INVERT MUDS HELP REDUCE SLOUGHING SHALE PROBLEMS

Don L. Myers
Canadian Hunter Exploration Ltd.
Calgary

Oil-based invert muds helped prevent sloughing shale problems and decreased the time to reach total depth in several wells in Canada.

The benefits of invert muds include increased penetration rates, reduced hole enlargement, more dependable open hole evaluation, and more efficient primary cement jobs. But invert muds have several pitfalls and potential problems.

Canadian Hunter Exploration Ltd. first used an oil-based invert mud in 1980 at Cutbank 11-7-68-12W6 in the Alberta Deep basin to combat serious sloughing problems. The invert mud was then used successfully on several other wells in the area.1 Since then, Canadian Hunter has drilled wells with oil-based muds in a number of fields in Alberta northeast British Columbia, and the western U.S.

In the past few years, a number of technical changes have improved the application of oil-based drilling fluid; specifically, improvements in bit design and solids control efficiency have helped reduce drilling time. Canadian Hunter's experience with invert muds has led to further optimization and performance improvements.

But the application of invert muds should be carefully analyzed for each drilling area because of new regulatory requirements for drilling waste cleanup and recent research on the impact of invert muds on hole instability and formation damage. These oil-based muds can be used in a cost-effective and environmentally acceptable manner to improve drilling operations.

INVERT MUD

Invert mud is an oil-based drilling fluid that generally consists of diesel oil with brine carried as the internal phase in a stable emulsion. Certain fatty acid compounds are used to emulsify the brine. Organophilic clays are often used for additional viscosity and fluid loss control, and barite is the common weighting material. Polymers are often used for additional control of rheology.

The brine exists as very tiny droplets dispersed throughout the oil by surface tension effects. Much like solid particles, these droplets create viscosity and contribute to filter cake buildup. A water content of 5-40% by volume may be used depending on the desired properties and component costs.

Although oil is the external phase, the water can transfer between the brine component of the fluid and the formation.2 3 To prevent hydration of water-sensitive shales, the brine must be of sufficient strength to overbalance the activity of the formation.3 4 Calcium chloride brine is most effective and is generally run with concentrations of 25-35% by weight.

ADVANTAGES

Oil-based invert mud has many potential benefits for specific drilling applications:

Invert mud is most commonly used to prevent costly sloughing shale problems.
The actual hole size (from caliper logs) of wells drilled with invert mud is usually much closer to gauge than that of wells drilled with water-based mud. The in-gauge hole reduces the cuttings volume discharged at the surface, reduces the cement requirements, and improves reliability in obtaining open hole drill stem tests (DST) and logs.
Invert fluid is very stable for a wide range of conditions, and normal contaminants, such as anhydrite, cement, and salts, have little impact.
The density of un-weighted invert fluid can be run as low as 900-920 kg/cu m (7.5-7.7 ppg). In many areas, this density is sufficient to control reservoir pressures and provides less overbalance for faster penetration.
Invert mud's low viscosity and gel strengths, even with weighted systems, helps minimize pump pressure and surge and swab effects.
Invert muds have extremely good lubricity to reduce torque and drag, particularly in high-angle holes.
Invert fluids are very effective in protecting the drillstring from corrosive attack and mechanical wear.
Oil muds prevent hole enlargement in salt sections, a problem common with water-based muds. This enlargement often leads to casing or cementing problems and a salt-contaminated sump.
Downtime from freezing in extremely cold weather is reduced with invert muds.
Invert muds can reduce formation damage in certain reservoir types.

DISADVANTAGES

Invert muds are expensive, with diesel costs alone averaging about $300/cu m. A new unweighted system can cost close to $400/cu m, and total mud costs near $25/m can be expected, even without any drilling problems.

A significant volume of invert mud is lost during each well. Total losses include the following: losses into porous formations or thief zones, volume left in the hole during cementing, and surface losses that cannot be separated and recovered from the drill cuttings and fine solids. Replacing the losses is a major portion of the total mud cost on each well.

In addition to the price of the fluid, extra handling costs are incurred for such things as tanks, transportation, pumps, and crew allowance. Storing recovered invert fluid between wells for extended periods before reuse is another potential cost.

