Introduction
Lignosulfonate Dispersed Systems
Lignosulfonate mud systems have been the most popular mud type used in Gulf of Mexico drilling for nearly thirty years. These muds provide excellent rheological properties and filtration control over a wide range of mud densities. They also tolerate high concentrations of drill solids and common contaminants such as salt and cement. Unfortunately lignosulfonate muds must be run at relatively high pH values because of the solubility and other characteristics of lignosulfonate itself. The most common pH range for lignosulfonate muds used in the Gulf of Mexico is 10.5 - 11.5. Lime treated lignosulfonate muds are run at even higher pH values (11.0 - 12.5).
The high concentration of hydroxyl ion (OH) contained in these muds results in their highly dispersive nature toward clay bearing formations such as shale. The hydroxyl ion serves to cleave clays, exposing more surface area to hydration by the fluid. As a result, lignosulfonate muds have long been associated with poor borehole stability. Washed out holes with poor caliper logs are a common occurrence. Many products and variations in formulation have been devised to improve this situation, but none have completely offset the strong effect of the hydroxyl ion on formation clays.
Non-dispersed Polymer Systems
The non-dispersed polymer muds have been more successful in this respect, since they do not require high pH in order to function. The weakness of these muds is that they tolerate drill solids poorly, and do not have good rheological properties and filtration control over the wide density range needed in Gulf Coast drilling. Stuck drill pipe and high treatment cost have plagued these non-dispersed muds throughout the Gulf of Mexico. This is particularly true at high mud weights.
Low pH Polymer Dispersed System
The low pH polymer dispersed mud system described here shows all of the many advantages of lignosulfonate dispersed muds and polymer muds, but without the adverse effects caused by high concentrations of hydroxyl ions. This system is the result of pioneering work by Ron Brazzel and colleagues at Tenneco Oil Co. Chemsperse/Desco are powerful mud thinners that are soluble at neutral pH. They also exhibit better high temperature stability and solids tolerance than lignosulfonate thinners. A Chemsperse/Desco thinned mud can be run at much lower pH (8.0 - 8.5) than lignosulfonate thinned muds. A low pH polymer dispersed mud has nearly identical rheological and filtration properties as a lignosulfonate thinned mud of the same density. The difference in effect of these muds on formation clays is dramatic however, and is closely related to the difference in hydroxyl ion and polymer concentration.
The low pH of the Chemsperse/Desco thinned mud makes it compatible with PHPA (partially hydrolyzed polyacrylamide) polymers. PHPA is a powerful inhibitor of formation clays. This increases the shale stability of the low pH polymer dispersed system. This system is also very economical to run. Penetration rates are improved because of reduced bit balling. In addition, low gel strengths with slick, thin filter cakes and excellent tolerance of solids are achieved. Excellent drilling results have been achieved with this type of mud in all areas in the Gulf of Mexico region.
System Guide
Introduction
The drilling of reactive shales can present a myriad of problems from hole washout and bit balling to borehole instability. Many systems have attempted to address such conditions. In many cases the cost of these alternatives is prohibitive. The low pH polymer mud addresses the above problems and can be efficiently run from a completely non-dispersed state on one end of the spectrum to a fully dispersed one. The mud's state can be tailored to individual well conditions. Having a cost comparable to lignosulfonate muds and familiar properties and appearance, its use is becoming more widespread.
pH, the most critical property for a properly run low pH polymer system, should be kept in the 8.0 - 8.5 range. Although generally run at chloride ranges of 4,000 - 6,000 mg/l, it can be run as a seawater system (10,000 - 21,000 mg/l). As with lignosulfonate muds however, chemical efficiencies are decreased.
The system can be safely run at the higher mud weight ranges of 18.5 ppg and temperatures of 340° F.
System Make-Up
A low pH polymer mud is usually placed in the hole after surface casing has been set. It can be done either as a complete system displacement or a 40 - 50 % whole mud dilution. Factors such as T. D. mud weight, number of days to T. D., and formation type to be drilled will influence the method chosen.
The polymer volume should be built using the attached formulations as a guide. Actual concentrations of products should be tailored to the individual well. For example, if the primary formation to be drilled is highly reactive shale, the higher concentrations of PHPA (1.5+ ppb) will be required.
After displacement or partial polymer mud dilution the mud should be sheared through the bit and shakers by-passed for at least one circulation to prevent polymer blinding of shaker screens. This can be avoided if a shearing unit is applied to the pits for a sufficient time prior to displacement.
Maintenance
The low pH polymer system is easily maintained using PHPA for encapsulation and inhibition, Chemthin (spa) and Chemsperse/Desco for rheological control, presolubilized lignite and PAC for filtration control, and sulfonated asphalt (Soltex) for borehole stability and HTHP control. Presolubilization of all chemicals adds to the chemical efficiency and improves overall mud performance. Shearing of polymers through a shearing unit also reduces viscosity humps associated with polymer additions and reduces the chances of polymer blinding of shaker screens.
The MBT of the mud system is normally run at 17.5 - 25 ppb equivalent. However, in situations where large and numerous sand sections are to be encountered, the MBT may be increased to 30 ppb (with LGS in the 6.0% range) with bentonite to provide the commercial quality clays necessary to adequately seal off the sands. Conversely, if shale is to be the primary formation drilled, xanthan gum would be supplemented for barite suspension and viscosity, and MBT's in the 17.5 - 20 ppb equivalent would be targeted.
Low gravity solids and MBT's can be controlled with solids control equipment (centrifuge in weighted systems) and water. However, if MBT's and solids are climbing rapidly with equipment functioning properly, this may be an indication of inadequate PHPA concentrations. Increasing PHPA concentrations in .25 - .50 ppb increments may be required. Low gravity solids should be maintained in the 6.0 - 7.0% range depending on the mud weight. No special solids control equipment is required for this system. As with other dispersed water based systems, chemical maintenance is reduced when solids are efficiently removed mechanically.
Solids control efficiencies in the 65 -70% range are considered the average and can be accomplished with a set of double deck rig shakers cascaded to Derrick linear shakers (flowline cleaners) along with desander and desilter in unweighted conditions and centrifuge in weighted conditions (> 12 ppg). If care is given to optimizing screen sizes on each shaker, maximum removal of encapsulated solids on the first pass can be achieved. When not removed, they deteriorate into finer particles as they are re-circulated. This resulting increase in solids, both in quantity and surface area, increases chemical maintenance costs and can lead to poor hole conditions.
Click here to download an Adobe Acrobat version of this article, which includes case histories and usage guide.