Qinglin Wu

Research at the LSU AgCenter includes fiber-reinforced plastic composites, which are widely used for such applications as building components and automobile parts. The AgCenter has patented a technology that uses composite blends reinforced by natural cellulosic and inorganic fibers to control what is termed “lost circulation” of drilling fluids in the oil and gas industry.

Drilling fluids, often referred to as drilling muds, are used in well drilling. The fluid, which may be water-, oil- or synthetic-based, circulates within the well bore, carrying cuttings to the surface, lubricating the drilling equipment and acting as a cooling agent. These fluids are lost when they enter into a porous or fractured formation and are not returned and reused. Many drilling hazards such as hole collapse, struck pipe and even blowout have been the result of lost circulation, costing the industry about $1 billion per year in the United States alone. And the lost materials themselves represent as much as an additional $250 million. High-performance lost circulation materials are needed to solve the problem.

The LSU AgCenter recently patented two composite lost circulation materials, TigerBullets and GEOUX HPHT. They are made of a thermoplastic polymer, cellulosic or inorganic fibers, and other additives for use in reducing lost circulation in drilling wells.

The TigerBullets product line is targeted for low- and moderate-temperature applications, while the GEOUX HPHT products are for high-pressure, high-temperature (HPHT) drilling applications. The materials are economical.  They seal fissures and cracks more rapidly, more efficiently and at higher temperatures than most commercially available lost circulation materials. They can be mixed with water to form slurries or muds shortly before being pumped into a well bore as part of the drilling fluid. In fractured formations the particles can settle, absorb water and swell in size, while maintaining rigidity. The swelling property, which is from the cellulosic-fiber-based blends, helps lock the particles into the fracture and seal it against drilling fluid leakage.

The use of the composite material allows more readily controlled distribution of the particles than mixtures of the individual components. In particular, the TigerBullets product line provides a significant value-added option for wood fiber resources in Louisiana.

Engineering
The composites can be manufactured using extruders with various ingredient combinations. Materials can include plastic and wood fibers (TigerBullets), inorganic mineral fibers (GEOUX HPHT), minerals such as calcium carbonate and other additives. The materials are mixed under heat and pressure to form composite particles, which are cooled and then pelletized or ground into granules of various sizes. For plastics-based TigerBullets blends, special extrusion techniques were developed. GEOUX HPHT products are compounded with engineering plastics, mineral fibers and additives in a single-step process at elevated temperatures. The compounds are then milled to form target particle size distributions.

Strength Properties
Lost circulation materials need to have sufficient strength to form strong bridges over fractures. The bridges will fail if the material is too weak and undergoes plastic deformation or brittle fracturing under the well-bore pressure. Thus, the maximum sealing pressure of a bridge or a plug is a function of the particles’ mechanical properties, such as compression strength, and their sizes and shapes relative to the fracture.

The PET-based TigerBullets are formulated with compression strength over 9,000 psi and GEOUX HPHT is formulated with compression strength of 12,000 psi, which are much stronger than most wood-based material (such as mixed nut shells), glass and concrete. Both laboratory and field test data show that these particles in combination with more flexible materials such as wood fibers form effective plugs.

Controlling Particle Size Distribution
For particulate-based lost circulation materials, particle size distributions, relative to the fracture size, particle composition, volume and viscosity of the mud, influence whether a seal is established in the fracture or at the fracture mouth. TigerBullets and GEOUX HPHT are made in various sizes through grinding and screening of the extruded materials. Different particle size distributions are formed by controlling screen size during grinding or by recombining the screened materials for different geological formations. Combinations include coarse particles to plug or bridge the largest opening in the formation and medium and fine particles to fill the voids between the coarse particles to produce a tight seal or filter cake.

Permeability Plugging Testing
Standard tests were performed with water-, synthetic- and oil-based mud systems. TigerBullets or GEOUX HPHT at various target loading levels were blended into the mud. Some basic mud properties influenced by the added TigerBullets or GEOUX HPHT were measured. The mixture was then loaded into a cylinder equipped with a slotted metal disc. A differential pressure of 1,000 psi was established, and initial spurt loss and filtration losses through the slot at various times were measured. Replicated tests demonstrated that TigerBullets and GEOUX HPHT had no negative effect on mud properties and provided quick sealing of the fracture by forming strong mud cakes.

Commercialization and Field Use
TigerBullets is currently being manufactured by Wallace Molding and Millwork Inc., in Columbia, La. – a traditional wood products company – and is marketed by MI-Swaco Inc., in Houston, Texas, (a Schlumberger company) and HolePluggers LLC in New Iberia, La. So far, more than 3 million pounds of material have been manufactured and sold. The material has been used by major oil companies, including BP, Exxon, Chevron, XTO, Pioneer and OXY in more than 300 oil wells across the United States. Markets for GEOUX HPHT are currently being developed for high-pressure, high-temperature drilling operations.

TigerBullets and GEOUX HPHT are designed to seal permeable formations, reduce differential sticking, increase lubricity, and control lost circulation. Additionally, they can be used as a lost circulation material for oil-well cementing operations. TigerBullets and GEOUX HPHT also can be used as a pretreatment to control differential sticking or seepage.

Both products use recycled green materials and are partially acid-soluble and nontoxic. They have combined rigidity and elasticity, controlled formulation, strength properties and particle size distributions, and serve dual functions as lubricants and lost circulation materials for minimizing differential sticking. The materials are compatible with synthetic-, oil- or water-based drilling fluids with improved field handling and performance. Most importantly, the TigerBullets product is based on renewable natural resources in Louisiana.

Qinglin Wu is Roy O. Martin Professor in the School of Renewable Natural Resources.

(This article was published in the fall 2012 issue of Louisiana Agriculture magazine.)