Both the oil and the brine salts in invert mud present potential environmental problems. The new Drilling Waste Disposal Regulations in Alberta require all oil-based muds to be surface spread and landfarmed. The oil content must be reduced to less than 0.5% in the affected surface soil. The initial spread rate of chlorides carried with the drilling waste should not exceed 800 kg/hectare (1 hectare = 10,000 sq m).

Oil-based fluid is more flammable than water-based mud and presents some health hazards. The solubility of hydrocarbon and H2S gases in oil muds can reduce reaction time somewhat during well control operations.5 Appropriate safety measures, however, can mitigate these problems.

Lost circulation is an expensive problem with invert muds. Water flows can upset the invert system by diluting the brine and emulsion strength. The cuttings can then become soft and sticky, and the hole can lose stability. With low density invert mud, thick coal seams can slough badly and can be costly to clean up. In some reservoirs, invert mud can cause serious formation damage. Invert muds are expensive to build high viscosity and yield point to clean large holes.

SHALE PROBLEM

The primary reason for using invert mud in western Canada is to prevent sloughing shale problems in the disturbed belt adjacent to the foothills of Alberta and northeastern British Columbia. In this area, drilling problems include extensive reaming and cleaning, stuck pipe, inability to run logs and DSTs, and lost hole sections.

The combined effect of the following two key factors determine the degree of problem in each particular well: the water-sensitivity of various shales and the magnitude and distribution of the in situ stresses acting on the well bore.

The absorption of water causes some shales to swell and lose compressive strength. Recent work by Santarelli, et al., provides new insights into the phenomenon.6 The specific composition of the shale in terms of clay minerals and the rock's permeability determines which formations are most susceptible.

The other factor controlling the sloughing problem is the ability of the rock around the well bore to withstand the in situ stresses after the hole has been cut. Near the mountains, tectonic forces result in greater horizontal stress in one direction and a significant imbalance in the maximum and minimum horizontal stresses acting normal to the well bore.

If the compressive strength of the rock is too low and the hydrostatic pressure of the mud column is not large enough to overcome the imbalance, well bore breakout occurs in the direction of least principle stress. Breakout can occur in any lithology, not just shales, and is readily seen on dual-axis caliper logs.7 8 For shales, the initial rock strength may be sufficient, but exposure to water can soften the rock, causing failure after a certain time delay.

Horizontal drilling, in which the imbalance between the vertical overburden stress and horizontal stress normal to the well bore can be substantial in any geographic area, has increased the efforts to understand the rock mechanics involved.9-11

In the disturbed belt in western Canada, the severity of shale sloughing can be very difficult to predict. Although the general mountain building thrust forces are pervasive and generally decrease eastward, many local thrust faults and compressional features lead to significant changes in rock stress over fairly short distances. Many operators have experienced the frustration drilling a well only a mile or so from a fairly trouble-free hole only to have much more severe sloughing problems. This lack of predictability makes choosing the optimum mud system difficult prior to each well.

SLOUGHING SHALE

Field experience and laboratory investigation have helped produce several methods to combat shale problems.

In water-based muds, reducing the fluid loss may slow water invasion somewhat and can delay the problem in certain cases. Increasing mud density to counter the stress imbalance can be effective, but increases of 200 kg/cu m or more are often required. High mud weights decrease rate of penetration, further increasing costs and risking sloughing from extended exposure time.

Various brines and cations, most notably KCI systems, have been used to limit the swelling of clays in the shales. However, they do not stop sloughing in all cases and have other drawbacks, including high cost, potential pipe corrosion, and waste disposal problems because of the high salt content.

The improved rheology of certain polymers has helped clean the sloughing material out of the well more effectively in both KCI and freshwater-based muds. The polymers help reduce the extent of bridging, reaming, and other related problems. If all the cavings can be pumped out as the hole sloughs, then it may be possible to finish drilling the well without any major difficulties.

Minimizing the mechanical agitation is also important. Reducing nozzle velocity, annular flow rates, and surge/swab pressures across sensitive shales may help prevent or delay sloughing in many cases.

