MIXED METAL HYDROXIDE DRILLING FLUID MINIMIZES WELL BORE WASHOUTS

Frederic Lavoix
Elf Aquitaine Production
Pau, France

Mark Lewis
International Drilling Fluids Inc.
Houston

The use of a mixed metal hydroxide (MMH) drilling fluid, instead of a conventional polymer-based fluid, improved well bore stability in troublesome formations in West Africa.

The unique flow and suspension characteristics of the MMH fluid improved cuttings removal and decreased well bore washouts. With fewer hole problems and better cleaning in the well, the operator reduced drilling time and cost for the well.

MMH compounds were developed and introduced to the drilling industry a few years ago. Initially their utility was limited by an inability to achieve reliable filtration control without destroying the unique fluid rheology. A fully functional drilling fluid system, based on this unusual line of chemistry, has been developed and used with great success in dozens of wells around the world.

MMH FLUIDS

The mixed metal hydroxide compound used to control well bore washout in the West African well contains approximately equal numbers of magnesium and aluminum ions in a hydroxyl ion matrix. The compound is synthesized to ensure that the crystalline product formed has a high net positive charge associated with it. The crystals are sheet-like and considerably smaller than the average bentonite platelet. The crystals are insoluble in water and environmentally benign.

When bentonite and MMH are dispersed together in water, the highly positive MMH crystals migrate and attach to the negative sites on the basal planes of the clay platelets. The new complex produces a slurry having quite unusual rheological and suspension characteristics.

RHEOLOGY

Previous publications have described in some detail the Theological characteristics of MM]k fluid and a possible mechanism by which such behavior may come about 1 4

The basis of the mechanism appears to be electrostatic in nature. Analytical measurements have shown that the simple slurry completes its gel structuring in less than 2 u sec from the time that motion ceases. In practical terms, this short duration means that gelling is instantaneous and that gels are not progressive.

At rest, the fluid demonstrates properties which are more typical of solids than of liquids, yet application of mechanical displacement overcomes this gelled state and turns the pseudo solid into a conventional liquid. Very low levels of energy input are required to effect this transition.

In its gelled state, the MMH fluid has an amazing capacity to suspend indefinitely solids as diverse as large shale cavings, lumps of concrete weighing several pounds, and metal debris produced during milling operations.

Moreover, there is substantial evidence from field experience that the MMH flow profile in the well bore is unique. Less than 100% of the fluid in the well bore is in motion during drilling, and there is a "dead zone" in the proximity of the well bore.

In this dead zone near the well bore, the fluid is in its gelled or pseudo solid state. The significance of this dead zone is that the well bore may become isolated from the harmful effects of the moving fluid. This lack of exposure to moving fluids is most significant for formations having low mechanical integrity.

Measurement of the MMH system's rheology by conventional means has given interesting results. According to Bingham plastic model terminology, the plastic viscosity is very low and the yield point very high. These values produce a very flat rheogram. Thus, the gels are numerically very high, and the readings taken at 3 rpm and at 6 rpm on a standard field viscometer are also significantly elevated.

The significance of the measured Theological data (on which the drilling industry typically relies) in practice is dependent on the mechanism by which the subject fluid develops its viscosity. The MMH fluid can tolerate much higher values for yield points, rheology at low shear rates, and gel strengths than a conventional type of fluid. The problems associated with conventional fluids exhibiting high values for these parameters do not apply with this MMH fluid type.

WASHOUT CONTROL

In West Africa, Elf Aquitaine had considerable washout problems in wells drilled through the Vandji sandstone formation.

Polymer-based drilling fluids had been used on three previous wells (designated Nos. 1-3) drilled in the area. These wells had problems with hole cleaning and hole enlargement.

Elf thought that the unusual flow characteristics of the MMH-based fluid could potentially solve the erosion problem. In addition, past experience suggested that this fluid would easily correct the hole cleaning problem. An MMH-based drilling fluid was selected because the exceptional cuttings transport properties of the fluid would allow a reduction in the flow rate, without compromising hole cleaning performance, as a means of further minimizing the chances of disturbing the fragile sandstone formation.

DRILLING AND CORING

Well No. 4 was drilled with the MMH-based fluid. The comparison offset well was drilled with a conventional polymer-based fluid.

