Comparison Study of Solid/Liquid Separation Techniques for Oilfield Pit Closures
Authors: Wojtanowicz A.K. , Field S.D. , Ostermann M.C. 
Published: December 2007

Summary.
Vacuum filtration, belt-press filtration, screw-press filtration, and centrifuging techniques were evaluated in full-scale experiments for use in oilfield waste volume reduction. Centrifuging and belt-press filtration proved applicable to oilfield pit cleanups. Also, an effective chemical conditioning (coagulation and flocculation) was found for deliquoring seven types of oilfield waste slurries before separation.

Introduction
Earthen pits have been the most commonly used impoundments for collecting oilfield wastes. Originally designed to serve as drilling reserve pits, they have been storing about 300 million bbl [47.7 Mm3) of drilling fluid wastes and cuttings each year. In most cases of water-based drilling fluids, these pits were closed by conventional methods, such as pit dispersing or pit pumping. In many cases. however, the drilling reserve pits were either abandoned or converted for oil production and testing and became suspected sources of long-term environmental contamination. There are, for example, about 12,000 abandoned pits in Louisiana that are suspected of chronic damage to the environment and to public health. A public survey reported environmental problems related to oil-waste disposal in Colorado, California, Texas, Nevada, Oklahoma, and Michigan. After the Solids Waste Disposal Amendments to the Resource Conservation and Recovery Act (RCRA) exempted drilling fluids and other associated wastes from the direct application of the federal RCRA rules, each state became responsible for regulating, the storage and disposal of oilfield waste. Regulations recently issued in Louisiana require that oil-waste pits be closed within 6 months by dewatering, followed by burial, trenching, land treatment, or solidification. Amendments to Texas Railroad Commission Rule No. 8 require closure of pits with chloride concentration levels below 6,100 ppm within one year and closure of pits with higher chloride concentration levels within 30 days after drilling operations end. Their recommended methods are dewatering, backfilling, burial, or land farming. Analysis of other local regulations regarding oilfield waste disposal indicates that reduction of a drillsite waste with solid/liquid separation (SLS) is the first and fundamental step toward solution of the disposal problem. Most literature on the environmental implications of oil exploration and production is dedicated to either an assessment of the environmental impact of oilfield waste and its characterization or the promotion of available cleanup systems. The environmental fate of oilfield offshore operations has been documented in more than 650 technical papers. Only within the past 10 years has emphasis been placed on the onshore environmental impact of oilfield waste disposal, sampling and characterization of drilling reserve pits, and pit closure procedures. Improvement in solids control and development of new techniques for oilfield-waste volume reduction are being investigated in several industrial laboratories. However, very little has been published to date. The research effort reported here is part of a continuing experimental program concerning the development and implementation of environmental techniques in petroleum engineering practices. The problems addressed are related to waste volume reduction resulting from SLS. Specifically, answers have been sought to the following questions.Is it possible to use typical oilfield solids-control equipment (centrifuges) for waste pit cleanup?Do other separation techniques-such as vacuum filtration, belt pressing, or screw pressing-provide a valid alternative to pit dewatering?To what extent is chemical conditioning (coagulation and flocculation) applicable to oilfield waste?How does the separation mechanism of oilfield waste differ from existing dewatering techniques (e.g., waste water treatment)?

Experiment Design
All experiments were performed at the Louisiana State U. Solids Control Environmental Laboratory. The laboratory consists of three units: a full-scale unit, an analytic unit, and a portable pit-sampling unit. The full-scale unit is designed for pilot experiments on SLS with handling, pumping, and chemical injection. Its capacity allows for a 200-bbl [31.8-m3] sample volume, a 110-gal/min [0.4 16-m3/min] feed rate, and an 8.4-gal/min [0.031-m3/min] on-line polymer injection rate. The analytic unit is designed for solid/liquid analysis and bench-scale simulation experiments. The pit-sampling unit is designed for field sampling and mapping of oilfield waste impoundments.
JPT
P. 845^