Partially hydrolyzed polyacrylamide (PHPA) polymer muds encapsulate shale cuttings and the well bore walls to prevent intake of water. These muds are very shear thinning and can be run with low solids content for excellent penetration rates. Recent results indicate this type of mud system may allow drilling and casing wells before sloughing becomes a problem in some areas. However, as with a basic reduction in fluid loss, this type of system should not be expected to ensure protection over longer exposure times.

Oil-based muds have proven the most dependable method of preventing sloughing shales. The water-sensitive shales simply do not come in contact with water, and the associated strength degradation does not occur. Additionally, the chemical potential of the internal brine phase may actually dehydrate the shale near the well bore, and a recent study detected significant increases in shale compressive strength as a result of this dehydration.2 3 This phenomenon may help explain how invert mud can maintain a stable, gauge hole in troublesome shales over long exposure times, even with low mud density.

If the brine strength is run too low, however, the chemical potential difference may be reversed causing the formation to pull water out of the invert mud. This problem can result in strength reduction and sloughing; thus, considerable work has been directed on balanced activity of invert muds.4 Although problems can occur with brine strengths run too high, it is best to keep the concentration high enough to prevent hydration of the most active shale in the well bore with some safety margin.

While invert mud generally provides maximum well bore stability, it is obviously not the most economic choice for all wells. The optimum approach to controlling sloughing shales depends on the anticipated severity and exposure time in each specific location. The amount of coring, testing, and logging in the open hole has a large bearing on the amount of protection required.

OPTIMIZATION

There are several ways to optimize the cost effectiveness of invert mud systems. Mud costs can be minimized by tailoring the formulation and properties for specific needs in the area and by minimizing the volume of fluid lost on each well. By eliminating hole problems, full attention can be focused on maximizing bit performance and reducing overall rig costs.

Many wells in the Deep basin trend have subnormal pressure gradients and can be drilled with unweighted invert mud to total depth (TD). Given the relatively in-gauge hole and absence of cavings, the hole can be cleaned with very low viscosity and a higher pump rate, much like water drilling.

Initial oil/water ratios of 70/30 were soon replaced with 85/15 or 90/10, and more recently 95/5 for some wells. Clay viscosifiers have been reduced to a minimum or eliminated altogether in unweighted systems. The CaC12 brine strength has been reduced to a range of 160,000-180,000 mg/l. Cl-. No additives are used to reduce seepage losses unless a serious loss zone is encountered.

A typical 90/10 formulation of invert mud has the following components: 890 1. diesel oil, 89 1. freshwater, 30 kg CaC12, 10 1. emulsifiers, 3 1. oil wetter, and 8 kg lime.

The following are typical mud properties:

density 920-970 kg/cu m, viscosity 32-35 cp, plastic viscosity (PV) 3-5 cp, yield point (YP) 0.5-1.0 lb/100 sq ft, gels 0.5/0.5 lb/100 sq ft, and chlorides 160,000 mg/l.

The cost is approximately $375/cu m.

Advantages of this system include the following:

Lower cost per cubic meter of new hole
Maximum penetration rate
Better solids separation and reduced surface losses
Reduced salt content.

The following disadvantages should be considered:

Greater sensitivity to freshwater influx
Little hole cleaning capacity at lower annular velocity
Additional viscosity required to weight up
Possibility for greater hole enlargement
High fluid loss and greater invasion into pay zones.

Theoretical work and field experience indicate brine strength is important. The concentration given above is for CaC12, as measured in field check terms of mg/l. Cl-, and is suggested as a minimal range for the geographic areas discussed based on field experience.

A few shale problems have occurred over the years in the Deep basin trend, and all have been associated with low brine strength.

An early indication of low brine strength is gummy rather than crisp shale cuttings. Low water content with minimal brine strength requires very quick reaction to any unplanned water influx.

EQUIPMENT

Sump pits are no longer used to contain invert drill cuttings and waste fluids. The shale shaker and centrifuge discharge into a steel catch tank. Liquid mud is skimmed off the settling compartments and put back in the active mud system through the centrifuge. The oily cuttings and solids are then stacked in a diked area to one side of the lease to await landfarming.