Both wells were drilled with rotary bits. Down to a depth of 5,825 m (9,266 ft) on Well No. 4 the 8 1 /2-in. bit was fitted with three 18/32-in. jets. Thereafter, three 20/32-in. jets were used on the bits to drill to total depth.

In comparison, the jets used on the corresponding section of the offset well ranged from three 13/32-in. jets to a combination of 11/32 in., 12/32 in., and 13/32 in.

The same rotary speed range of 100-130 rpm was used on both wells. The weight on bit range for the subject well was 20,000-44,000 lb, and for the offset well the range was 29,000-40,000 lb.

The fluid flow rate, 600 l./min (160 gpm), on Well No. 4 was substantially lower than for the offset, which required flow rates around 900 l./min (240 gpm). Moreover, the standpipe pressure with the MMH fluid was only 500 psi, compared to 950 psi for the offset. Table 1 shows comparative hydraulics data for the wells.

Even at the lower flow rates, the MMH fluid gave a significant improvement in hole cleaning and well bore stability. The offset well had problems with torque, drag, and sloughing shale, yet no such problems occurred on Well No. 4. The cuttings returns to the shakers were consistent with the drilling rate.

The good, consistent lifting performance of the fluid made identification of fine layers of sandstone and limestone much more definitive.

One negative effect of the low fluid flow rate was the less efficient heat transmission away from the bit. This inefficiency resulted in overheating of the bit. A bit tripped out of the hole had developed the characteristic blue color associated with steel which has been exposed to high temperatures. Fortunately, the bearings were not adversely affected, and the durability of the bit was unimpaired.

The coring operations on Well No. 4 encountered fewer problems as compared to Well No. 3. On Well No. 4, three coring runs were required to cut 40 m (130 ft) of core with 100% recovery. On Well No. 3, fifteen coring runs were required to cut a similar length of core with an 84% recovery rate.

With the coring time subtracted from the total time taken to complete the equivalent sections on the offset wells, no appreciable difference in drilling times is evident. Yet, the footage drilled on Well No. 4 exceeded that of the offset by 20%. Table 2 summarizes some of the operations data for the 8 1/2-in. hole section.

SOLIDS CONTROL

MMH-based fluids demonstrate exceptional shear-thinning behavior. These fluids give plastic viscosities which are substantially lower than normal for fluids with such elevated yield points (Table 3).

In addition, the unique mechanism through which the viscosity and suspending performance are developed causes the fluid to behave in an unusual fashion on vibrating screens. The thick, gelled appearance changes dramatically when the fluid comes in contact with the vibrating screen. The structure appears to burst, providing a true liquid which has very little viscous character and which easily passes through the screens.

Unlike polymer fluids which adhere to surfaces, the MMH fluid has no such tendency. There is little chance of the fluid itself inducing screen blinding. Thus, the shale shakers can operate with screens that are finer than normal.

The three high-speed shakers used on Well No. 4 were fitted with 140-mesh, 175-mesh, and 175-mesh screens, respectively. The offset well had to use 84-mesh, 110-mesh, and 110-mesh screens.

The low flow rates and the highly shear-thinning nature of the MMH fluid allowed processing of the entire circulating fluid volume over one shaker for most of the well. The only other solids control equipment available was a mud cleaner fitted with a 200-mesh screen, Solids removal was very efficient.

Whole fluid dilution was used to limit buildup of fine drill solids. The total maintenance volume used on Well No. 4 was substantially lower than that on Well No. 3 despite the increase in length of the hole section.

FLUID PROPERTIES

Table 3 lists the properties for freshly mixed fluids and the fluid properties during drilling for both wells. The data show unusually high Theological values for the MMH fluid at low shear rates. The fluid performance in this shear rate range is of critical importance and is therefore always recorded. These data do not have as much value for polymer fluids, and therefore the equivalent information was not recorded.

In a drilling operation, however, the polymer fluid gave over double; the value for plastic viscosity compared to the MMH fluid. Despite the higher yield point, the polymer fluid gave poorer hole cleaning performance. In accordance with previous field applications the exceptionally high gel strengths shown by the MMH fluid did not give the problems normally associated with such values in conventional fluids.

The properties of the MMH fluid showed much less variation than those of the of the polymer fluid. The rheology of the latter fluid suffered from incorporation of drilled solids, whereas the MMH fluid had little reaction to the drilled solids.

