MIXED METAL HYDROXIDE MUD IMPROVES DRILLING IN UNSTABLE SHALES

June 10, 1991
Donald P. Sparling Oryx Energy Co. Oklahoma City Don Williamson International Drilling Fluids Inc. Houston A mixed metal hydroxide (MMH) mud reduced some of the hole problems common to offset wells in the Arkoma basin. By specially engineering the MMH rheology, cuttings removal efficiency increased, and well bore problems were minimized.
Donald P. Sparling
Oryx Energy Co.
Oklahoma City
Don Williamson
International Drilling Fluids Inc.
Houston

A mixed metal hydroxide (MMH) mud reduced some of the hole problems common to offset wells in the Arkoma basin.

By specially engineering the MMH rheology, cuttings removal efficiency increased, and well bore problems were minimized.

Wells drilled in the Arkoma basin frequently have experienced problems associated with hole instability: excessive reaming, stuck pipe, packing off, and difficulty in obtaining open hole logs. These problems often occur in the massive shale intervals of Atokan age.

The causes of such problems are generally thought to be related to the dissolution of the reactive shales and clays in the Atoka interval, particularly along microfractures created by the tectonic stresses associated with overthrust environment.

There exist insufficient data regarding the borehole stress states, primarily the minimum and maximum horizontal stresses. It is also possible that shear failure at the borehole wall is a result of the stress imbalances.

In this area, wells are frequently air-drilled to 5,000-8,000 ft, resulting in significant borehole enlargement, which compounds the problems.

Operators typically approach the problem in two ways. Oil muds are used worldwide to reduce the trouble time associated with shales. They have been particularly successful in the Arkoma basin for over 25 years.

Water-based muds also are used with varying degrees of success in situations that preclude the use of oil muds.

In general, when well bore stability problems arise while using water-based mud, the traditional response is to lower the API fluid loss to about 4 cc/30 min, and to increase the viscosity tremendously: plastic viscosity (PV) and yield point (YP) values greater than 100 are not uncommon.

The assumption is that API fluid loss measurements accurately represent the quantity of filtrate to which the shale would be exposed, and that minimizing this parameter will reduce the degree of sloughing.

These higher mud rheologies are thought to serve two purposes: hole cleaning is enhanced; and this very viscous mud sets up into a semirigid mass in washed-out areas and suspends cuttings and sloughings outside the effective borehole.

It may appear that an oil mud is always preferable to a water-based mud. However, oil muds also possess a number of disadvantages, including those related to environmental issues. Additionally, there are isolated cases of wells drilled with oil muds requiring rheologies similar to those on the most troublesome water-base mud wells because of hole stability and cleaning problems.

These problem wells are often located in areas of significant faulting, which suggests that hole instability might be related to the tectonic stresses in the immediate borehole area as well as to clay dissolution.

OBJECTIVE

In certain situations, mechanical failure of the borehole may be unavoidable, regardless of whether the chemical activity of the shales is balanced by the mud chemistry.

One method of addressing the problem is to use a fluid capable of transporting all cuttings and sloughings to the surface, rather than allowing accumulations at the bottom of the hole or in "possum bellies" along the borehole wall.

The ideal transport fluid should possess thixotropic properties such that regions away from a source of shear would be quiescent, thereby stabilizing the borehole wall by minimizing erosion from the viscous drag of the moving fluids. Also, the mud system should not experience much fluid loss.

The fluid should behave as a pseudo-elastic solid rather than a true fluid while at rest, i.e., API fluid loss would not be used as an indicator for mud treatment.

A water-based mud system using MMH in conjunction with conventional bentonite slurries has some of these desirable properties. Oryx Energy Co. used an MMH system for its Cable No. 1-1 in the Arkoma basin in Pittsburg Country, Okla., to field test an MMH mud.

The intention was to control hole stability primarily by mud rheology rather than chemistry. Oryx anticipated drilling through approximately 8,000 ft of Atoka shale. The well was considered to be sufficiently displaced from the Arkoma basin's major thrust faults to provide a reasonable trial for this application of the MMH mud.

MMH THEORY

Mixed metal hydroxide mud is a blend of complex chemicals fundamentally different from conventional muds. There are two unique additives to the MMH system: a mixed metal hydroxide of aluminum and magnesium and an organic fluid loss reducer developed specifically for the system.

