Development and Implementation of a Production Water Master Plan

Abstract

ABSTRACT Petroleum Development Oman (PDO) currently produces 206,000 m3/day of water together with its oil output of 120,000 m3/day. The volume of production water has been steadily increasing over time and is predicted to rise to about 460,000 m3/day by the year 2005, representing an increase of 130%. Approximately 38% of the produced water is presently utilised for reservoir pressure maintenance by injection, mainly into the Yibal field in North Oman. The remaining 113,000 m3/day cannot be used industrially and is presently disposed of into shallow (150-400 m) sub-surface formations at 11 oilfield sites in North Oman. The majority of this water is presently disposed of into shallow non-potable aquifers. Although the sub-surface disposal of production water is an accepted practice within the oil production industry, it is essential to ensure that the disposed water is adequately sealed from exploitable aquifers and cannot break through into other formations or to the surface. The production waters are highly saline and contain a number of contaminants that prohibit their usage without further treatment. Principal contaminants include traces of crude oil, trace heavy metals and residuals of production chemicals. At present a very small quantity of oil, some 40 m3/day or 0.04% of production, remains in the disposal stream; mainly due to de-oi!ing difficulties encountered in the heavier oil field accumulations in South Oman. Detailed analyses have been made of production water quality and the natural trace element concentrations in me various oilfield waters have been quantified. Production waters from South Oman, which originate from older Permian formations generally have lower concentrations than those that originate in the Cretaceous formations of Central and North Oman. Over recent years PDO has investigated ways and means of improving production water quality by improved dehydration and de-oiling technologies. PDO is co-sponsoring research on the application of hydrocyclones, gas flotation, membrane filtration and media filtration with the intent of improving both oil recovery and disposal water quality. Nevertheless the costs of the different technologies to improve disposal water quality remain the important factor when viewed against the background of petroleum economics in Oman. The interior of Oman is an arid desert environment with no surface water resources and subject to only rare rainfall events. There are however some extensive sub-surface Tertiary aquifers containing groundwater that varies from potable in the south of the country to highly saline in the north. The regional hydrogeology has been well defined from the E&P operations and this information has been used to ensure that PDO's current practice of shallow sub-surface disposal does not contaminate any potable aquifers. There are no specific regulations in the Sultanate of Oman for disposal of production water generated by the petroleum industry. Waste water disposal regulations in Oman are directed at the conservation of drinking and irrigation water as a national resource. In addition, one of the objectives of the regulations is to ensure the safety of any waste water that may become accessible to humans or livestock. Waste water is defined in the regulations as "any polluted liquid discharged from an domestic, trade or industrial or institutional premises". PDO's Corporate Objectives includes the obligation to conduct its operations so as to "protect the environment, prevent pollution and seek improvements in the efficient use of natural resources". The Company has therefore compiled a Production Water Management Plan (PWMP) with specific proposals for each disposal centre with the prime objective of minimising any environmental impact. The PWMP calls for a number of new initiatives. The basic philosophy that has been applied has been first to reduce the volumes of production water, then to re-use within the industry any produced water and finally, to dispose any surplus water in an environmentally acceptable manner in accordance with the Omani legislation. The strategy to be utilized in all cases will be, in order of preference:- Minimise the volumes of water produced during oil extraction, as much as possible.Maximise on re-use of such produced waters.Phase out the use of shallow disposal wells and prevent disposal into usable or exploitable aquifers (ie. where total salinity < 35,000 mg/1).Return waters to the producing reservoir.Dispose surplus waters to formations which have salinity > 35,000 mg/1 in conjunction with case specific monitoring programmes. Costs of the Plan are significant, up to US$ 85 million over the programme period 1993-97, and it will require a number of evaluation studies to be carried out. The Plan has been presented to the regulatory authorities in Oman and an Inter-Ministerial Working group has recommended the Plan for approval by higher Government level.