections or operational downtime. Otherwise a day’s worth of drilling could result in solids build up down hole and/or cuttings settling once fluid circulation is stopped. Both can cause significant downhole problems. In both static and circulating fluid, proper cuttings suspension promotes effective hole cleaning by improving cuttings removal and rate of penetration (ROP). Enhanced cuttings transport coupled with proper bit hydraulics allows the bit to constantly cut new formation rather than re-grind previously un-removed solids. A great deal is expected of a drilling fluid in addition to cuttings transport. The same fluid must suspend the drilled solids that are in active transport when circulation and rotation ceases.
Although gel strength is a necessary property for efficient drilling, it is ultimately a compromise; it should be controlled since it is directly related to pressure to break gels when fluid circulation is reestablished. Excessive gel strength can also lead to retention at the surface, thereby complicating solids control efforts or resulting in undue pressure losses, fracturing of the formation, and loss of fluid down hole. A gel strength that is too low will not adequately suspend the cuttings resulting in a build-up of the cuttings bed within the bore path causing an increase in potential for stuck drill pipe. This issue can also cause structural damage as the formation matrix can be disturbed while attempting to free stuck pipe. In a horizontal drilling application, the pumps are turned off frequently during connections. Furthermore, when pumping, the flow rate of fluid is not consistent through the entire bore, so as the fluid flows through the bore and picks up drilled cuttings, the gel strength should suspend the cuttings in areas of little to no flow for reliable transport out of the hole. The optimal gel strength for horizontal drilling is one that forms relatively quickly, build to an appropriate level and then remain constant. The resultant gel structures, while adequate for suspension of cuttings, should also be fragile and easily broken by initial rotation of the drill string and slow initiation of the pump to return the fluid to a pump-able and flow-able state. Conversely, the optimal gel strength for a vertical application should be lower than those selected for horizontal drilling operations. This is a result of the high annular velocity, lower active solids content and the distance that cuttings have to fall back to the bottom of the borehole in a vertical environment. Elevated gel strengths in vertical scenarios are undesirable as cuttings deposition at the surface will be negatively impacted thereby increasing the potential for permanent entrainment of drilled solids into the drilling fluid and re-circulation of those solids.
As previously mentioned, a viscometer is used to measure gel strength. For evaluation, the fluid is first subjected to shear for a period of time in order to break any existing gels and get an accurately timed measurement. Then, it is allowed to sit undisturbed for a set amount of time (typically 10 sec, 10 min and /or 30 min). After the desired amount of time, the viscometer is turned on at low shear (3 RMP) and the reading is taken. There is a spike on the viscometer that designates the pressure required to initiate movement of the fluid; this spike or highest point reached by the dial is the measurement that is recorded as gel strength. Some fluids exhibit flat gel strengths which differ little with the amount of time that the fluid is static: a 10 sec and a 10 min reading would be nearly identical. Other fluids will exhibit progressive gel strengths, and after 10 sec, 10 min and even 30 min gel strength will continue to increase either linearly or exponentially. Although some increase is often experienced and can be beneficial, severe exponential increases in gel strength can exert excessive subsurface pressure when flow is reestablished making them highly problematic.
Engineered fluids used in vertical applications typically require lower gel strength than fluids for horizontal applications. In a vertical environment, solids typically have a longer interval to settle before deposition is an issue, so immediate suspension provided by a static fluid is not as critical. Also, cuttings transport and suspension are aided by fluid velocity as fluid flow works directly against gravity. The force provided by fluid velocity works in conjunction with high gel strength to carry cuttings. In addition, cuttings removal is often achieved through gravity settling at the surface, so high gel strengths are not desired. This is not to imply that there can be an absence of gel strength in vertical applications, because static fluid should provide solids suspension to prevent the deposition of a high concentration of solids which results in stuck pipe or a clogged bit.
