Floyd Harvey
Milpark Drilling Fluids
Houston
The potentially severe borehole problems intrinsic with horizontal drilling can be minimized dramatically with the selection of a nondamaging drilling fluid system possessing optimum hole cleaning and lubrication capabilities.
Drilling fluids for vertical wells are often chosen primarily for their ability to control the formations surrounding the reservoir. By modifying the fluid properties and dynamics, these same fluids, in many cases, can be used to drill horizontally.
This fourth of an eight-part series describes special considerations that must be made for using these fluids in highly deviated holes.
In this regard, it is important to devote particular attention to the cleaning, lubrication, and maximum pay zone protection capabilities of the fluid system.
SELECTING A FLUID
As with vertical wells, deciding which fluid system, or combination of systems, to use in a horizontal application depends upon the geology of the prospect, historical successes and failures, environmental constraints, and the experience of the mud company and operator.
When planning a drilling-fluids program for a horizontal well, it is often advantageous to view the prospect in the same manner as the drilling engineer-from the downhole objective up.
The drilling fluid should be designed to minimize damage to the reservoir, and if the proposed design conflicts with fluid designs uphole, then a change in the casing program may be warranted. The carrying capacity, inhibition, and lubricity of the fluid should suit the formations drilled and the prospective well profile.
Furthermore, such rig capabilities as pump output and solids control equipment, and the effect of the fluid on downhole motors and sensors, must be addressed. Underlying these factors are the common concerns of environmental acceptance and economic feasibility.
FLUID TYPES
As shown in Table 1, the most common systems used in horizontal drilling over the past 2 years have been sorted from industry surveys into five categories.1 2 Each system has distinctive advantages and disadvantages.
While these are currently the most popular systems, choices are not limited to these categories as other innovative fluids and combinations continue to evolve.
Invert-emulsion systems, or oil muds, have been the primary choice in past horizontal drilling programs. Oil muds typically exhibit superior lubricity, inhibition, and formation protection, and also are tolerant of contaminants and solids. These attributes make oil muds a favorite choice for an initial venture into horizontal drilling.
However, from an operator's perspective, oil muds may present economic and environmental limitations in some areas. Fluid and cuttings disposal and pollution control may preclude their use; however, mineral oil-based muds are being used successfully in some sensitive environments.
Oil muds generally provide the driller with less downhole concern. But since formulation and dilution of oil muds are expensive, oil muds should not be used in areas with probable lost circulation zones, such as the Pearsall field of South Texas.
However, low-weight invert systems can be used to drill subnormally pressured formations, as demonstrated by Edlund3 in the successful drilling of the Spraberry formations in West Texas, which used a 7.4 ppg oil mud.
Polymer fluids are the most popular water-based fluid systems used in recent high-angle drilling operations. The polymer-based drilling fluid category is somewhat broad, encompassing both natural and synthetic polymers in fresh and salt water.
Polymer systems can also exhibit good inhibition and lubricity, generally at a lower cost and with less environmental liability than oil muds. While offering some formation protection, polymers are moderately expensive and require good solids-control practices.
In some areas, such as Indonesia and Saskatchewan, a simple bentonite slurry has proven sufficient for drilling horizontal wells in carbonate reservoirs. This system is inexpensive, simple to formulate and maintain, and environmentally safe. Yet, it offers little or no formation protection or lubricity.
Water, or brine, is often used in association with viscous sweeps to drill probable lost-circulation zones, or those formations where inhibition is not required. Since these fluids possess no gel strength, sweeps are usually pumped before making a connection or tripping to prevent cuttings from settling to the bottom of the hole. Due to their low fluid weight, the application of clear fluids is limited to low-pressured formations, or those which are normally drilled underbalanced.
Evolving from workover and completion operations, sized salt systems have been used to drill producing intervals.4 This system is composed of saturated brine and a mixture of sized salt and polymers, which yields a fluid with a high degree of lubricity and formation protection. Downhole cleanup of the sized salt-bridging agents is accomplished by circulating subsaturated brine.