The basic composition of the MMH system is indicated in Table 1.

The rheologic profile of the mud is determined using a six-speed rheometer. The high 3 rpm and 6-rpm readings and the relatively low 300 rpm and 600-rpm readings result in a flat rheologic profile, quite different from that of a conventional mud (Fig. 1).

Traditional drilling fluids with rheology values as high as those of the MMH system generally produce high gel strengths. High gel strengths require much energy to initiate circulation and generate high equivalent circulating densities (ECDS) once flow is established.

Because neither of these is desirable, the rheology of conventional drilling fluids is normally controlled to minimize high YP and 3/6-rpm rheometer values.

By design, the MMH system produces high rheologic values. However, ECD values and energy requirements to start circulation are low. Data from lab testing and field trials support the low energy requirement and hole cleaning capability. This rheology provides a high level of hole cleaning and static suspension of cuttings.

The rheology mechanism is directly related to the interaction between the MMH compound and clays, both added and drilled. It is theorized that the viscosity results from the interaction of water envelopes that fully encompass the MMH/clay complex, which reacts in an optimum pH range of 10.2-10.5.

Conventional drilling fluids commonly obtain their rheological characteristics from the interaction of charged clay platelets which orientate under static conditions. This kind of gelation requires a much higher level of energy to disrupt the structure and initiate fluid motion than for the simple water/water interface thought to occur in the MMH system.

Laboratory evaluation of various drilling fluids using the back pressure readings from a capillary viscometer indicates energy levels required to maintain motion may be as much as 52% less for MMH mud than for conventional drilling fluids. This lower resistance to flow can be translated into lower ECDS, higher effective energy at the bit, and possibly improved performance of downhole motors.

The Huxley-Bertram rheometer measured rheology parameters under elevated temperatures and pressures ranging from ambient conditions up to 190 F. and 2,500 psi. These data were plotted in Fig. 2. The MMH fluid has a relatively high shear stress intercept and a near-linear rheologic profile as opposed to the downward curve shown for the partially hydrolyzed polyacrylamide (PHPA) fluid.

The higher shear stress correlates to improved hole cleaning and better static suspension characteristics for the MMH fluid.

FLOW PROFILE

There are several theories and models of flow dynamics for various drilling fluids. These models have traditionally been based on common drilling fluids, with little or no work conducted to model fluids with Theological properties similar to those of MMH muds. As a result, for Cable No. 1-1, there were questions as to the validity of standard industry calculations for flow characteristics and general hydraulics.

Several experiments were conducted to evaluate the MMH system's response to applied force. A force was applied to the surface of the fluid while noting the distance at which a surface response could be observed. The MMH fluid responded in an extremely local manner to the applied force. The gel structure was broken only within close proximity to the applied stress.

In actual application, it is theorized that the fluid flows immediately next to the area where shear or stress is applied. While drilling, the stress is generated by the circular motion of the drillstring. Field data have indicated a definite trend for the fluid to channel during circulation, further supporting this theory (Fig. 3).

Based on experiences from this drilling project, the degree of channeling can be adjusted directly by modifying the rheologic profile through variation of the 3/6-rpm rheometer values. As these lowend rheology values increase (from lows near 10 to highs above 40), fluid velocity slows with respect to distance from the applied stress, in this case the drillstring.

Fluid motion at the well bore face approaches zero as the 3/6-rpm rheometer values increase. The advantages of this kind of flow profile include a reduction in dynamic fluid loss. Hole cleaning improves because the decreased hydraulic diameter results in higher flow velocities.

CONTAMINANTS

The MMH drilling fluid progresses through two stages. In the first, or fresh-mix stage, the system is more sensitive to some contaminants than in the latter stage in which drill solids have been incorporated into the system.

Freshly mixed MMH fluid is sensitive to anionic contaminants. Excessive hardness, especially magnesium, in makeup water can interfere with MMH mud performance. Moreover, the mud system will not tolerate conventional fluid loss reducers or thinners such as lignite, lignosulfonate, and phosphates.

A dramatic thinning reaction occurs with very minimal amounts of anionic materials. One 50-lb sack of lignite will thin 1,000 bbl of freshly mixed MMH fluid. In contrast to conventional muds, polyanionic cellulose (PAC) material may be used to thin fresh MMH mud. Care must be taken to add only products which have been thoroughly pilot tested.