Fluid loss control was easily maintained with both fluids, and both were run at about the same pH.

The polymer fluid used in Well No. 3 was weighted with barite and had a solids content of 10-12%. The MMH fluid had a solids content of 16% because of the use of calcium carbonate as a weighting agent.

GAS INTRUSIONS

On Well No. 4 gas cutting of the fluid was a constant factor below 2,715 m, and the measured gas content ranged from 60% to 100% below this depth. Fluid having a specific gravity of 1.26-1.27 (about 10.5 ppg) at the suction had been gas cut to 1.20-1.22 (about 10.1 ppg) by the time it reached the flow line.

Towards the end of the drilled section the fluid was weighted up to a specific gravity of 1.29, and the gas content dropped to less than 50%. Much of the gas came from the Sialivakou formation, a predominantly marl formation which contains some silty clay and sandstone layers.

The unusual organizational structure of the MMH fluid causes it to retain gas while the drilling fluid is at rest or moving slowly. No spontaneous degassing occurs when the fluid arrives at surface.

The behavior has two significant consequences:

The potential for more complete and quantitative gas detection exists because the gas detection unit can analyze a more representative sample of the gas reaching the surface in the fluid.
The fluid must be thoroughly degassed before being pumped back down the hole. Failure to degas the fluid fully can cause problems with recycling of the gas. Fortunately, the MMH fluid responds very well to gas separation operations, and no problem with gas removal occurred at any time.

One effect from the increased initial gas retention was that the gas detectors became saturated, a phenomenon not encountered with conventional fluids.

CALIPER RESULTS

The caliper data confirm the successful application of the MMH fluid.

The two major formations drilled in the interval are the Sialivakou/Djeno formation which comprises mainly marls and the Vandji formation which is primarily a sandstone.

On both wells, 8 1/2-in. bits drilled these formations. The average hole diameter of the section was 9 5/8 in. for Well No. 4 and 12 1/2-in. for Well No. 3.

The most dramatic improvement in hole diameter occurred in the sandstone section where a diameter of 8 3/8-in. was achieved with the MMH fluid compared to diameters ranging from 13 in. to 20 in. with the polymer fluid.

The well bore diameter improvements may have also resulted partly from a reduction in bit hydraulic power and partly from encountering the Vandji formation in a more compacted form than on the offset wells.

The MMH fluid also outperformed the polymer fluid in the marl where hole diameters reached 10 1/4-in. and 11 3/4-in., respectively. Data from Well Nos. 1 and 2 also showed more substantial washout.

One of the most significant improvements seen on the caliper log for the MMH fluid is the reduction in the variation of the hole diameter.

COST

The direct cost per well for drilling fluid was reduced by more than 10% by switching to the MMH fluid (Table 1). A smaller volume of MMH mud was required because of less seepage losses downhole and better solids removal and tolerance. The total fluid volume consumed while drilling the section was 25% less on Well No. 4 than on the offset.

Furthermore, approximately the same number of drilling days were used to drill 20% more footage. Coring time was greatly reduced. The avoidance of hole problems, the ability to remove and suspend drill solids, and the success with stabilizing the well bore contributed to this improvement in drilling time.

REFERENCES

Burba, J.L. III, Holman, W.E., and Crabb, C.R., "Laboratory and Field Evaluations of Novel Inorganic Drilling Fluid Additive," IADC/SPE paper 17198, presented at the IADC/SPE Annual Drilling Conference, February 1988, Dallas.
Burba, J.L. III, Tehan, W.F., Hamilton, F.D., Holman, W.E., Porzucek, C., Christiansen, C.P., and McKenzie, J., "Field Evaluations Confirm Superior Benefits of MMLHC Fluid System on Hole Cleaning, Borehole Stability and Rate of Penetration," IADC/SPE paper 19956, presented at the IADC/SPE Annual Drilling Conference, February 1990, Houston.
Sparling, D.P., and Williamson, L.D., "Mixed Metal Hydroxide Mud Improves Drilling in Unstable Shales," OGJ, June 10, 1991, pp. 29-33.
Fraser, L.J., "Unique Characteristics of Mixed Metal Hydroxide Fluids Provide Gauge Hole in Diverse Types of Formations,"