Control of fine solids is difficult with this fluid and the disposal of saturated brine may pose problems in some areas. Also, when compared to the simpler systems, sized salt is moderately expensive.
HOLE CLEANING
While considerable literature has been assembled on cuttings transport in a vertical well, these principals are not directly applicable to the drilling of highly deviated holes.
Basically, hole cleaning in a vertical well is a matter of overcoming the slip velocity of the cuttings by incorporating sufficient annular velocity and carrying capacity into the drilling fluids program. Theoretically, any fluid velocity greater than the settling velocity of the largest cutting should eventually lift all the cuttings to the surface.
If the annular cuttings transport velocity is too low, cuttings will tend to accumulate downhole. This increases the possibilities of lost circulation and stuck pipe.
In deviated holes, Okrajni, et al., has shown this basic approach to hole cleaning to be altered significantly as axial particle slip shifts to radial particle Slip.5 As deviation from vertical increases, radial slip increases, forcing cuttings to the low side of the borehole. From a hole cleaning perspective, three general ranges of hole inclination are considered, as illustrated in Fig. 1.
The first inclination range is from 0 to 45 from vertical. In this range, hole cleaning differs only slightly from vertical holes, which use the traditional laminar flow and carrying capacity principles. Turbulent flow is typically avoided to prevent borehole erosion and instability.
The second and most critical range is from 45 to 55 from vertical, where cuttings transport is more dependent on fluid annular velocity and momentum. As turbulent and laminar flow have similar effects in this range, hole cleaning becomes more difficult as cuttings tend to fall to the low side of the borehole. This may instigate the formation of a cuttings bed.
At inclinations in the second range, cuttings beds can slide downhole against the annular flow, packing off the hole or sticking the drillstring when the pump is stopped. Therefore, in the 45-55 range, the drilling fluids program should be designed to move the cuttings bed effectively with maximized annular velocity.
In the third range, which is recognized as 55-90, the eddying effect of turbulent flow has been shown more effective in cleaning the hole. Turbulent flow and higher annular velocities have a destructive effect on cuttings beds by forming dunes that dynamically erode and reform uphole. Cuttings beds that form in this deviation range do not tend to slide downhole.
Adequate annular velocity is very important for cuttings transport. In a highly deviated hole (inclinations in the second and third range are greater or equal to 45), an annular velocity of 3 fps, or greater, is desirable to minimize the concentration of cuttings downhole.6
Research indicates that at very high annular velocities (greater than 5 fps) little or no cuttings bed will be formed. When a cuttings bed is formed due to low annular velocity (varies but generally is less than 3 fps), cuttings may continue to accumulate in the bed. The accumulation reduces annular volume and increases fluid annular velocity until a velocity high enough for cuttings transport is attained. A state of pseudo-equilibrium may then exist where the rate of cuttings bed erosion equals deposition.
When pump rates are limited by rig capabilities, larger diameter drillstring tubulars may be considered to boost the annular velocity.
Drillstring eccentricity in highly deviated holes contributes to problems if fluid velocity is decreased in the narrow portion of the eccentric annulus. As a cuttings bed forms, the drillstring may actually lie in the cuttings bed, decreasing the removal rate of the bed, increasing torque and drag, and creating the possibility of stuck pipe.
An examination by Martin, et al.7 has shown that smaller cuttings are entrained in the fluid boundary layer of the cuttings bed, effectively limiting their removal. Therefore, cuttings transport can be enhanced by either decreasing the boundary layer, or maintaining larger cuttings.
Hydraulic hole cleaning may be supplemented by disturbing the cuttings bed mechanically, such as employing frequent wiper trips and drillstring rotation. Although not essential, top drives are very beneficial when drilling highly deviated holes, as they allow circulation and drillstring rotation to continue during a trip.
