Bingham plastic fluids more effectively clean horizontal holes

Jack Estes, Bill Randall
Environmental Drilling Technology Inc.
Tulsa

Ken Bridges
Environmental Drilling Technology Inc.
Lafayette, La.

Drilling horizontal wells with water can flood the reservoir and decrease oil production. The bentonite can "mud up" the reservoir, which also reduces production potential. Alternatively, the use of oil-based muds, particularly offshore, can give an operator significant environmental liability. The use of oil-based mud in these directional wells facilitates "sliding," and generally results in less torque, drag, and stuck pipe than when water-based muds are used.

Conventional water-based muds do not have the proper rheological properties to drill a directional hole, although such muds may drill vertical holes economically and relatively trouble free. Operators sometimes take the environmental risks of using oil-based muds in favor of increased drilling efficiency.

However, a properly engineered water-based mud can actually allow the drilling of a directional or horizontal hole better than an oil mud, without incurring the associated environmental liability.

Laminar and turbulent flow are the two most commonly observed flow velocity profiles for the fluids used in drilling. Flow in the drill pipe is almost always turbulent; laminar or plug flow best describes the annular fluid flow profile. Both water and oil behave as Newtonian fluids, and the transition between laminar and turbulent flow can be determined by the Reynolds number, which is a function of the fluid flow rate, density, Newtonian viscosity, and the hole diameter.

Low-solids, oil-based muds tend to be Newtonian, and as such do not exhibit shear thinning as much as low-solids, water-based muds, which tend to be pseudoplastic. This makes the comparison of hydraulic efficiency, cuttings transport, and the transition between turbulence and laminar flow in oil-based and water-based muds a bit complex.

Some drilling hydraulics computer programs calculate the efficiency and transition, but most do not calculate cuttings transport. For cleaning cuttings out of an annulus, turbulent flow generally gives more efficient transport. Unfortunately, turbulent flow also increases fluid loss into permeable formations and erodes shales, causing hole enlargement. Where there is hole enlargement there will usually be lower fluid velocities and, as a consequence, the fluid may revert to laminar flow.

If the mud viscosity is not high enough, the laminar flow will result in poor cuttings transport or pockets of cuttings left behind. It is important to note, however, that plastic viscosity is not a good measure of a drilling fluid's ability to transport cuttings.

The mud's yield point is a much better indicator of hole cleaning capacity. For hole sizes from 81/2 in. to 14 in. where the mud is in laminar flow, a general rule of thumb is that the yield point should be maintained between 10 and 20 lb/100 sq ft, as measured by a rotational viscometer designed for drilling fluids. This yield point is only a calculation and does not represent an actual viscosity value for a typical power law-type drilling mud.

This rule of thumb is only a guide for vertical holes and only if cuttings are less than 0.25 in. diameter.

Laminar flow

A laminar flow velocity profile can be depicted by velocity vectors (Fig. 1 [107367 bytes]). The fluid that wets the boundary of the hole has zero velocity. As the distance from the side of the borehole increases, the velocity of each "layer" of the mud flow increases until reaching a maximum in the center of the annulus. The velocity decreases as the flow approaches another boundary, the drill pipe.

Fast-rotating drill pipe also imparts a rotational helix velocity spiral on the flow stream. This greatly increases the path length of cuttings transport, and the centrifugal force of the rotation causes many of the cuttings to stay in the slower flow streams nearer the side wall of the borehole.

The result is slower average cuttings transport velocity than would be predicted by an ideal laminar flow profile. Because the velocities are changing across the cross-section of the borehole, the mud is being shear-thinned by its own flow, so its viscosity is less than that of static mud.

A cutting will fall under the influence of gravity, but encounters with the upward mud flow slow the rate of fall. If the flow is fast enough, the cutting is transported upward. The relative motion between the cutting and the flow stream is called slip velocity.

Most cuttings would probably stay in the hole if they remained close to the side wall where the mud velocity is lowest. However, at slower drill pipe rotation, the Bernoulli effect causes the cuttings near the side wall to be sucked into the faster moving fluid in the center of the annulus.

In vertical or near vertical holes, the cuttings have the entire length of the borehole to fall; thus, there is little accumulation of cuttings at the side wall. Large shale sloughings, however, may fall at a rate nearly equal to the upward velocity of the mud and may not be transported out of the borehole until they are ground smaller. Increasing the fluid's yield point may cause the accumulated cuttings to be transported out all at once, making the hole unload. This is generally good for cleaning the hole.

Gel strength

Drilling mud gel strengths are an indication of a mud's at-rest resistance to movement. Gels are measured at 10 sec, 10 min, and occasionally at 30 min after stirring. If the gel properties of the mud are inadequate, the larger particles will settle to the bottom of the borehole during pipe trips.

