GREEN CANYON DRILLING BENEFITS FROM ALL-OIL MUD

Lindsay J. Fraser, A.J. Gerbino, Bill Hurst
International Drilling Fluids Inc.
Houston

Jerry P. Allamon, Chris Beato, Mike Shaughnessy
Conoco Inc.
New Orleans

With the recent introduction of high-performance, oil-mud viscosifiers and filtration-control products, all-oil drilling fluids have proven to be practical.

A number of wells in the deepwater Green Canyon Block 184 have been successfully drilled with the all-oil fluid. When compared to wells in the area previously drilled with invert-emulsion fluids, clear advantages of the all-oil fluid are evident.

OIL MUDS

The first U.S. oil-mud patent was issued to J.C. Swan in 1923.1 The all-oil drilling fluid principle proposed by Swan never really found acceptance. It suffered from too many shortcomings, including high cost, water intolerance, inconsistencies in properties, high flammability risk, and poor drilling performance.

The development of emulsifier technology,2-6 mainly in the 1940s, provided a solution to the water-intolerance problem. It was discovered, moreover, that positive advantages (in terms of viscosifying the oil and reducing filtration rates) were gained when water was emulsified in oil.

The addition of water or brine to oil muds became commonplace, and the "invert emulsion" drilling fluid philosophy was established.

With the introduction of organophilic clays7 around 1950 and amine treated lignites and humates8 in the mid-1960s, improvements for independent control over rheology and fluid loss were achieved.

Developments in emulsifier and wetting agent technology and the introduction of the balanced activity concepts9-11 further advanced invert-emulsion drilling fluids.

INVERT EMULSIONS

Most of the objections that led to the early demise of Swan's oil-fluid design have been overcome. With the introduction of high flash point, low-toxicity oils, the flammability and safety aspects have been addressed.12

Invert-emulsion fluids are cost effective13 in many cases and can also be engineered to provide faster penetration.14 15 Use of "relaxed" inverts to provide high penetration rates is, however, somewhat restricted.

For all the successes achieved by invert-emulsion drilling fluids there are still aspects of the performance that can be improved. The three major areas of concern are as follows:

Interaction between the brine phase of the fluid and the drilled formation can still occur. Hole instability may result.
Rheologically, the emulsified brine droplets act as fine solids and endow the drilling fluid with undesirably high plastic viscosities.
Failure of the emulsion is an ever present risk.

ALL-OIL FLUID

The overall objective in designing the all-oil fluid was to provide the advantages of an invert-emulsion fluid while minimizing or eliminating the inherent shortcomings of the invert system.

By removing the aqueous internal phase of the fluid, the emulsion's contribution to viscosifying the fluid and to filtration control is lost. More efficient viscosifiers and filtration-control additives, therefore, are required to compensate for these shortfalls. The fluid also has to accommodate, on a routine basis, small influxes of water.

A means whereby fluid properties can be easily and independently altered was also addressed during development work.

The system routinely comprises:

A base oil (either mineral or diesel)
A novel organophilic clay that yields in any oil without reliance on contact with water or other polar activator
An emulsifier package used for routine accommodation of water influxes
A wetting agent used to maintain all solids in an oil-wet state
A fluid-loss reducer
A deflocculant for use in producing major reductions in rheology, when such reductions are necessary
Lime for treatment of acidic gases
Barite (if appropriate).

The base oil used throughout the project discussed was Conoco's low-viscosity LVT mineral oil. The ability to provide viscosity in an all-oil environment, under the low-shear and low-temperature conditions found at the average mud-mixing plant or rig site, is crucial to the successful application of the concept. The new organophilic clay provides this capability.

Conceptually, the system is simple, with each constituent product fulfilling a single function. This independence of function imparts a high degree of flexibility in manipulating fluid properties.

DRILLING PROJECT

On Conoco's Jolliet project in Green Canyon Block 184, a tension-leg well platform (TLWP) is being employed. This deepwater drilling project involved the drilling of 20 wells at maximum angles ranging from 30 to 60 from vertical. The first 12 wells are referenced in this article.

The use of an 1,800-ft riser with an internal diameter of 19.75 in., combined with the critical nature of the angles of borehole inclination, necessitated that the drilling fluid provide very efficient hole cleaning.

When drilling in deepwater environments where high rates of penetration are easily attained, drilling rates are controlled as a matter of policy.

The lithology in Green Canyon Block 184, over the applicable depths, is dominated by quartz and illite. Analysis by X-ray diffraction shows the presence of hydrateable and dispersible clays and shales in significant quantities. Appreciable quantities of halite have been identified in cuttings from the deeper hole sections.

Data shown in Table 1 are typical for the depths drilled.

