Injection-Water Salinity, Formation Pretreatment, and Well-Operations Fluid-Selection Guidelines

Abstract

Summary Injection-water/ formation-clay compatibility criteria that are based onwater and formation-clay analyses were developed to determine whether aninjection water has sufficient total cations and/or divalent cations to preventformation-clay dispersion and permeability impairment and to determine whenformation pretreatment is required to prevent pretreatment is required toprevent impairment during the transition to injection water. These criteriaalso were incorporated into a scheme to determine the type of brine to be usedfor completion, stimulation, and workover operations to prevent formationclaydeflocculation or to eliminate formation pretreatment. Introduction Two factors affect whether injected waters/ brines will causeformation-clay-related impairment: the water must have an adequate total cationconcentration (TCC) to prevent deflocculation of formation clays, and duringthe transition from one water/brine to another, cation exchange must not reducethe divalent cation concentration below the level required to keep claysflocculated. This paper discusses (1) the salinity-related formation damage mechanism, (2) injection-water evaluation, (3) formation pretreatments to prevent claydeflocculation pretreatments to prevent clay deflocculation during water/watertransitions, and (4) well-operations brine-selection guidelines to prevent claydeflocculation and to eliminate the prevent clay deflocculation and toeliminate the need for formation pretreatment. Field application guidelines andfield test results are presented. presented. Salinity-Related Formation Damage Mechanism Formation clays respond to inadequate-salinity waters by swelling and/ordeflocculating (dispersing). Both mechanisms cause clay particles to detachfrom each other and from particles to detach from each other and from porewalls. Migration of, and pore-throat pore walls. Migration of, and pore-throatplugging by, deflocculated clays is the plugging by, deflocculated clays is thedominant impairment mechanism related to salinity-sensitive clay. The followingsections describe how water salinities and compositions, especially divalentcation concentrations, influence clay deflocculation. Flocculation Salinity. Clays have a net negative charge because ofsubstitution of lower-valence cations in the clay lattice structure--e.g., Alfor Si and Mg for Al. To maintain electrical neutrality, the resulting negativelattice charge is balanced by positive cations (counter-ions) located on andnear the clay surface. The spatial distribution of counter-ions is determinedby the opposing forces of electrostatic attraction to the negative clay latticeand diffusion into the surrounding bulk solution. This diffused concentrationof cations around the clay particle is often referred to as the counter-ioncloud or diffuse double layer. In dilute brines, diffusion forces are large andthe counter-ion cloud expands. When mutual repulsion between the chargedcounter-ion clouds exceeds van der Waals attractive forces, the particlesdisperse. Increasing salinity causes the counter-ion cloud to contract. Flocculation salinity (as defined here) is the cation concentration at whichthe counter-ion cloud contracts sufficiently that short-range van der Waalsattractive forces begin to cause clay particles to attach to each other and topore walls (flocculate) or just begin to deflocculate (disperse) as brinesalinity is slowly decreased. (Although flocculation and dispersion probablyoccur at slightly different probably occur at slightly differentconcentrations, our tests were not sufficiently sensitive to make such adistinction.) Effect of Clay Type. Flocculation salinity increases with increasing claycation exchange capacity (CEC). For example, the NaCl flocculation salinitiesfor Wyoming montmorillonite (CEC = 76 meq/100 g dry clay), IMt-1 illite (CEC=15meq/100 2), and KGa-2 kaolinite (CEC=3 meq/100 g) are 600, 200, and 17 meq/L, respectively. Chlorite has essentially no CEC and is not deflocculated by freshwater. The composition and flocculation properties of the clays and corestested in this study are summarized in Tables 1 and 2, respectively. Effect of Cation Type- The clay-flocculating power of cations is a functionprimarily of power of cations is a function primarily of valence andsecondarily of the specific cation. Within a given valence, flocculating powerdecreases with increasing hydrated power decreases with increasing hydratedionic radii. Thus, flocculating power decreases in the order Cs+ greater thanRb+ greater than NH, K+ greater than Na+ greater than Li+ for monovalentcations and Ba++ greater than Sr++ greater than Ca++ greater than Mg++ fordivalent cations. As discussed later in Well-Operations Brine Selection, K+ isabout six times more effective in flocculating clays than is Na+. NH has thesame hydrated ionic radius as K+, and H+ is smaller than K+. Thus, these ionsare considered to have clay-flocculating properties equivalent to K +. Depending on clay type, divalent cations are 50 to 100 times more effectivein flocculating clays than monovalent cations, and increasing Ca++ ionconcentration sharply reduces the flocculation salinity. JPT P. 836