A field contractor has developed a device called a "thumper box," a 1 cu m steel box sitting on a cam shaft arrangement that runs through a worm gear and electric driver. The entire box is lifted and then dropped sharply about six times per minute. The cuttings and sludge are transferred from the buried tank into this box with a small backhoe. The thumping action compacts the solids and lets the oil rise. The backhoe can then tip the box, decanting the oil back into the tank for recovery. Field tests have shown this unit can reduce the oil content of cuttings by an additional 30-40% (Table 1).

A steel box filled with gravel, sitting directly under the shaker discharge and above the buried tank, has also worked well. Small holes in the side of the box allow excess liquid to drain into the buried tank before the cuttings are scooped by a front-end loader.

It is important to have a good large drip pan mounted on top of the flow nipple under the rig floor and good drain trays under the pipe stand back area. Care must be taken to ensure that mud tank gates and fittings are tight.

By using an effective solids-handling system, surface mud losses can usually be kept to about one times the nominal hole volume drilled. Actual losses depend on the amount of cuttings generated (that is, calipered hole volume), the size distribution of these solids, the amount of barite stripped from the mud and the efficiency of the centrifuge.

Table 2 lists surface and downhole losses from three recent wells. On most wells, field experience indicates that unless lost circulation or very high permeability zones are encountered, 80-90% of mud losses occur at the surface.

FIELD EXAMPLES

In the Chedderville area, the well drilled at 6-16-37-7W5 in 1990 used a freshwater gel polymer system. Drilling progressed quickly and reached 2,165 m in 10 days. Sloughing shales soon resulted in tight connections, bridges on trips, and reaming. The mud weight was eventually increased to 1,240 kg/cu m. An attempted DST was aborted. Logging tools became stuck and had to be fished. Casing was finally set to TD at 2,650 m, and the rig was released after 29 days.

An offset well was drilled with invert mud (maximum mud weight 950 kg/cu m) at 8-18-37-7W5 in 1992 (Table 3). The shale problems were eliminated, and drilling performance was optimized. The well reached core point at 2,621 m in 11 days from spud. After 16.9 m of core was taken, the well was drilled to 2,677 m TD, logged, and cased in 15 days (Fig. 1). Although mud costs were much greater than for well 6-16-37-7W5, the total well cost was $140,000 less, and quality evaluation data were obtained.

Southwest of Elmworth, the last well drilled with water-based mud was at 7-1770-13W6 in 1988. Sloughing started almost immediately after the Shaftesbury shale was penetrated. Tight hole and stuck pipe problems occurred. Mud density was increased to stabilize the hole, but lost circulation was also a problem. The hole bridged repeatedly during logging attempts. Casing was finally set at 2,286 m TD without logging. Total time on the well was 34 days.

A recent well drilled to the same formation at 10-10-70-13W6 with invert mud was finished in 12 days and included three DSTS, logs, and production casing (Fig. 2). The maximum mud density was 950 kg/cu m. The target zone was identified from logs and successfully tested by DST. The well was cased, and the zone was completed with production of 19 MMcfd, indicating the importance of open hole evaluation. The total cost on this well was $280,000 less than that for the well drilled with water-based mud (Table 4).

The Wapiti area has also had tremendous success with invert muds (Table 5), Three wells (7-29-65-9W6, 620-65-10W6, and 15-33-65-11W6) drilled in 1992 reached 2,600 m in 13,7, 14.7, and 13.8 days, respectively. Mud densities in this top hole section ranged 900-1,000 kg/cu m. The oil/water ratio was 95/5 on two wells and about 88/12 on the third. The chloride content in the brines ran about 160,000-170,000 mg/l. The mud weight was increased to 1,170-1,200 kg/cu m in all wells to penetrate the overpressured pay zone and to conduct various evaluation work. Coring, testing, and logging were conducted with no problems.

Two wells (Elmworth 15-16-68-13W6 and Kakwa a-83-H/93-1-16) have been drilled from surface casing to TD with oil muds containing no brine phase. These wells were drilled to 3,099 m TD and 2,790 m TD, respectively, in an area where sloughing is common.

