Water Quality Control and Its Importance in Waterflooding Operations

Abstract

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Introduction The term "water quality" has many meanings, most of which are defined by theend use of the water in question. In oilfield waterflooding, water quality isusually defined in terms of the plugging tendency of the water. Ideally, thequality of the water should be such that there is no reservoir plugging, andhence no loss of injectivity during the life of the flood. In addition, the injection system must be protected against corrosion topreserve its physical integrity and to prevent the generation of insolublecorrosion products. Any insoluble material in water, either solid or liquid, can contribute toplugging. This includes formation solids (sand, silt, or clay), corrosionproducts, water-formed scales, bacterial growths and algae, oil (both crude andlubricating), and undissolved treating chemicals. The contaminants primarily responsible for plugging fall into threecategories:Present at the source. Some of theprimary contaminants commonly present at the source include: in produced water- oil, corrosion products from the production system, bacteria; in water supplywells - formation solids, corrosion products, bacteria; and in surface waters -dissolved oxygen, bacteria, suspended inorganic solids, marine organisms.Generated within the injectionsystem. Contaminants generated within the system may includecorrosion products, bacterial masses, biogenic hydrogen sulfide, andscale.Added to the injectionsystem. Sometimes, intentionally added materials ultimatelycontribute to plugging. For example, contaminants such as dissolved oxygen, bacteria, suspended solids, and usually oil are the inevitable results ofpumping trucked water or pit water into an injection system. Improperlyselected corrosion inhibitors that are not sufficiently soluble in theinjection water can contribute to plugging. In addition, contaminants may inadvertently enter the system. One of theprimary problems in most injection systems is the entry of dissolved oxygenthrough leaking pump seals and open hatches on water tanks.1 Pumplubricating oil may also enter the system in amounts sufficient to contributeto plugging. We can either accept the water quality at the source or improve it bymechanical and/or chemical processing. Next, the challenge is to preserve thequality by controlling contaminant generation or addition. The difficulty of preserving water quality is essentially a direct functionof the length and complexity of the injection system. The result is that thequality of the water reaching remote injection wells in long systems is oftenconsiderably worse than it was at the source. Water Quality Requirements When the quality of the injection water is inadequate, reservoir pluggingresults. The consequence of plugging is reduced sweep efficiency, which resultsin decreased recovery and, ultimately, loss of revenue. In addition, operational costs are increased because of workovers and system repairsrequired to restore injectivity. The quality required for a given reservoir will be primarily a function ofreservoir permeability, assuming true matrix injection into the reservoir porespace. Tight, low-permeability zones generally require better-quality waterthan higher-permeability zones. If natural fracture systems are available for fluid transport or ifinjection is carried out under parting conditions, then much poorer quality canbe tolerated with few apparent injectivity problems. Injection throughfractures, however, offers a high probability of reduced sweep efficiency. Despite numerous attempts to correlate water quality parameters withreservoir rock properties,2,3 we are currently unable toquantitatively predict the minimum water quality required for trouble-freeinjection into a given formation. Onsite core flow testing has been used toestimate quality requirements,4 but in most cases minimum water qualitystandards evolve through experience. Formation Sensitivity Some sandstones contain clays that swell upon contact with low-salinitywater. Although not caused by suspended matter in the injection water, reducedpermeability and injectivity are the results. The susceptibility of sandstoneformations to this type of impairment should be assessed before the initiationof any water injection project.