RIASSUNTO
Abstract
Deepwater drilling in the Gulf of Mexico is a challenging opportunity for drilling fluid performance. Synthetic-based muds (SBMs) applied in the Gulf of Mexico have overcome performance shortfalls of currently available water-based and oil-based muds. This paper will compare environmental, health, and drilling performance factors of polyalphaolefins (PAO) and internal olefins (IO) SBMs with emphasis on rheological profiles, acute toxicity, benthic toxicity, bioaccumulation, and biodegradation rates. Field case histories of PAO and IO SBMs used in deepwater projects are compared to water-based muds (WBM) and oil-based muds (OBM).
Introduction
The environmental and operational demands of offshore drilling on drilling muds have produced a need for new and better technology. Depletion of producing formations in existing fields has driven operators to search for new reserves in deepwater locations. Increasing environmental awareness has resulted in government agencies developing strict discharge regulations. In response to the need for better technology, advances in water-based mud (WBM) performance and oil-based mud (OBM) disposal technology have provided operators tools to meet the challenges of deepwater drilling. Synthetic-based muds (SBMs) offer a new option for meeting the environmental, drilling performance, and economic challenges of deepwater drilling.
Many of the drilling problems that are encountered in the Gulf of Mexico with WBM can be attributed to the interaction between the drilling fluid and high clay content formations that are frequently encountered. In addition to the problems normally associated with offshore drilling with WBM, deepwater drilling offers the additional challenges of gas hydrate formation in blowout preventers, and hole cleaning problems in the large diameter riser sections.
OBM has been used to minimize problems with difficult shale formations encountered in Gulf of Mexico drilling operations. However, offshore discharge of OBM is prohibited in the USA and severely restricted in the North Sea because of environmental concerns. In addition to the problems of drilling with OBM offshore, deepwater drilling offers the additional challenges of gas solubility in oil-based mud, logistical transportation problems for waste disposal, and oil spill liability from emergency disconnects of the riser.
Introduction of SBMs offer solutions to the limitations of traditional oil-based and water-based muds. Limitations of WBMs are addressed by SBM because the formation is exposed to a synthetic liquid and not water, the high clay content shales do not hydrate and expand into the wellbore. Gas hydrates are suppressed in SBM and the stability and flexibility of rheological profiles allows proper hole cleaning of the riser. Limitations of OBM use are addressed because SBMs can be discharged into the marine environment thereby eliminating logistical problems of OBM disposal and oil spill liabilities from emergency riser disconnects. In addition, gas is less soluble in SBM than in mineral and diesel oil-based muds allowing for easier detection and control of gas kicks.
Since their introduction into the Gulf of Mexico in 1992, SBMs have proven their cost effectiveness in the current framework of US and Mexican offshore discharge regulations. Because the cost per barrel of SBM is much higher than WBM or OBM, loss of whole mud to the formation is a significant concern. Consequently, high equivalent circulating densities (ECDs) caused by high viscosity SBM is an operational concern compared to OBM and WBM.
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