LAMINAR VS. TURBULENT
In the more vertical range, research has shown laminar flow, combined with higher yield values, exhibits the best cuttings transport. Laminar and turbulent flow generally have similar effects in the intermediate, critical deviation range. In the more horizontal range, turbulent flow suppresses cuttings-bed formation and is shown to be more effective for transport.
Regardless of the flow regime and deviation, fluid annular velocity continues to have a profound effect on hole cleaning.
RHEOLOGY
Adjustments in rheology may improve hole cleaning, depending on the dominant flow regime. Higher yield point and plastic viscosity (YP/PV) ratios provide better cuttings transport in laminar flow, while rheology has a minimal effect on transport in turbulent flow. This appears to hold true for all inclination ranges.
Fluid rheology can also be used to control flow regime. Generally, the higher the viscosity of the fluid, the higher the velocity or shear rate required to attain turbulent flow. Therefore, viscosity may be decreased to allow the fluid to become turbulent at a lower velocity, or may be increased to allow laminar flow at a higher velocity.
Sweeps may be used to supplement hole cleaning in laminar flow regimes. Low-viscosity sweeps may reach transitional or turbulent flow to disturb the cuttings from the bed. Sweeps with high YP/PV ratios, on the other hand, increase the carrying capacity in the laminar regime.
While the immediate concern is to clean the highly deviated hole, cuttings transport in the upper hole sections should not be neglected. Lower annular velocities existing in larger hole geometries may require additional circulating time or viscous sweeps to ensure hole cleaning.
TORQUE AND DRAG
A variety of factors can affect torque and drag, including hole cleaning, formation inhibition, filter-cake quality, and hole deviation. In highly deviated holes, the compaction of the cuttings bed and the degree which the drillstring extends into the cuttings bed has a major impact on both torque and drag. Using a drilling fluid with a high degree of lubricity can alleviate this problem to some extent.
Invert emulsion oil muds generally have a very low lubricity coefficient as compared to other systems, whereas oil-in-water emulsions contribute little lubricity.
A polymer-based fluid containing 3 ppg of a high molecular weight partially hydrolyzed polyacrylamide has been shown to possess a significantly lower coefficient of friction than that of a clay-based lignosulfonate fluid. The improved lubricity of a polymer-based fluid is inherent in the behavior of the polymers themselves, as the hydrated polymer chains slide by one another.
A wide variety of commercial lubricants is available in today's market, and performance of these lubricants varies just as widely. Careful testing and evaluation should be conducted before adding lubricants to a system, as some products may have detrimental effects on the fluid properties or formation protection.
BOREHOLE STABILITY
Boreholes fail by collapsing, which is due to either compressive or shear failure, or by fracturing, which is precipitated by tensile failure. Drilling fluids can impact these mechanisms in a number of ways, i.e., chemical reactions with the formation, fluid hydrostatic pressure, and annular hydraulics. Furthermore, borehole stability can be affected by hole inclination and, in some cases, geographical direction.
When drilling formations under compressive stress, the stresses are induced on the surrounding rock. If compressive stress exceeds the compressive strength of the formation, compressive or shear failure can result. Drilling-fluid hydrostatic can help offset these compressive stresses by increasing well bore pressure. It must be cautioned, however, that if well bore pressure is too great, tensile fracturing of the formation may occur.
Well bore pressures above the compressive failure pressure and below tensile failure pressure represent the permissible borehole operating pressure range.8
Fig. 2 illustrates the three principal in situ stresses, s1, s2, and s3, in a normally stressed region (arrows are proportionately sized to indicate magnitude of stress). The veritcal, or overburden, stress is s1. The horizontal stresses, s2 and s3, are equal in a tectonically neutral state. If the vertical and horizontal stresses are not equal, changes in hole inclination alter the stress subjected to the rock surrounding the borehole.
Bradley found inclined wells may collapse at higher pressures and fracture at lower pressures than will vertical wells.9 This more narrow operating range could require additional casing strings in highly deviated wells.