Because of the necessity to wash to bottom during the trip into the borehole, trip time increases. Cuttings accumulation in the annulus also increases drag, which decreases drilling rates.

A good mud for suspending cuttings, thereby avoiding their accumulation at the bottom of a vertical hole, has 10-sec gels between 10 and 20 lb/100 sq ft, with a 10-min gel of no more than triple this value. The 30-min gel should not be much greater than the 10-min gel, otherwise the progressive gel will require high pump pressures to break circulation.

This situation can be avoided or reduced by instructing the driller to start the rotary table with the bit just off bottom, thereby breaking the gels, before he starts the pump.

Poor cleaning

The physics of cuttings transport capacity in a vertical hole change for a horizontal or highly directional hole. A horizontal annulus is depicted in Fig. 2.

In a horizontal hole the drill pipe lays on the side wall of the borehole. Turbulent flow will keep the cuttings in agitated flow and will transport them out. Turbulent flow requires either a low-viscosity thin mud or a high fluid flow rate.

If the mud is thick or if the hole is eroded, the flow profile in the horizontal hole may become laminar. This laminar flow can lead to the following undesirable effects:

• Cuttings, which may be moving along the upper wall, are still falling because of gravity. Instead of having the entire hole length in which to fall as in a vertical well, they have only inches to fall.

• The Bernoulli effect still pulls cuttings into the higher-velocity stream. In a vertical hole, there are two velocity vectors acting in opposite directions: gravity pulling downward and flow velocity pushing upward. If the cuttings are small enough, the viscosity is high enough, and the fluid flow is high enough, then the resultant vector is upward. These two velocity vectors are still present in a horizontal hole, but they act 90° apart. The resultant vector is downward.

• The laminar flow velocity vectors also cause the mud to shear-thin. This lowers the viscosity acting on the cuttings and decreases the cuttings transport capacity.

• The cuttings, which have only inches to fall downward in a horizontal borehole, accumulate around the drill pipe. These cuttings beds greatly increase drag, hindering the drill pipe from sliding. This drag can limit the possible lateral borehole extension by hundreds of feet.

• Accumulation of cuttings at the side wall acts as a filter aid, increasing the dynamic fluid loss or whole mud loss into the reservoir. This fluid invasion can damage the formation and reduce the production potential. Stuck pipe or lost circulation may result. This fluid invasion may result in large amounts of water production later.

The culmination of these effects results in lower drilling rates and increases the nondrilling time spent on trips and hole problems. Given the usual mud properties and flow rates, conventional water muds are totally inadequate for use in horizontal or highly directional holes.

Plug flow

These undesirable effects from cuttings accumulation on the low side of horizontal or high angle holes can be alleviated by a change in mud properties by using a properly engineered water-based mud, with the correct flow rates for the hole size. A change is needed in the rheological behavior of the mud.

Typical mud systems approximate power-law fluid behavior where the yield point, a property of Bingham plastic fluids, is not an actual measurement but only a calculation. The definition of a Bingham plastic fluid is that the yield point is the force required to initiate shear or movement.

A Bingham plastic fluid is typically much more shear-thinning than the more conventional muds. However, a true Bingham plastic mud has very high, low-shear-rate viscosities and has higher effective viscosities for improved cuttings transport in the annulus than a typical power-law mud. Its excellent shear-thinning property will give the fluid low viscosities in the drill pipe for a good pumping efficiency and increased hydraulic horsepower to clean the drill bit. It will also shear-thin at the side wall, giving low equivalent circulating density (ECD).

A low ECD becomes a critical factor in high angle or horizontal holes because it takes less pressure to fracture the formation rock in a horizontal hole than in a vertical hole. In a vertical borehole, the increase in annular circulating pressure due to depth is compensated for by an increase in formation overburden pressure with depth.

In horizontal holes, the true vertical depth stays the same as the length of the hole increases. The increasing circulating pressure in the annulus is thus more likely to exceed the formation fracture gradient as the horizontal hole is extended.

A Bingham plastic fluid does not have a velocity profile that is parabolic, as shown in Figs. 1 and 2. Most of the velocity vectors immediately away from the side wall are of equal value, giving a plug flow velocity profile (Fig. 3 [57490 bytes]). The highest shear rate occurs immediately at the side wall, resulting in a low ECD regardless of the mud thickness in the rest of the annulus.

Thus, a Bingham plastic fluid is less likely than power-law or pseudoplastic fluids to fracture the borehole. Bingham plastic plug flow has the following desirable effects on cleaning in a horizontal borehole:

• With plug flow, the highest shear rate is right near the side wall of the borehole. Shear-thinning takes place there instead of across the annulus. This results in a very low effective viscosity at the side wall and consequently a low ECD.