As the system matures and drilled solids are incorporated into the fluid, it becomes more tolerant of contaminants. However, lignite may be required in low concentrations as a thinning agent. The amount of thinner required is directly related to the low-gravity solids (LGS) content.

The most common contaminants found in drilling fluids are solids. The MMH system is an extremely solids-tolerant drilling fluid. However, abnormally high LGS concentrations can decrease the cost benefit of the fluid.

Other common sources of drilling fluid contaminants are carbonates. Whether drilled or the result of degradation of mud additives, carbonates can create serious problems for most water-based fluids. Thus far, the MMH system has been tolerant of carbonate contaminants.

Lab studies have contaminated the MMH system with 150,000 mg/l. of carbonate, as determined by the Garrett gas train. No adverse effects were noted, but the pH dropped as expected. The acid gas in this reduced the pH of the fluid, resulting in a drop in the Theological properties. As the pH was brought back to the optimum range (10.0-10.5), the rheology returned to its original value.

Further testing included the treatment of carbonate contamination with lime. The carbonates could be treated out with small additions of lime with no adverse effects on the drilling fluid. However, there is little reason to treat out the carbonates because they have little to no effect on fluid performance, provided the pH is maintained in the proper range.

Hydrogen sulfide (H2S) gas was not encountered on Cable No. 1-1. However, H2S can be tolerated and treated with normal scavengers such as zinc carbonate. The only unusual requirement is the use of a chelating agent in conjunction with the zinc carbonate. This maintains pH while rapidly treating out the sulfides.

The degree of cement contamination is directly dependent upon the amount and kind of solids present. Should severe contamination be expected, pretreatment with sodium bicarbonate is recommended (as with most waterbased muds). False alkalinities may be a problem with cement contamination. Even with a pH as high as 11.5, additional caustic may be necessary to obtain fluid rheological performance in the presence of cement.

Early in the drilling project, shortly after the MMH system was put into use, a lignite stringer was encountered. The mud system's rheology fell drastically, necessitating dilution with fresh whole mud.

Because the MMH mud is sensitive to anionic compounds, the lignite stringers acted as natural thinners. To date, natural coal and lignite are the only contaminants known to affect the system in a manner for which there are few remedial measures short of system dilution.

SYSTEM MAINTENANCE

MMH was added through a chemical barrel where the material was also prewet. Because MMH has a very low solubility in water, it was prewet to improve dispersion in the mud. MMH is commonly maintained at a concentration of 1 lb/bbl and adjusted as indicated by fluid profile and cuttings integrity.

Prehydrated commercial clay was maintained at a concentration of 10-12 lb/bbl. Floplex was added through the hopper as needed for fluid loss control. Initially, PAC was used to control the system rheology. As drilled solids built up, it became necessary to switch to lignite for rheology control. The fluid properties are presented in Table 2.

CHEMISTRY

Chemical maintenance of the MMH system is quite simple. Initially it was thought that the pH of the system might need to be adjusted as solids content changed with the addition of barite and the accumulation of drilled solids.

Early lab testing indicated that the fluid performed more appropriately from a rheological perspective when the pH was maintained near 10.5 for low-solids, whereas a pH of 9.5 was recommended for higher levels of solids. Yet, field experience quickly indicated optimum fluid performance for a pH of 10.0-10.5.

As pH fell, a loss of lowend rheology was experienced, indicating that alkalinity acted as a catalyst for the reaction between MMH and clay in water. Other mud parameters, such as phenolphthalein and methyl orange alkalinity end points (Pf/Mf), chorides, hardness, and carbonates, were monitored, but variances in these parameters did not appear to affect system performance. For the most part, simply monitoring the mud and maintaining its pH around 10.5 with either sodium hydroxide or potassium hydroxyde promotes optimum product performance and maximum system stability.

RHEOLOGY

The rheology of the MMH system is its most unique characteristic. As drilling began, little was known about the long-term maintenance of a true MMH mud. Based on lab data and early field tests, the 3/6-rpm rheometer measurements, the 10-sec gel, and YP were kept at equivalent values. These properties are collectively referred to as the Theological indicators.

As drilling progressed, these properties were allowed to range from values as low as 5 to as high as 94, but all the Theological indicators were kept at equivalent values. As deviations occurred, MMH or clay was added according to pilot tests.