In tectonically stressed areas, the principal stresses are not necessarily oriented in the vertical and horizontal planes and are generally all of different magnitude. Fig. 3 illustrates how borehole stability in these areas may not only vary with inclination, but also with orientation.
Borehole stability appears to be improved in these areas by inclining the borehole in the same direction as the least in situ stress.10 Because axial stress has little effect on borehole stability, orienting the borehole in this direction negates the effect of the least principal stress and subjects the borehole to the lesser differential of the intermediate and maximum principal stresses.
Therefore, in evaluating offset data, it should be remembered that wells of different inclination and direction may not correlate directly with regard to mud weight and hole stability.
When considering well bore pressures, it is important to remember that the annular pressure losses in the circulating system exert pressure on the open hole. This effect is even more pronounced in a horizontal or a highly deviated hole.
Although the annular pressure loss is estimated along the measured depth of the hole, its effect is calculated relative to the true vertical depth. Consequently, at the recommended higher annular velocities, highly deviated holes with long displacements can experience significantly higher equivalent circulating densities than those experienced in vertical holes of the same measured depth.
A similar effect will result in a formation overbalance when using the conventional wait-and-weight kill method on a highly deviated or horizontal well. The conventional kill method reduces back pressure on the pump at a constant rate as kill mud is pumped to the bit.
In a highly deviated well, additional hydrostatic is gained in the drillstring at a greater rate as the kill mud travels from the surface to the kick-off point, as opposed to traveling from the kick-off point to the bit. If back pressure on the pump is reduced at a constant rate, the formation will be increasingly overpressured as the mud approaches the kick-off point.
After the kill mud passes the kick-off point, the overbalance decreases until kill mud reaches the bit and the formation is again balanced. Accordingly, the pumping schedule for a highly deviated hole should be recalculated to allow for a more rapid pressure drop to the kick-off point and a slower pressure drop from the kick-off point to the bit.11
Other fluid properties can also affect borehole stability in a horizontal well. For instance, lower filtration rates may be required in some areas to improve formation integrity and strength. Inhibiting agents, asphaltics, or lubricants may also have applications on particular exploration or development wells.
Additionally, special attention should be given to solids control by the continual monitoring of solids-removal efficiency. Poor solids control adversely affects mud properties and filter-cake quality, which serves to increase dilution and mud treatment costs.
FORMATION PROTECTION
With as much as an order of magnitude more of the formation exposed to the drilling fluid in a horizontal well, careful scrutiny must be given to selecting a fluid system that will not damage the formation of interest. From an economic standpoint, a 50% damage, or permeability loss, in a potential 500 bo/d horizontal well represents a considerably greater loss in production than in a 100 bo/d vertical hole. Therefore, the drilling fluid and contingency products should be examined to prevent any damage to the producing zone.
Sandstone is susceptible to damage or permeability loss during both drilling and completion operations. While it is arguable that some formation damage can be precipitated by poor drilling practices, an inadequate fluid design may lead to severe impairment of the productivity of the reservoir. Production losses attributable to the fluid can be prompted by one or more of the following:
- Particle transport from the mud and into the producing zone, subsequently plugging the pore throats
- Mud filtrate reacting with expandable clays in the rock to decrease pore throat diameters and/or fractures
- Particle movement with the permeable rock due to dispersion of clays and other minerals from quartz surfaces
- Wettability changes of the formation from exposure to the drilling fluid filtrate
- Interaction of the mud filtrate with formation fluids to form water-insoluble precipitants.
With these potential occurrences in mind, a thorough examination of the formation should be made to determine its susceptibility to damage and to identify the sources of likely damage. It is recommended that a core from the formation of interest be examined by petrographic analysis, and mineral composition by X-ray diffraction and scanning electron microscope analysis.
These data should then be coupled with information from return permeability tests of core plugs to choose the least damaging fluid.