• Because the velocity vectors in plug flow are about equal, the mud will remain thick across most of the cross section of the annulus. The cuttings will tend to stay suspended in this central plug. This reduces their accumulation along the lower side wall, and it is more likely that they will be transported out of the hole.

• Because almost no shear occurs within the plug flow portion of the annulus and this mud is moving above the average annular velocity, there is a very high velocity differential and shear-thinning around the drill pipe. Rotation of the drill pipe adds to this velocity so that local turbulence occurs along the low side of the hole. This agitation of the cuttings causes more of them to be picked up by the plug flow and transported out of the hole.

The change to Bingham plastic mud properties results in less accumulation of cuttings around the drill pipe laying on the lower side wall. Because less annular circulating pressure is required than in turbulent flow, there should be less hole erosion and lower differential pressure between the hole and formation fluids. There should also be lower fluid loss, less chance of differential sticking, less chance of initiating shale sloughing or lost circulation, and lower torque and drag. These should all contribute to a more stable borehole, and the directional tools will work better.

The standard rotational viscometer used in drilling fluid analysis is an ideal instrument for measuring Bingham plastic mud viscosities. Typical characteristics of an unweighted Bingham plastic fluid include the following:

• Low plastic viscosity

• A yield point that is higher than the plastic viscosity

• Nonprogressive gel strengths that are near in value to the yield point.

Low-solids, water-based muds with Bingham plastic rheological characteristics can be used in place of bentonite or oil-based fluids to drill horizontal holes. For brevity, the statements in this article were not referenced, but there is extensive research, numerous technical publications, a large number of field tests, and routine operations reports to substantiate the conclusions.

Bingham plastic muds

Inexpensive, simple, water-based bentonite muds are satisfactory for most routine drilling operations. Oil-based mud solves most borehole problems in directional drilling, but their use may be restricted because of environmental considerations. Both mud systems are relatively easy to maintain.

Bingham plastic fluids, on the other hand, require more careful attention to maintain properties and performance. Additionally, they also "look strange." Bingham plastic muds have a flocculated appearance, and indeed, some flocculated conventional muds exhibit Bingham plastic rheological behavior.

For true Bingham plastic muds, the American Petroleum Institute (API) static fluid loss is higher; the rheology requires that the yield point be higher than the plastic viscosity; and the gel strengths must not be excessive. Using only conventional materials, these requirements are somewhat difficult to maintain.

The authors have studied the rheological properties of more than 100 drilling fluid formulations flowing in real drill pipe, including the cuttings and shale sloughings transport in 4-in., 6-in., 12-in., 16-in., and 24-in. annuluses. Newtonian, power-law, dilatant, Bingham plastic, and viscoelastic fluids were studied. This experience has been used to optimize mud product formulations for better directional or enhanced lateral oil recovery drilling.

As a result, rig hands can more easily make up and maintain these muds because the chemical technology is well understood. They are therefore able to use just two or three products that provide the hole cleaning properties of Bingham plastic fluids.

To achieve the desired rheological properties, however, these fluids have to be properly engineered. It is important that the ratio of materials be maintained precisely. This would be difficult to do using the dozen or so materials that are required out at the rig site. Using custom-made drilling mud products, with the correct ratios already in the bags, the job of make up and maintenance is made much easier.

Over the years of applying optimized mud technology, the cost of custom engineered mud systems has been recovered by increased drilling efficiency, fewer hole problems, and fewer days until production. In offshore and remote areas, transportation and rig inventory are costly, and the freight frequently costs more than the value of conventional mud products going into the hole.

Custom-engineered mud products put more value-added engineering into the hole, with less freight cost. The better the mud, the fewer hole problems are likely, and the more mud cost is justified.

Since 1970, properly engineered and maintained low-solids drilling fluids have consistently reduced drilling costs, even on some of the deepest wells ever drilled.

Table 1 [7603 bytes] shows the justification for buying better mud that can cost $20-60/bbl more than conventional mud. The data assume 1,500 bbl of mud to total depth, with five different rig costs.

Such considerations make it obvious that the most economical fluids are generally low-solids, water-based muds that exhibit true Bingham plastic rheology.

The Authors

Bill Randall is a staff engineering consultant for Environmental Drilling Technology Inc. in Tulsa. He is formerly an Amoco research supervisor (1975-1982) and drilling training director for Gupco in Egypt (1982-1986). Randall holds 20 patents and has authored 55 papers in drilling, rig hydraulics, and well control. He was the principal rheologist on Project Moho, for the development of coring fluids to 50,000 ft.
Randall did original work in polymer extenders and is the principal inventor of Kwik-Seal. Between 1964 and 1982, he conducted directional and rheological research which resulted in the development of state-of-the-art nonNewtonian fluid flow equations and kick control procedures. His directional calculations were adopted as API standards in 1971. Randall has a degree in chemical engineering from Oklahoma State University.