Treatment of Oil-Based Drilling Waste Using Supercritical Carbon Dioxide
Authors: Street C.G. , Guigard S.E. 
Published: March 2010

Abstract
Oil-based drilling fluids are essential for challenging drilling operations. However, their use requires costly handling, treatment and disposal. Supercritical fluid extraction is herein investigated as a novel technology to treat this waste. Supercritical fluid extraction employs a substance above its critical temperature and pressure as a solvent. In this state, the substance has both liquid- and gas-like properties that can be controlled by the pressure and temperature of the extraction process.
In this paper, results of studies using supercritical carbon dioxide to remove the base oil from drilling waste are presented. Current work investigates the extraction of hydrocarbons (i.e. base oil) from a synthetic oil-based centrifuge underflow drilling waste. Extraction efficiencies as high as 98% have been observed. Additionally, results of both past and current studies indicate that the hydrocarbons are unchanged by the extraction process and that they may be recovered and potentially reused.

Introduction
In rotary drilling, drilling fluids are essential to lubricate the drill bit and circulate the drill cuttings to the surface. It is well documented that oil-based drilling muds (OBMs, sometimes referred to as non-aqueous drilling fluids or NADFs) have several advantages over their water-based counterparts(1, 2). OBMs have a higher natural lubricity, making them suitable for challenging drilling operations. Also, OBMs are less reactive with clays and shales, thereby preventing hole enlargement, resulting in smaller overall waste volumes. However, OBMs must be carefully handled and treated prior to disposal due to their potential negative environmental effects.
There are a number of options available to treat and dispose of OBM drilling wastes (for example, land spreading and landfilling), however the cost of handling and disposing of drilling waste is increasing. The drilling industry is now turning to novel approaches for the treatment and disposal of drilling wastes in order to meet more stringent environmental guidelines(3).
Supercritical fluid extraction (SFE) is an extraction technique that uses substances at or above their critical pressure and temperature as solvents. In the vicinity of the critical point, the liquid and vapour phases of the substance merge, producing a fluid with gas-like diffusivity and viscosity and liquid-like density(4, 5). These properties provide for favourable mass transfer of soluble waste components (i.e. hydrocarbons) from solid matrices to the bulk supercritical fluid. The density of the fluid is defined by the pressure and temperature; small changes in processing conditions can fine-tune the solvating power of the fluid(6). Additionally, supercritical fluids have zero surface tension, thereby allowing easy penetration into most matrices(4).
Several studies have documented the treatment of OBM drilling waste using SFE. In 1984, a patent by Eppig et al. detailed a system suitable for the removal of organic contaminants from inorganic matrices(7). This patent specifically lists the treatment of oil contaminated drill cuttings as an application of the technology, and indicates that propane, Freon and carbon dioxide would be suitable supercritical fluids for this purpose.
A later study by Eldridge investigated the use of a pilot-scale SFE system to treat oil contaminated drill cuttings from North Sea drilling platforms(8).