In contrast, velocity does offer the same advantage in a horizontal bore because it does not directly counteract the force of gravity on the cutting. When drilling horizontally, the fluid’s gel strength is extremely important for retaining the cutting in a state where it can be transported to an entrance or exit pit. The suspension requirement can be determined by assessing the type and size of solids removed. In order to efficiently clean the hole, solids must be removed at a rate equivalent to the rate they are being produced. Efficient hole cleaning helps prevent differential sticking of the drill rod to the wall of the bore and the accumulation of cuttings that could cause pressure build up and frac-outs.
Also, the shallow depth of a horizontal bore means the overburden is less consolidated, so the formation can be more fragile and can easily collapse or fracture. Preventing destabilization can be directly related to solids removal, and as stated before, failure to remove cuttings effieciently builds pressure and pressure will eventually be released. This fractures which in turn cause hole collapse, partial or complete loss of fluid, or a combination of all of these things.
The type of bentonite used in horizontal drilling applications is different than that of vertical drilling applications. A lower yield (less viscous) bentonite is often selected for horizontal drilling due to its enhanced ability to suspend cuttings, develop optimized gel strengths, enhanced borehole stabilization and filtration control, while producing a lower resultant viscosity. Low yield simply means there is less polymeric enhancement of the material than a high yield bentonite. A lower yield bentonite allows for more platelets to be present per unit of fluid with lower viscosity. Thus, there are more edge to face orientations of the bentonite platelets creating more robust gel structures and enhanced filter cake characteristics. In simplest terms, a higher active bentonite concentration resulting in preferential properties, see the pictorial example below.
In order to explain how the gel strength of horizontal drilling fluid is different from that of fluids used for vertical applications, a typical fluid used in vertical applications was compared to a typical fluid used in a horizontal application. The following data was obtained in a lab under ideal mixing conditions, using a high yielding bentonite for vertical drilling fluid formulation and a low yielding bentonite for the horizontal drilling fluid formulation. As many formulations contain polymers for filtration control and clay/shale stabilization, the formulations compared here also contain a PAC polymer and a PHPA polymer. The formulations were made assuming both applications would be taking place in a formation containing sand/small gravel and clay with no large fracture zones in need of a lost circulation material.
Table 1: Gel strength comparison between fluids used for horizontal and vertical disciplines.
Vertical: The bentonite range is common in vertical drilling. Notice the gel strength increases but is substantially lower than the horizontal fluid at all intervals. This fluid is designed such that drilled solids can be settled at the surface, velocity directly counteracts the force of gravity on a cutting, longer intervals exist down hole before solid deposition is problematic, and high viscosity (resistance to flow) can be tolerated by the surface conditions.
Horizontal: This fluid is typical for an application in sand, clay and small gravel. The bentonite tested is typically used in horizontal drilling because it gels quickly and continues to increase, such that solids will remain suspended long enough to be transported to surface by the slower moving fluid and will remain in the fluid over a longer term since areas of little to no flow exist throughout a horizontal bore. The increased suspension can be seen at all intervals, as the gel strength of the vertical fluid.
Horizontal drilling can require fluids with higher gel strengths than fluids used in vertical applications. The ability of a fluid to hold and move solids out of a horizontal borehole is crucial to the completion of any project. Vertical applications have the benefit of fluid velocity to aid in direct counteraction of the force of gravity. By understanding gel strength and associated rheologic properties a fluid can be engineered to cater to the needs of any drilling application. For a horizontal application, this can be a vital tool that can lead to the successful completion of a bore; lack of adequate gel strengths can lead to stuck pipe, loss of circulation, subsidence, or the dismantling of the bore. Drillers should understand the properties of the fluid being used in order to operate as efficiently as possible. By utilizing the gel strength property provided by bentonite and enhancement products, horizontal applications can be made more successful.
This picture shows a low yield bentonite fluid suspending sands and gravel. The piece in the center is about 1.5 centimeters in diameter.
This figure shows the same low yield bentonite fluid as as above, suspending a handful of sand and small gravel.
Heather Otell is a Knowledge Broker and Joshuah Hathcox is field sales representative with Baroid Industrial Drilling Products, based in Houston.