INVERT FLUID PERFORMANCE

The first four wells on the Jolliet project were drilled using conventional invert-emulsion fluids having oil/water ratios in the range 80:2085:15.

Drilling fluid properties were typical for fluids of this type over the range of densities encountered. Averaged drilling fluid properties for Wells 3 and 4 are shown in Table 2.

All four wells reached target depths without major problems. Nevertheless, observations made while drilling these four wells highlighted some areas for concern. The pore-pressure profile lay very close to the fracture gradient, and herein lay the source of most of the problems. The four problems were:

Hole cleaning-There were instances of drag on trips, lost circulation, and fluctuations in standpipe pressures. In addition, cuttings-box fill rates often fell short of values calculated from the volume of hole drilled. These occurrences were attributed to cuttings buildup with the attendant increase in annular mud density.
In particular, there was evidence that accumulations of cuttings in the riser may have been responsible for some of the lost-circulation problems.
Those problems were combatted by use of reduced drilling rates, short trips, and higher-density sweeps. Yield points were altered to values far above those originally programmed in an attempt to achieve more consistent and efficient hole cleaning.
Reduction of rheology-In the delicately balanced pressure situation, mud-loss problems typically arose at casing points.
The approach adopted to minimizing whole-mud losses sustained while running and cementing casing was as follows:
- The hole was cleaned of cuttings in as efficient a manner as possible.
- The yield point, gels, and low-shear-rate rheology were reduced as far as possible.
There is inherent in an invert-emulsion fluid a contribution to rheology which is derived from the presence of emulsified-brine droplets. Although chemical treatment was used successfully in reducing that part of the rheology associated with solid-solid interactions, the emulsion-derived component of the rheology could not be successfully depressed.
Consequently, chemical treatments had to be supplemented by dilutions with mineral oil to achieve reductions of the required order.
The need for dilution was not only costly in terms of rig time and materials, but it also provided logistical problems in the form of excessive volumes which had to be stored and returned to shore at the end of drilling.
Osmotic transfer-There was evidence that on at least one occasion a balance of activity between the formation and the brine phase of the drilling fluid had not been achieved. The resulting formation destabilization was felt to have been partly responsible for some occurrences of packing off.
Intrusions of halite from the formation further complicated the water phase chemistry and calculations. The inaccuracy of the field methodology available for estimating sodium chloride content necessitated shipping of samples to an onshore analytical facility for an accurate analysis.
Cost-Major costs were incurred at casing points. Dilution was required to depress the rheology, and significant volumes of whole mud were lost to the formation during casing and cementing operations.

In light of these concerns, and the fact that such problems are largely indigenous to invert emulsion fluids, it was decided to investigate the possibility of using an all-oil fluid to circumvent the problems.

NEW FLUID OBJECTIVES

The success or failure of the all-oil fluid was to be gauged by its ability to meet the following five objectives (set in advance of application of the system):

Improved hole cleaning-Improved hole cleaning was sought through provision of shear thinning properties and high rheologies in the low-shear-rate ranges. The aim was to eliminate the need for short trips, use of high-density sweeps, and pressure fluctuations caused by inadequate cleaning of the riser.
Improved Theological control-By elimination of the water component of the fluid, the viscosity would be derived primarily from the organophilic clay. The resulting low-solids fluid was expected to provide improved yield point-to-plastic viscosity (YP/PV) ratios. In addition, it should prove possible to reduce the rheology at casing points in a more efficient and timely manner than had been the case with the invert fluid.
The improved shear-thinning characteristics anticipated for the new fluid were expected to allow using fiber screens on solids-control units, thereby improving efficiency in solids removal.
Easier maintenance-in the absence of an emulsified aqueous phase, stability of the emulsion, which normally calls for careful monitoring and maintenance, would no longer need to be continually addressed. The single functionality of each of the system components should also make engineering more straightforward.
Improved logistics-it was expected that preparation of the fluid could be carried out routinely in a single tank, on location, without specialized equipment. Shipping of premixed maintenance volumes of mud from the mud plant would be eliminated. By greatly reducing the consumption of emulsifiers and wetting agents and by eliminating the need to use calcium chloride, drilling fluid inventories carried on the rig should be reduced. Overall, a significant improvement in logistics was sought.
Reduced costs-By improving hole cleaning and by providing the ability to easily reduce the rheology at casing points, it was expected that major savings in rig time, and therefore in overall drilling costs, would be achieved.

In addition, it was expected that the costs directly associated with drilling fluid maintenance would also be reduced.

By negating the requirement for maintenance of a stable emulsion (as is the case with an invert emulsion fluid) a major part of the normally accepted maintenance costs should be eliminated. It was anticipated that consumption of both emulsifiers and wetting agents would be greatly reduced.

