Offshore Water Injection System: Problems and Solutions

ABSTRACT Offshore pressure maintenance programs involve handling large quantities of seawater in predominantly carbon steel systems. Problems such as effect of the seawater on the formation, corrosion and deterioration of materials of construction caused by seawater, scaling, oxygen corrosion, and bacteria, must be solved. This paper will address problems encountered in the North Sea in operating a seawater injection system, and solutions to these problems. INTRODUCTION Water injection is performed to maintain reservoir pressure for production purposes, and to dispose of produced water or waste water. This discussion concerns some of the problems encountered and means of solving them in seawater pressure maintenance systems, including the use of chemicals. EFFECT OF INJECTION WATER ON THE FORMATION Injection systems must be designed to meet the requirements of the reservoir. Reservoirs must be evaluated for the effect of injection water and suspended solids. Each reservoir has specific water quality requirements, aside from general quality requirements common to all reservoirs.1 The water that is injected will be of a different composition than that found in the reservoir. When seawater is injected substantial incompatibilities can arise2. For example, seawater contains large amounts of sulphate ion, and many formation waters contain little or no sulphate but do contain barium or strontium. The result is barium sulphate scale, particularly when the seawater injection front "breaks through" into the producing wells. 3 Therefore, tests must be run on formation cores, water analyses must be made, and compatibility studies run between the different waters before beginning water injection. Core studies are performed on actual formation material retrieved by coring operations. Standard porosity, permeability, and pore size distribution tests are run, then the cores can be flushed with formation water to determine flow rates. Injection water is then flushed through the core to see if flow reduction occurs. The core can then be inspected visually or by X-ray tomography for further information. X-ray diffraction and chemical analyses of the formation material will identify the different minerals in the formation. The information derived from the tests can be summarized as:Pore size analysisPorosity and permeability of the formationPermeability reduction due to injection waterEffect of mobile fines on permeabilityTypes of clays and silts in the formationClay swelling problems Many formations contain shales and clays which can swell and disintegrate when contacted by different waters. There are also "mobile fines" present in the formation. These are small particles of sand or silt that are not consolidated, and can move with water flow. Formation clays, when contacted with lower salinity water can deflocculate and/or swell. Migration of deflocculated clay particles to pore throats is reported to be the main cause of salinity sensitive clay related formation damage.4 EFFECT OF SUSPENDED SOLIDS Seawater and surface water contain suspended solids. They are inorganic particles such as finely divided silicates, clays, and silt, and organic materials. These include organically derived waste solids, microorganisms, algae, plankton, and degradation products of live or dead organisms.