A formation damage test involves a sequence of flow analysis in a liquid permeameter as shown in Fig. 4. A core is mounted in a Hassler-type cell, evacuated, and then saturated with simulated formation water.
The initial oil permeability, k (i, oil) is determined by flowing a filtered mineral oil through the core under constant pressure until irreducible water content is obtained. The state is then indicated by a constant flow rate and stabilized differential pressure. The downstream, or borehole, face of the core is afterwards exposed to the test fluid under pressure of a predetermined period.
Oil flow is then initiated in the original direction of flow (formation to borehole) under the same initial pressure until the flow rate and differential pressure stabilize. At this point, the permeability of oil is the return oil permeability, k (r, oil). The ratio of k (r, oil) to k (i, oil) is expressed as the percent of return permeability, %k (r, oil).
Therefore, a return permeability of 90% is considered attractive since a permeability loss after mud-off of 10% or more is commonly experienced even with the least damaging fluids.
Droddy, et al., used the analytical procedures for fluid selection on an argillaceous sandstone in the Wilcox formation.12 Coupling the findings of petrological and mineralogical analyses with return oil permeability tests enabled the investigators to make a satisfactory selection of the drilling fluid for this particular formation.
Generally, with all variables being equal, formations with lower permeability are considered more susceptible to damage.
There is no specific permeability that the problem of pore plugging becomes acute, but rocks with permeabilities below 100 md are widely regarded as the most sensitive to damage.
REFERENCES
- Moritis, G., "Worldwide horizontal drilling surges," OGJ, Feb. 27, 1989, pp. 53-63.
- Moritis, G., "Horizontal drilling scores more successes," OGJ, Feb. 26, 1990, pp. 53-64.
- Edlund, P.A., "Application of Recently Developed Medium Curvature Horizontal Drilling Technology in the Spraberry Trend Area," Paper No. 16170, SPE/IADC Drilling Conference, New Orleans, Mar. 15-18, 1987.
- Wilkirson, J.P., Smith, J.H., Stagg, T.O., and Walters, D.A., "Horizontal Drilling Techniques at Prudhoe Bay, Alaska," Paper No. 15372, SPE Annual Technology Conference and Exhibition, New Orleans, Oct. 5-8, 1986.
- Okrajni, S.S., and Azar, J.J., "The Effects of Mud Rheology on Annular Hole Cleaning in Directional Wells," Paper No. 14178, SPE Annual Technology Conference and Exhibition, Las Vegas, Sept. 22-25, 1985.
- Tomren, P.H., Iyoho, A.W., and Azar, J.J., "Experimental Study of Cuttings Transport in Directional Wells," Paper No. 12123, SPE Annual Technology Conference and Exhibition, San Francisco, Oct. 5-8, 1983.
- Martin, M., and Georges, C., "Transport of Cuttings in Directional Wells," Paper No. 16083, SPE/IADC Drilling Conference, New Orleans, Mar. 15-18, 1987.
- Hsiad, C., "A Study of Horizontal Wellbore Failure," Paper No. 16927, SPE Annual Technology Conference, Dallas, Sept. 27-30, 1987.
- Bradley, W.B., "Mathematical concept stress cloud can predict borehole failure," OGJ, Feb. 19, 1979, pp. 92-98.
- Aadnov, B.S., and Chenevert, M.E., "Stability of Highly Inclined Boreholes," Paper No. 16052, SPE/IADC Drilling Conference, New Orleans, Mar. 15-18, 1987.
- Mueller, F.S., and Grayson, R.E., "Worksheet Helps Control Slanted Hole Kicks," Drilling Contractor, June/July 1990, pp. 68-69.
- Droddy, M.J., Jones, T.A., and Shaw, D.B., "Drill Stem Fluid and Return Permeability Tests of South Texas Wilcox Cores," Paper No. 17159, SPE Formation Damage Control Symposium, Bakersfield, Calif., Feb. 8-9, 1988.
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