A drop in 3/6-rpm rheometer values with an opposing rise in YP indicated a trend toward a more conventional fluid profile. It was determined that the MMH compound was depleted as drilling progressed; it was replenished accordingly.

Funnel viscosity had no significance in the engineering of the MMH system. Funnel viscosity measurements in excess of 200 sec/qt were not uncommon.

As the rheology indicators dropped below a value of 20, hole cleaning became a problem, resulting in tight connections and fill on trips. Conversely, as the values rose above 40, the amount of solids retained along the well bore increased exponentially; this is also an undesirable mud attribute.

Optimum fluid performance occurred when the rheological indicators ranged around 30-40.

SOLIDS

Solids control is of paramount importance to the MMH system. As a general rule, the cation exchange capacity (CEC) should be maintained around 12-20 lb/bbl equivalent.

In contrast, the commercial gel content should be maintained at 10 lb/bbl, according to the concentration sheet. Accurate product concentrations are best obtained by using water meters and accurate material balance calculations.

As with most water-based muds, LGS should be kept to a minimum by using centrifuge equipment and diluting with whole mud as necessary. During this well, LGS levels reached 12% which is far above the desired level to control the system. The level of LGS should remain below 6%, particularly on wells with high mud weights and subsequently less water available to wet LGS surfaces.

SURFACE PIT

Because of the unique rheological properties of the MMH system, a number of operational problems did not lend themselves readily to conventional field remedies.

In particular, circulation of the MMH system within the surface pits was a constant concern. Even with mechanical stirrers and mud guns in all pits, "dead spots" of stagnant mud were prone to appear within minutes in any unagitated area. This caused continual problems with the chemical treatments.

To control variation in rheology, chemicals were added in routine concentrations but with unexpected results. The addressed property fluctuated wildly over the first two to three circulations. Additional treatments were made to counteract the suspected overtreatment, which in turn caused other properties to fluctuate out of the desired ranges.

The channeling in the surface pits prevented the entire surface mud volume from treatment in a normal time frame. In effect, chemical additives sufficient to achieve a desired concentration in the calculated active system volume were, on a short-term basis, actually a significant overtreatment. After six to eight full circulations, the mud properties generally reacted in the expected manner, indicating homogenization of the treatment throughout the system.

This severe channeling also resulted in an inability to route the entire mud system volume through the degasser, resulting in incomplete removal of gas from the mud. The dead spots in the pit corners sometimes retained small amounts of gas for several days.

Future applications of MMH systems need to address these surface handling problems. In particular, the pit agitation setup should be analyzed and modified to minimize stagnant areas. The flow patterns in the pit indicated that only the mud within or immediately adjacent to the area swept by the stirrer blades was in motion. improvements should lead to more efficient degassing and beter response time for chemical treatments.

SOLIDS CONTROL

The initial solids-control equipment consisted of a linear motion shale shaker and a decanting centrifuge for the anticipated low mud densities (< 10 ppg). The shear thinning property of the MMH mud allowed use of this equipment with acceptable results until mud density was increased with barite to 10 ppg. The centrifuge was then shut off to avoid stripping barite from the mud.

Solids-control efficiency suffered from tears in the shaker screens, which might be deemed minor in a conventional mud system. Also, the greater retention of drill solids could be attributed to the almost complete absence of particle settling (Stokes' law) in the pits.

Only one shaker was used, following the practice of similar water-based drilled wells in the area. Separation of drilled solids from the return mud was acceptable. When rheologies increased above optimum values, whole mud was lost across the shaker due to screen blinding. Although overall solids removal was satisfactory, future MMH jobs should have excess solids-control equipment.

CEMENTING

Four cement jobs were performed with the MMH system in the subject well: the 9%-in. surface casing, the 7-in. intermediate casing, one sidetrack plug, and the 4 1/2-in. liner. The MMH mud rheology caused concern about adequate displacement of the mud with good quality cement.

It is theorized that the cement tends to be "stiffer" than the MMH fluid, thereby providing proper displacement.

On the 7 and 4 1/2-in. strings, mud displacement was enhanced by additional circulating time with the casing on bottom. This ensured that the mud was thinned to the greatest extent practical.

The cement slurry was preceded with approximately 2,000 annular ft of chemical flush followed by 1,000 annular ft of viscous spacer fluid. The casing was run with spiral-vane, rigid centralizers and reciprocated and rotated throughout the job.

