EVALUATION OF SEEPAGE FROM EVAPORATION PONDS OF PRODUCED WATER IN SOME SUDANESE OILFIELDS

Abstract

Produced water associated with crude oil production is considered as waste water. In most of the Sudanese oilfields processing facilities, evaporation ponds are used to store the produced water for evaporation or further treatments and reuse. The objective of this work is to study the probable effects of seepage into soil from existing evaporation ponds that were constructed many years ago to store produced water from field production facilities (FPFs) in five Sudanese oilfields.
Seepage assessment methodology involved of electrical resistivity measurements, drilling boreholes and collecting soil samples at different depths, geotechnical laboratory tests, water and soil tests and finally incorporates all testing results and geophysical interpretations for each location into a computational model to predict seepage front in vertical and
horizontal directions. Electrical resistivity method basically the vertical electrical sounding (VES) has been employed during this study. The geophysical work was carried out with sensitive Mini-Sting Resistivity meter manufactured by Advanced Geosciences Incorporation. More than 60 VES measurements were conducted using Schlumberger array.
Geotechnical investigation program was carried out included field work and laboratory testing. The investigation started with the field work by drilling 10 boreholes, collecting representative disturbed and undisturbed soil samples. Soil profiles and the engineering properties of the subsoil were tested and interpreted.
Water and soil samples were tested in two laboratories for environmental properties, pH, TDS, oil content and heavy minerals. Historical data about water quality from the evaporation ponds as well as from nearby water wells were collected and compared with the lab results. It was found that, the majority of these tests showed normal concentration of different chemicals in evaporation ponds, soil till depth of 15 meters and other oil wells till depth of 400 m. Thorough comparison of these test results indicates no seepage evidence in these waters and/or soils. Comparison is made by
investigating old data with new data from several laboratories for TDS, pH and other constituents to see if there is considerable change with time. When comparing the test results to the national and international limits for pollution discharge, the obtained results are far below standard limits.
Computational seepage model was created to predict seepage advancement in each location. It has been found that both the current seepage level and the predicted one will not reach the ground water static level in all five locations. Risk assessment was performed for five different scenarios. Generally, and according to the laboratory test results for the
five locations, the likelihood for vertical or lateral seepage to contaminate ground/surface water and soil is rare, i.e. extremely unlikely or impossible.