An Environmental Study To Assess the Effect of Drilling Fluids on Water Quality Parameters During High-Rate, High-Volume Discharges to the Ocean
Authors: Ayers Robert C. , Meek Robert P. , Sauer Theodor C. , Stuebner David O. 
Published: December 2007

A drilling mud dispersion field test showed that as a result of rapid settling and dilution, suspended-solids and tracer-metal concentrations in the water column decrease rapidly with distance from the discharge source. It is concluded that drilling fluids have a negligible effect on open-ocean water quality even during high-rate, less than volume discharges.

Introduction

Upon completion of an offshore well, it is often necessary to discharge excess drilling mud. In exploratory wells, this is done rapidly to avoid costly delays in moving the rig off location. This paper presents results from a drilling fluid dispersion study conducted to determine mud-component concentrations in the water column during high-volume, high-rate mud discharges representative of the type that normally occur when drilling is completed. Previous investigators have studied the effects of drilling discharges on water quality. However, most of this work was directed toward determining effects during low-rate cuttings discharges or was designed primarily to answer specific questions about a particular drillsite. These studies did not directly address the effect of high-volume, high-rate discharges. In addition, the development of improved sampling equipment designed specifically for this type of test allowed a more complete description of plume properties in the water column. The field test was conducted in Summer 1978 from the jackup rig Penrod 63 in the Gulf of Mexico. The rig, under lease to Exxon Co. U.S.A., was located in East Cameron Block 121, approximately 96 km south of Cameron, LA in a water depth of 23 m. The test was carried out using a high-density mud discharged under calm sea conditions to demonstrate a worst-case condition for dispersion of drilling fluids offshore.

Methods

During each discharge test, the mud was pumped from the mud pits through a system of pipes across the mud ditch and down a discharge pipe immersed 3 m below the surface. The rig's mud pumps were used to pump the mud at a constant rate. Two discharge tests were performed on consecutive days. On the first day, 250 bbl of mud were discharged for 54.5 minutes at a constant rate of 275 bbl/hr and, on the second day, 389 bbl of mud were discharged for 23.3 minutes at a constant rate of 1,000 bbl/hr.

Currents, Winds, and Waves

Current data were obtained with an array of three self-contained ENDECO Model 105 ducted impeller current meters. The meters were positioned approximately 15 m from the platform's bow support leg at depths of 7, 14, and 21 m. The data cartridges (film) were developed, analyzed, and computerized at ENDECO Data Services. A Weather Measure Corp. Model W103 anomometer and vane were used for wind-velocity measurements. The wind speed and direction system was installed on the rig 35 m above the water surface. Continuous strip-chart recordings were averaged for 15-minute intervals throughout each discharge sampling. Wave height data were recorded with a Senso-Metrics Inc. Model SP-91 pressure transducer. Measurements of wave-induced water motions (speed and direction) were collected with a Marsh-McBirney Inc. Model 511 electromagnetic current meter (EMCM). Both of these systems were placed at 3-m depths. The installation was designed to minimize the influence of wave refraction from submerged platform structures. The resulting analog tapes were digitized and formatted on computer tape and transformed and analyzed to produce estimates of spectral density as well as steady currents.

Photography

JPT

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