The 7-in. casing was continuously reciprocated while circulating and thinning the mud for 9 hr. At this point the casing became stuck. Some returns were lost while attempting to free the casing. The pipe was worked free after regaining circulation, and the cement job was started. While cementing, circulation was partially lost, and pipe drag increased significantly.

The intermittent nature of the drag and quantity of cuttings recovered at the shale shaker indicated that the annulus was packing off during the job.

The 4 1/2-in. liner job had similar problems. The liner hanger was set without difficulty, and rotation was initiated at 14 rpm with 2,400 ft-lb of torque. Rotation continued for approximately 2 hr while cement was batch mixed. Cementing operations proceeded normally until approximately half of the cement volume had cleared the liner shoe. Surface pressure then increased 400 psi, and all circulation was lost.

Rotational drag stalled the power swivel at the preset maximum of 5,500 ft-lb. For the remainder of the cement job, returns were lost. After bumping the wiper plug and shutting down the cementing pumps, rotation was initiated once again with 3,000 ft-lb of torque.

Qualitative analysis of cement bond logs run in the 7 and 4 1/2-in. casing supported the assertion that the MMH system does not preclude adequate mud displacement to obtain a good cement bond.

On both strings, the bond logs found distinct cement tops corresponding to the top of cement calculated from the point of lost returns. Bonding below the top of cement was considered good or excellent with no remedial work required in the cemented areas.

DRILLING PERFORMANCE

In the 8 3/4-in. hole interval (5,600-8,700 ft), directional control was not a primary concern and the penetration rates ranged around 15-16 ft/hr. This compares favorably with penetration rates of 10-18 ft/hr for a recent (1988) air-drilled offset.

Oryx used conventional power law models for daily calculations of bit hydraulics, pressure losses, and surge/swab pressures. Although the differences between predicted and actual pressure losses were not quantified, the calculations seemed accurate because the results were well within the limits of conventional field measurement practices.

Table 3 compares surface pressures predicted by power law models to the actual field measurements for one bit run.

The MMH fluid performed as expected in regard to hole cleaning and cuttings suspension. Considerable amounts of material were circulated from the well bore, with particle sizes up to several inches.

Even after extended periods without circulation, cuttings remained suspended in the annulus and did not fall back.

Although there was little doubt that all cuttings were removed from the hole as it was drilled, occasionally the well bore collapsed after bit trips, and extensive reaming was required.

Thus, the MMH fluid did not provide complete well bore stability.

Examination of a subsequent log resistivity-vs.-depth plot gave credence to beliefs that borehole stresses contribute significantly to well bore stability difficulties in the Arkoma basin (Fig. 4).

A direct relationship exists between pore pressure (in situ stress) and mud density (borehole pressure). Each instance of well bore instability can be related to a change in the general character on Fig. 4:

  • At 5,677 ft, air drilling was halted because of an inability to make connections.

  • A single bit run from 5,677 ft to 8,677 ft encountered difficulty only in an interval of instability at 7,608 ft where the drill pipe annulus packed off intermittently. Mud density ranged from 9.6 to 9.8 ppg until 8,677 ft, where it was increased to 11.0 ppg to control gas influx.

  • From 8,677 ft to 9,328 ft, drilling progressed normally with mud density around 10.8-11.0 ppg. No significant amount of fill was noted after trips even though larger than normal volumes of sloughing shale were circulated out of the hole.

  • After a bit trip at 9,328 ft, fill was tagged at 8,565 ft and 29 hr were required to reach bottom.

  • Drilling from 9,328 ft to 10,058 ft presented no problems other than minor lost circulation. The mud density was allowed to gradually fall back to approximately 10.8 ppg. On the subsequent bit trip, fill was tagged at 8,197 ft, and approximately 5 days were spent reaming back to bottom.

  • The MMH fluid system performed as expected with regard to its cuttings suspension, hole cleaning, and rheologic properties.

  • Valuable field experience was gained with respect to the homogenization of chemical additives into the system, the apparent applicability of current hydraulic models, the system's tolerance to common contaminants, and the ability to obtain adequate cement jobs.

  • The ability to continuously remove cuttings and sloughings from the well bore does not necessarily guarantee a stable well bore.

ACKNOWLEDGMENT

The authors would like to thank the staffs of Oryx Energy Co. and International Drilling Fluids Inc. for their help with this article.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.