The improvements anticipated in solids removal (as a result of the shear thinning properties claimed for the fluid) would also provide a means of reducing costs. The reduction in drill solids buildup would reduce the cost of maintaining the fluid.

It was intended that the all-oil fluid should equal or better the performance of the inverts in all respects.

INITIAL PERFORMANCE

The first application of the all-oil fluid on the project was on Well 5. In total, over 6,000 ft of hole was successfully drilled. Fluid densities varied from 11.5 to 15.4 ppg.

Some formation water was incorporated, but the water content rarely exceeded 2% by volume. Laboratory tests showed that 25-40% of the total system water was associated with oil-wet drill solids and therefore was relatively inaccessible.

There was a noticeable improvement in the all-oil fluid's Theological characteristics as compared to the invert fluids used previously. The all-oil fluid gave YP/PV ratios significantly higher than those observed for invert fluids having the same density. In addition, the all-oil fluid was observed to give substantially higher rheology in the low-shear-rate ranges.

The improved Theological characteristics of the fluid were credited with the reduction in the standpipe pressure, with the improvement in the cleaning performance in the hole and riser, and with the ability to run finer-mesh screens on the flow line cleaners.

Use of pills and reaming to mobilize cuttings accumulations were eliminated.

Not all the stated objectives were achieved, however. Regulation of the viscosity proved to be difficult. This was attributed to a lack of full understanding of the unique characteristics of the fluid. In addition, water wetting did occur during displacement, and API HTHP (high-temperature, high-pressure, fluid-loss test) control was unstable.

SUBSEQUENT DEVELOPMENTS

Based on performance on this initial well, it was clear that the Theological performance of the all-oil fluid was significantly better than that of the previously used invert-emulsion fluid. Development work continued on overcoming the shortcomings experienced in the first well.

Reformulation of the wetting agent eliminated water-wetting problems which had previously occurred during displacement. Moreover, in combination with a better educated approach to utilization of the organophilic clay viscosifier, the wetting agent provided the means for controlling the rheology under drilling conditions.

The need to effect substantial reductions in rheology (in advance of running casing) was addressed through introduction of an improved deflocculant. The necessity for diluting the fluid with oil was therefore eliminated.

While some success was achieved in providing fluid-loss control in the circulating fluid, this control was lost after hot rolling samples of the active mud.

In the light of problems being experienced, the entire approach to controlling fluid loss in fluids of this type was reevaluated. An intensive development project was undertaken and a new polymeric fluid-loss reducer was introduced on Well 9. This product proved to be successful in controlling the system's API HTHP filtration rate, both before and after hot rolling. Introduction of this product did not adversely affect any other fluid properties.

PERFORMANCE OVERVIEW

As drilling proceeded with the fluid on subsequent wells it became clear that all five initial objectives were consistently achievable.

Hole cleaning-The rheological profile of the all-oil fluid differs significantly from that of the traditional invert fluids, showing a much flatter Theological profile (Fig. 1). The differences are very pronounced in the low-shear-rate ranges with the all-oil fluid giving significantly improved performance (Fig. 2). It has become standard practice with this fluid to use the 6-rpm rheometer reading as the primary Theological parameter.
Approximately 10% improvement was made in daily footage rates by eliminating the short trips and the circulation of pills previously used to clean the riser. This improvement was achieved in spite of applying the same control over drilling rates with both invert and all-oil fluids.
Rheological and filtration data averaged over Wells 912 are shown in Table 3. Analysis of Bingham plastic model data shows significant decreases in PV values for the same YP's taken across the fluid-density ranges encountered.
Invert-fluid data are averaged over Wells 3 and 4. All-oil fluid data are averaged over Wells 7 and 8. While the YP was maintained in the same range for both fluids (Fig. 4), the PV's measured for the all-oil fluid were consistently much lower than for the invert (Fig. 3).
Standpipe pressures had been reduced and fluctuations in pressure eliminated. Higher circulating rates were therefore achievable.
Rheological control-The viscosifying clay is used, as required, to increase both the YP and low-shear-rate rheology of the fluid without significantly affecting the PV. The rheology derived from both the viscosifier and drill solids can be routinely depressed by use of the deflocculant. (Containment is achieved, while drilling, by use of the wetting agent.)
Recovery is easily achieved by addition of more viscosifying clay.
This flexibility reduced the time spent on conditioning the fluid prior to beginning casing and cementing operations. Moreover, the desired depression in rheology was now fully achievable. Whole mud losses sustained during casing and cementing operations were greatly reduced.
The shear thinning nature of the fluid was considered to be responsible for improvements in solids removal at surface through use of finer screens. (With the invert emulsion fluid, 110-150 mesh screens had been used. With the all-oil fluid, 175-210 mesh became the standard.) This improved solids removal also contributed to the improved Theological control.
Fluid maintenance-Each component in the system has a single primary function. Alteration of any given property is therefore simply achieved by addition of the appropriate product.
In practice, most of the material consumed while drilling with the all-oil fluid is associated with the routine need for increases in volume of fluid. (Those fluid additions are required due to increasing hole volume as drilling proceeds and loss of fluid adhering to cuttings removed at surface.)
As with an invert emulsion, the wetting agent is a consumed item and must be routinely replenished.
Logistics-Maintenance volume was routinely prepared in a single tank on the rig. No special mixing facilities were required.
Overall product consumption had been significantly reduced, and the need to stock calcium chloride on the rig site was eliminated. The result was a considerable reduction in the inventory carried at the rig site.
Maintenance costs-The plant makeup costs for the invert and all-oil fluids proved to be very similar (although, given the different characteristics of the fluids, it is difficult to make a strict comparison).

