Characterization of a New Green Material for Offshore Well Completions and Downhole Treatments

Abstract

In oil and gas well construction during the drillouts or wellbore cleaning process, one of the most critical functions of land and offshore completions fluids is the ability to suspend solids effectively under extreme downhole conditions. Conventional agents such as xanthan gum, HEC and numerous other polymers have historically been used to accomplish this function, albeit with limitations. Functionally, these commonly used polymers depend primarily upon viscosity rather than elastic characteristics to suspend solids and require intensive chemical processing that leads to high deployment cost. Recently microfibrillated cellulose (MFC) has been investigated as a prospective suspending agent in carrier fluids for extreme downhole conditions. MFC is a unique type of superfine cellulose fibrils obtained from fully sustainable sources that have been subjected to proprietary treatment procedures, resulting in fibril bundles consisting of lateral dimensions in the sub-micron scale and lengths up to micron scale with abundant terminal hydroxyl functional groups. When dispersed into aqueous solutions, the resulting fluid has been characterized to have several favorable rheological, chemical and mechanical properties. Rheological measurements show the viscoelasticity of MFC dispersions is dominated by their storage modulus (G′ > G″) even with fluids formulated with as low as 0.25 wt% (about 20 lbm MFC /1000 gallons). The result is a suspension that exhibits superior particle suspension properties compared on a mass basis to conventional materials such as guar, CMC, HEC and xanthan gum. In addition, MFC solutions exhibit comparatively high viscosities at low shear rates but thin by several orders of magnitude at high shear, a characteristic that implies less work on surface equipment while having the ability to suspend solids at very low pump rates. MFC dispersions also have an excellent brine tolerance, demonstrating stable suspensions over weeks in fluids containing up to 150,000 mg/L TDS. The dispersions are stable at downhole relevant temperatures, applicable at low and high pH levels and resistant to shear degradation. Finally, MFC originates from natural resources and is environmentally benign and biodegradable. This paper describes the comprehensive characterization of the rheological and suspension properties that distinguish MFC from other conventionally used materials and make it fit-for-purpose as a robust, environmentally benign and high-performance suspending agent for downhole applications.