Calcium carbonate (CaCO3) has been used to make high-density drilling muds. However, when drilling 5⅞-in. hole, sticking occurs because of the high concentration of CaCO3 required. To minimize sticking, barite (BaSO4) is added to reduce the amount of solids needed. However, barite is damaging because it does not dissolve in commonly used acids. Drilling fluids have been developed at a wide range of densities using calcium chloride (CaCl2) salt with manganese tetroxide (Mn3O4). The small particle size, spherical shape, and high specific gravity of Mn3O4 make it good weighting material to reduce solids loading and settling compared with CaCO3 and BaSO4.
Introduction
Several factors need to be considered when designing drilling fluids. For example, formation permeability determines filtration characteristics. Temperature- or water-sensitive formations determine the type of polymer and type of drilling fluids needed. Important considerations in designing drilling fluids include rheological properties to provide good carrying capacity and minimum filtration control loss.
Different polymers are used for filtration control, viscosity modification, and shale stabilization. Different types of polymers can provide different performances in rheology and filtration. Some polymers, such as xanthan gum and starch, lose viscosity at high temperatures because of degradation and instability at harsh conditions. Some polymers are not effective in salt solutions because salt inhibits hydration of polymers, affecting their functions. Other polymers are less sensitive to salt drilling fluids. Polymer compatibility with drilling fluids is important to achieve good suspension, rheology, and filtration control to ensure good hole cleaning and less formation damage.
Designing drilling fluids depends heavily on the selection of the suitable size of weighting materials that will work as bridging materials as well. Once the solids invade the formation, they cannot be removed by natural flow.
The requirements for water-based drilling fluids for horizontal wells include physical stability, cuttings transport, lubricity, and formation-damage control. The hydrostatic pressure of the drilling fluids must be high enough to control the formation pressure but not so high as to fracture the formation and lose circulation. The use of bridging materials is important to minimize filtrate invasion, minimize fines migration, and improve hole stability. In deviated wells, cuttings accumulation and settling while drill-in fluids are static are major concerns. The drilling fluids should have good rheological properties to prevent solids and cuttings settling. Coreflood testing also is important to evaluate drilling-fluids formation damage, as is laboratory evaluation at reservoir temperature and pressure.
In 2007, a potassium chloride (KCl) water-based drilling fluid weighted with Mn3O4 and a small amount of CaCO3 was developed. CaCO3 was added to control the filtration properties of the drilling fluid. The need for using a drilling fluid with high rheological properties was achieved using Mn3O4 particles.
The aim of this study is to discuss laboratory work that was performed to design water-based drill-in fluids using CaCl2/Mn3O4 at 95 lbm/ft3 and 300°F.
CaCl2 Salt
CaCl2 can be used in combination with gypsum (calcium sulfate) when drilling evaporates sections. If the drilling fluid is not saturated, it will be washed out. In this formulation, hydroxyl ethyl cellulose (HEC) is used to build up viscosity. HEC polymers are nonionic derivatives of the cellulose polymer, modified to impart water solubility to the cellulose molecule. The nonionic substitution in HEC polymers makes them very tolerant to high salt environments, including divalent calcium and magnesium. However, HEC polymers alone are considered non-thixotropic. Xanthan-gum polymer functions quite well in CaCl2 fluids as long as the polymer is properly sheared in the initial mix. Xanthan-gum polymer is one of a very few polymers that will build gel structure. This, therefore, makes xanthan-gum polymer a key ingredient when solids suspension is required.
Experimental-Studies Materials
All additives used in this study are typical materials for drilling fluids, such as polyanionic cellulose polymer, starch, and lignite.
Starch was the first organic polymer used in large quantities in drilling fluids. It is used as a filtration-control additive. Starch is the primary component of the seeds of cereal grains and a significant component of foodstuff such as rice, wheat, corn, and potato. The assigned formula is (C6H10O5·H2O)n. Starch is degraded at temperatures of 250°F; it is used in concentrations from 2 to 10 lbm/bbl.
Xanthan gum is a water-soluble polysaccharide produced by bacterial action (genus Xanthomonas) on carbohydrates. Laboratory tests showed that xanthan-gum polymer has better suspension properties compared with any other type of polymer. Xanthan gum is used in concentrations from 0.2 to 2 lbm/bbl.
Polyanionic cellulose polymer is used as a filtrate reducer and supplementary viscosifier. It is effective in fresh water, seawater, and salt-saturated fluids. Polyanionic cellulose polymer can be used where extra viscosity is not required and is used in concentrations from 0.2 to 5 lbm/bbl.
Calcium hydroxide is a hydrated lime. It is used in high-calcium-ion drilling fluids to increase the pH. It is used in concentrations from 0.5 to 20 lbm/bbl.
Potassium hydroxide is used to increase pH in KCl drilling fluids. It is used in concentrations from 0.5 to 3 lbm/bbl. CaCO3 is used as weighting material because the filter cake that is formed on the productive formation can be removed by treatment with hydrochloric acid. Its low specific gravity (2.6 to 2.8) limits the maximum mud density to 12 lbm/gal.
The most important ores of manganese are pyrolusite and hausmannite (Mn3O4). Mn3O4 is prepared by heating manganese dioxide in air at 1000°C. Mn3O4 is brown to black in color, with a molar hardness of 5.5 and a specific gravity of 4.8. The properties of these (D50=1 µm)—spherical shape, high specific gravity (4.8 g/cm3), and solubility in acids—make them good weighting material compared with CaCO3 (2.78 g/cm3 and D50=10 µm) and BaSO4 (4.25 g/cm3 and D50=20 µm).