Very low mud densities and viscosities were used (Table 6). In the Elmworth well, the purpose of the fluid was to cut a series of pressure cores in the Falher section without altering the in situ connate water. The drilling fluid was diesel viscosified with an organophilic clay and weighted with CaCO3 to about 950 kg/cu m.

At Kakwa, the plan was to use a very low density, cheaper fluid to reduce lost circulation costs. Unfortunately, significant losses still occurred, but in both wells this fluid prevented any shale problems and allowed fast drilling (Fig. 3). Apparently, the chemical potential of an internal brine phase was not required to help maintain shale integrity in these wells. In fact, the Elmworth well was later redrilled because of a casing failure, and the standard invert mud was used with no difference in results. Wider application of oil muds with no brine phase should be possible.

FORMATION DAMAGE

Invert muds can either minimize or worsen formation damage. For oil-wet reservoirs, invert mud can allow excellent completions with little or no damage. Invert mud was a key to the development of Cardium oil field (township 61/62, range 4/5, W6M). Water from a drilling mud or cement damaged this zone so badly that DSTs looked tight and completion stimulations were difficult. Drilling with invert mud and cementing with a pack-off collar above the zone allowed successful completions.

In other reservoir types, invert muds can be detrimental. The emulsifiers and oil filtrate can cause serious emulsion blocks. Low density/low brine content invert muds have high fluid loss and can result in deep invasion into permeable zones. The result can be expensive stimulation work, possibly not needed otherwise.

Recently, Canadian Hunter developed and successfully used a nondamaging oil-based fluid for horizontal drilling and barefoot vertical completions. This fluid contained no water or emulsifiers. A similar fluid has also been used by other operators in horizontal wells.

WASTE DISPOSAL

During the past few years, the oil industry has been working to find acceptable and cost effective methods of cleaning up invert mud drilling wastes. Surface landfarming appears to be the best method.11 13 Canadian Hunter has landfarmed invert wastes for several years with apparent success based on oil degradation and revegetation results. However, the new Alberta Waste Disposal Guidelines set more stringent requirements.

Table 7 lists the analyses of cuttings and sludge wastes left on eight locations drilled with invert mud in northeast British Columbia in 1990. These data provide some insight into the cleanup problem.

The waste composition from each of the eight sites varied considerably. The amount of solids depends on the actual volume (from caliper logs) of the hole drilled, the amount of solids stripped from used invert mud, and any barite that may be removed. The oil and chlorides content depends on the composition of the mud and the efficiency of the solids control equipment.

The total bulk volume of waste material averaged 204 cu m per location. Spread evenly over 1 hectare, that volume would be 2 cm thick. Using discs to mix the waste into the top 15 cm (6 in.) of soil can dilute the material to 1:7.5 by volume. The weight dilution depends on the soil density.

The average oil content of these samples was 11% by weight and represented about 47 cu m of total oil assuming a specific gravity of 0.84. Therefore, the oil content could be reduced to about 2% just by initial mixing over 1 hectare. Fig. 4 gives field measurements of some oil degradation rates. On some sites, the wastes were spread over an area much less than 1 hectare and initial oil content was near 5%. However, if the maximum area on the lease is used to keep initial oil below 2%, then reduction to the 0.5% limit appears possible within 1 year.

The chlorides content of these wastes was a greater concern because the average total of 1,495 kg is greater than the new Alberta guidelines of 800 kg per hectare for initial spreading. One hectare is about the maximum area that can be used for landfarming if the material is to remain on the well site lease, which is certainly desirable in terms of minimizing cost and surface land disturbance. A few sites actually fell below the 800 kg limit, but some were much higher.

Many of the locations had a much higher ratio of chlorides to oil than was present in the mud itself. Because the brine droplets behave much like solids particles, the separation method is important. The discharges from the shaker, thumper box, and centrifuge were analyzed on one well (Table 1). The centrifuge obviously concentrated the brine in the discharge by spinning some of the heard brine droplets out with the fine solids. The amount varies with the mud properties and how "dry" the cut is on a specific centrifuge operation.

