ABSTRACT
The paper reviews and analyses performance, toxicity and cost data generated during the development and field application of a low-toxicity (LT) oil mud system. During the development phase approximately twenty "white mineral oils" were studied and evaluated as prospective diesel-oil substitutes. Their mud-formulating properties and toxicity to a variety of aquatic species were compared with those of diesel oil. The LT oil muds, to date, have been used successfully to drill over seventy wells.
The advantages of drilling with an oil-based mud are well documented. These include improved well bore and formation stability, high lubricity, fluid stability at high temperatures and pressures, corrosion protection and tolerance to contaminants. However, the LT oil muds offer an additional advantage, namely, environmental compatibility, not hitherto available, with diesel-based systems.
Government regulations restricting the discharge of oil and other toxic substances into the marine environment have precluded the use of diesel-oil systems in certain areas. Further, operators have often found cuttings-wash systems to be ineffective for removing diesel mud from cuttings. Recently, several governments have approved the direct discharge of LT oil mud cuttings which makes these systems cost effective and thus highly attractive.
The white mineral oils were found to contain significantly less aromatic hydrocarbons than diesel oil, were less toxic and fluoresced to a lower degree.
Kinematic viscosities ranged from 6.6 to 1.0 at 40°C providing great flexibility in adjusting the rheological properties of the mud to meet specific conditions. Approximately eleven oils have been used in mud formulations and successfully employed in drilling wells.
The LT oil muds were found to perform as well as diesel-based systems with a marked improvement in penetration rate in many cases.
Toxicity studies were conducted by several reputable laboratories in Europe and the United States. The results indicated that the non-toxic muds had LC50'S which ranged from 10,000 ppm to 50,000 ppm for the brown shrimp Crangon crangon; and 100,000 ppm for the plaice, Pleuronectes pletessa. Corresponding results for the diesel oil systems were 160 ppm C. crangon and 4,700 ppm P. platessa. Several of the LT systems have been approved by European governments for the direct discharge of unwashed cuttings.