Lignite is a product designed to provide filtration control and rheological stability over a wide range of temperatures. It can be used in water-based drilling fluids up to 400°F. It is notably effective in high-density mud. Normal treatment concentrations range from 2 to 6 lbm/bbl, depending on the degree of filtration control desired.
Vinyl amide/vinyl sulfonate copolymers are terpolymers consisting of N-methyl-N-vinylacetamide, monomer acrylamide, and vinyl sulfonate monomer 2-acrylamide-2-methyl-1-propane-sulfonic acid. They are used as a high-pressure/high-temperature (HP/HT) filtration-control additive for water-based muds.
Vinyl amide/acrylic polymer is an extremely-low-viscosity synthetic polymer for filtration control in water-based drilling fluids. Vinyl amide/acrylic polymer is effective in calcium chloride concentrations up to 100,000 ppm and brines containing Na, K+, and Mg++ concentration up to saturation and is stable in a wide range of pH environments.
Procedure
Many attempts and tests were made to formulate Mn3O4 water-based drilling-fluid formulations in order to achieve the final formulations suitable for deep drilling. The tests needed are
- Rheological properties
- American Petroleum Institute (API) and HP/HT standard filtration
- Thermal stability
Intensive laboratory work was conducted on the basis of API Recommended Practice 13 procedure to design Mn3O4/CaCl2 water-based drill-in fluids at a density of 95 lbm/ft3 at 300°F.
Results and Discussion
The presence of oxygen in drilling fluids can accelerate corrosion rates and degradation of water-soluble polymers. An oxygen scavenger can be used to remove the oxygen. Sodium sulfite is an example of an oxygen scavenger.
The drilling-fluid properties measured included plastic viscosity, yield point, filtration (API and HP/HT), filter-cake thickness, and drill-in-fluid pH. To assess thermal stability, the drill-in fluid was aged for 16 hours at 300°F and the properties were measured again.
Polyanionic cellulose polymer was used to formulate Mn3O4/CaCl2 drilling fluid. Good rheology properties were obtained, but having controlled fluid loss was the concern. High concentrations of CaCl2 were used, so calcium bromide was added to minimize recrystallization.
The CaCl2 concentration was reduced, and the Mn3O4 amount was increased to investigate the compatibility of polyanionic cellulose polymer with Mn3O4/CaCl2 drilling fluid. Starch and CaCO3 were added to improve fluid-loss control. However, no control was ob-served even before hot rolling.
Vinyl amide/acrylic polymer was used to replace starch to control the fluid loss. Good rheology properties were obtained, but no fluid-loss control was achieved even after adding more solids (CaCO3 and Mn3O4) and reducing the CaCl2 salt concentration. Vinyl amide/acrylic polymer was not compatible with the Mn3O4/CaCl2 drilling fluid.
Lignite was used in the Mn3O4/CaCl2 drilling fluid in an attempt to control fluid loss. Mn3O4 was used only in the formula without CaCO3 to evaluate its filtration performance with lignite. Xanthan-gum polymer and polyanionic cellulose polymer were used to evaluate their rheological performance. Exposure of drilling fluids to reservoir temperatures might change the fluid’s rheology and filtration. Good fluid-loss control was observed before and after hot rolling at 300°F and 300 psi for 16 hours.
Lignite was used again but with lower concentration of CaCl2 salt and with higher concentration of Mn3O4 and only 5 lbm/bbl of CaCO3. Good rheology and fluid-loss control before and after hot rolling were observed with this formulation.
Finally, vinyl amide/vinyl sulfonate copolymers were tested without CaCO3 and high concentrations of CaCl2. The best drilling-fluid properties were obtained when using vinyl amide/vinyl sulfonate copolymer in the Mn3O4/CaCl2 drilling fluid, even after hot rolling at 300°F and 300 psi for 16 hours.
In this study, the authors showed that lignite and vinyl amide/vinyl sulfonate copolymers are better than polyanionic cellulose polymer and starch in providing good rheological stability and filtration control for Mn3O4/CaCl2 drilling fluids.
Conclusions
In this study, Mn3O4/CaCl2 drilling fluids (95 lbm/ft3) were designed and tested. On the basis of the results, the following conclusions can be drawn:
- An oxygen scavenger was added to polyanionic cellulose polymer to extend its stability and provide good rheological properties. However, the filtration control was not good.
- Adding starch and CaCO3 and polyanionic cellulose polymer did not solve the fluid-loss-control problem.
- Vinyl amide/acrylic polymer was not compatible with Mn3O4/CaCl2 drilling fluid. No fluid-loss control was achieved, even after reducing the CaCl2 concentration.
- Lignite and vinyl amide/vinyl sulfonate copolymers are better than polyanionic cellulose polymer and starch in providing good rheological stability and filtration control for Mn3O4/CaCl2 drilling fluid.
- The use of a small concentration of CaCO3 (5 lbm/bbl) with Mn3O4 (203 lbm/bbl) improved the filtration and reduced the filter cake compared with using Mn3O4 alone.
This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 169315, “Innovative Manganese Tetroxide/CaCl2 Water-Based Drilling Fluids for HP/HT Wells,” by Abdullah S. Al-Yami, SPE, Abdullah Al-Nimer, SPE, Abdulaziz Bubshait, SPE, Omar Al-Fuwaires, SPE, and Ali Al-Wabari, SPE, Saudi Aramco, prepared for the 2014 SPE Latin American and Caribbean Petroleum Engineering Conference, Maracaibo, Venezuela, 21–23 May. The paper has not been peer reviewed.