ABSTRACT Sea water often reacts with the formation water in offshore fields to produce barium, calcium and strontium sulfate deposits that hinder oil production. Newer fields often have more difficult to control scale problems than older ones, and current technology scale inhibitors are not able to control the deposits as well as needed. In addition, ever more stringent regulations designed to minimize the impact of inhibitors on the environment are being enacted. Three new inhibitors are presented that overcome many of the problems of older technology scale inhibitors.
INTRODUCTION Water flooding is the most commonly used technique to maintain oil production on offshore platforms. In this process, sea water is injected under pressure into the reservoir via injection wells, which drives the oil through the formation into nearby production wells.
Since sea water contains high concentrations of dissolved salts such as sulfate, chemical reactions with the formation water can give rise to unwanted inorganic deposits that can block the formation, tubing, valves and pumps. These deposits are primarily composed of BaSO4, CaSO4, and SrSO4. In some fields, CaCO3 is a major problem.
In the North Sea area, formation water chemistry can vary enormously. In the Central North Sea Province, Ba levels can vary from a few mg/L to over 1000 mg/L and pH varies from about 4.4 (often due to high partial pressures of CO2, such as in the Ula and Gyda fields) to over 7.5. In the Northern North Sea province, pHs as high as 11.7 have been measured. In the Southern North Sea, the waters are high salinity, sulfate rich and acidic.
Recently, there has been a push for low toxicity chemicals to handle scale and corrosion problems . New inhibitors must now conform to guidelines specifying maximum usage for materials having an impact on the environment.
effective scale control at low inhibitor concentration.
compatibility with sea and formation water.
balanced adsorption - desorption properties allowing the chemicals to be slowly and homogeneously released into production water at concentrations that provide effective scale control.
high thermal stability.
low toxicity and high biodegradability.
low cost.
Since the conditions vary widely and the regulations governing the use of scale and corrosion inhibitors arc becoming ever more stringent, it is impossible today for a single inhibitor to meet all the requirements at a commercially feasible cost. The ideal inhibitor that could be used in both downhole squeeze treatments and topside application would require the following properties:Currently there are a variety of scale inhibitors available such as polyacrylic acid, phosphinocarboxylic acid, sulfonated polymers and phosphonates. None of these are fully satisfactory for the demanding conditions currently encountered in the North Sea.
To meet these requirements, we have developed three new scale control chemistries that we believe come closer to fulfilling the requirements of an ideal scale inhibitor:
An improved general purpose scale inhibitor for moderate conditions.
A high performance scale inhibitor for harsh conditions.
A low toxicity, biodegradable inhibitor for environmentally sensitive areas.