The shaker cuttings had only about half the chlorides-to-oil ratio present in the mud because much of the oil is contained in the filming layer around each particle that tends to exclude the brine. More work is required to learn how to obtain maximum oil recovery with minimum chlorides concentration in the discharged wastes.

The following are several ways to reduce chlorides load:

Reduce both the water content and brine strength. If less than 800 kg of chlorides are used per well in an ongoing program, that amount should be the maximum that could end up in each waste pile. However, this value is a difficult target in most cases without sacrificing desired mud properties.
Monitor solids equipment discharges more closely, and try to minimize brine stripping by centrifuging.
Run oil muds with no brine phase when feasible and cost effective.
Substitute other nonchloride salts to maintain the necessary brine activity level.

The overall impact of exceeding the 800 kg/hectare loading rate should be closely considered if landfarming occurs on the existing drilling lease. Field experience indicates that both oil biodegradation and revegetation can occur successfully with higher chloride loading. Also, dilution and leaching by rain and snow could well mitigate the concern over time. These factors should be weighed against the impact of using additional land not previously affected by the drilling operation just to take the chlorides load below the required level.

If the total waste volume can be kept to a minimum and adequate landfarm area can be used, then landfarming appears to be the most practical means of invert mud waste disposal, considering both cost and environmental impact. Meanwhile, the oil industry is continuing to improve the efficiency of oil recovery from and clean-ups of invert mud drilling wastes. Incineration, washing, and distillation are several methods under study.

RECOMMENDATIONS

The oil industry still needs oil-based mud; it is the most efficient drilling fluid in some areas prone to serious shale sloughing. Several other properties of oil muds make them a good choice for many difficult drilling problems.

The potential environmental impact of invert mud drilling waste is a serious concern. Landfarming can achieve successful biodegradation of the oil component. However, more work is required to ensure chlorides loading falls within acceptable limits.
Reducing both water content and brine strength can reduce invert mud costs, drilling time, and the chlorides disposal problem in many areas.
Centrifuging tends to concentrate brine and therefore chlorides in the waste discharge. More work is required to reduce this impact on waste disposal.
Oil muds with no brine phase have been successfully used in the disturbed belt in Northwest Canada, and wider application of these muds should be considered to help reduce environmental impact. These muds may have potential in reducing formation damage in some reservoirs.
The substitution of more environmentally acceptable salts to replace chlorides in invert emulsions should be pursued.
The oil industry must take care not to overuse invert mud. New freshwater systems, such as PHPA muds, may offer more effective alternatives in less severe problem areas.

REFERENCES

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Hale, A.H., and Mody, F.K., "Experimental Investigation of the Influence of a Chemical Potential on Wellbore Stability," IADC/SPE paper 23885, 1992.
Chenevert, M.E., "Shale Control with Balanced Activity Oil-Continuous Muds," Journal of Petroleum Technology, October 1970, pp. 1309-1316.
Thomas, D.C., Lea, J.F. Jr., and Turek, E.A., "Gas Solubility in Oil Based Drilling Fluids: Effects on Kick Detection," SPE paper 11115, 1982.
Santarelli, F.J., Chenevert, M.E., and Osisanya, S.O., "On the Stability of Shales and Its Consequences in Terms of Swelling and Wellbore Stability," IADC/SPE paper 23886, 1992.
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Bell, J.S., and Gough, D.I., "Northeast-Southeast Compressure Stress in Alberta: Evidence from Oil Wells, Earth Planet," Science Letters, 1979, pp. 45, 475-482.
McLean, M.R., and Addis, M.A., "Wellbore Stability Analysis: A Review of Current Methods of Analyses and Their Field Application," IADC/SPE 19941, 1990.
Hsiao, C., "A Study of Horizontal Wellbore Failure," SPE 16927, 1987.
Woodland, D.C., "Borehole Instability in the Western Canadian Overthrust Belt," SPE Drilling Engineering, March 1990.
Zimmerman, P., and Robert, J., "Landfarming Oil Based Drill Cuttings," CADE/CAODC Spring Drilling Conference, April 1991.
Hugo, K., and Lizama, H., "Bacterial Degradation of Oil Based Mud Cuttings," CADE/CAODC Spring Drilling Conference, April 1991.