The cost distributions (by product function) for the two fluids were found, however, to be significantly different.

The all-oil fluid showed slightly increased elements of the makeup cost being centered on oil, barite, and fluid-loss additives. The major differences between the fluids, as regards makeup costs, are centered on viscosifiers and surfactants (in which are included all emulsifiers and wetting agents).

The invert fluids showed a much higher percentage of the cost going to surfactants. The all-oil mud showed a large cost bias towards the viscosifier.

Neither fluid system showed a high-maintenance cost for viscosifiers under normal drilling conditions. The invert fluids required high levels of surfactant additions while drilling, whereas surfactant requirements for the all-oil fluid were very much lower.

The overall picture of relative maintenance costs is dominated by the difference in surfactant-consumption levels.

The major reduction in surfactant costs with the all-oil fluid endow it with lower maintenance costs than was the case with the invert.

Another substantial cost saving was realized on the project through reduction of the mud losses at casing point.

This reduction in whole-mud losses is attributed to the ease with which rheology is manipulated with the all-oil system.

ACKNOWLEDGMENTS

The success of this project resulted from a team effort, depending on conscientious and dependable input from a large number of people. The authors would like to thank the numerous representatives of Conoco Inc., International Drilling Fluids Inc., and Sonat Offshore Drilling Inc. whose cooperation and combined efforts ensured the success of the project.

REFERENCES

Swan, J.C. Method of Drilling Wells, U.S. Patent No. 1,455,010, May 15, 1923.
Mazee, W.M., Non Aqueous Drilling Fluid, U.S. Patent No. 2,297,660, Sept. 29, 1942.
Dawson, R.D., and Blankenhorn, C.F., Non Aqueous Drilling Fluid, U.S. Patent No. 2,350,154, May 30, 1944.
Self, E.S., Oil Base Drilling Fluid, U.S. Patent No. 2,461,483, Feb. 8, 1949.
Anderson, F.M., "Oil Base Drilling Fluid," Oil Weekly, June 30,1947, pp. 43-50.
Fischer, P.W., Oil Base Drilling Fluids, U. S. Patent No. 2,612,471, Sept. 30, 1942.
Hauser, E.A., Modified Gel-Forming Clay and Process of Producing Same, U.S. Patent No. 2,531,427, Nov. 28, 1950.
Jordan, J.W., Nevis, M.J., Sterans, R.C., Cowan, J.C., and Beasley, E.A. Jr., N. Alkyl Ammonium Humates, U.S. Patent No. 3,281,458, Oct. 10, 1966.
Mondshine, T.C., and Kercheville, J.D., "Shale dehydration studies point way to successful gumbo shale drilling," OGJ, Mar. 28, 1966, pp. 194-205.
Mondshine, T.C., "New technique determines oil-mud salinity needs in shale drilling," OGJ, July 14, 1969, pp. 70-75.
Boyd, P.A., Whitfill, D.L., Carter, T.S., and Allamon, J.P., "Low Viscosity Base Fluid for Low Toxicity Oil Mud Systems," SPE Drilling Engineer, September 1987, pp. 218-228.
Holder, B.J., "Oil Mud Aids in Reducing Problems and Cost of North Sea Platform Development Drilling," JPT, June 1982, pp. 1199-1203.
O'Brien, T.B., Stinson, J.P., and Brownson, F., "Relaxed Fluid Loss Controls on Invert Muds Increases R.O.P.," World Oil, August 1977, pp. 31-34, 70.
Simpson, J., "A New Approach to Oil Muds for Lower Cost Drilling," SPE 7500, 1978.