Clay Stabilization in Low-Permeability Formations

Abstract

Summary The most popular clay stabilizers used recently in well-treating solutionsare classified as cationic organic polymers (COP's). Recent studies have shownthese stabilizers to be ineffective in microdarcy to low-millidarcy sandstones. Recent research led to the development of a stabilizer applicable to formationswith permeabilities of 0.010 md and higher that also provides enhancedload-water recovery and more efficient placement from gelled-water solutions.placement from gelled-water solutions. Introduction Most oil- and gas-producing formations contain clay minerals that eitherwere originally deposited during sedimentation (detrital clay); were formedlater by the action of heat, pressure, and time on minerals already present; orwere precipitated from fluids flowing through the matrix (authigenic clays). The importance of these minerals in oil and gas production and their potentialpermeability damage have been investigated widely. The two major mechanisms bywhich these minerals cause permeability damage are swelling and migration. Inswelling, clay imbibes water into its crystalline structure and subsequentlyincreases in volume, plugging the pore in which it resides. In migration, clayminerals can be dispersed by contact with a foreign fluid or can be entrainedby produced fluids and transported until a restriction is encountered (usuallya pore throat), where the entrained particles bridge and restrict flow porethroat), where the entrained particles bridge and restrict flow in thecapillary. The migration of clays and other fine minerals has also beenexplored extensively. Advances in the treatment of clay-bearing formations haveled to the development of numerous clay-stabilizing treatments and additives. Most additives used during the last 15 years can be classified as COP'S. Recentstudies have shown, however, that these COP stabilizers are ineffective instabilizing formations with permeabilities below about 30 md, depending on themolecular weight permeabilities below about 30 md, depending on the molecularweight of the COP. These findings indicated the need for further research intoclay stabilizers. This research led to the synthesis of a new class ofclay-stabilizing chemical additives capable of successfully stabilizing claysin very-low-permeability (10-md) sandstones. These additives provide additionalbenefits when used with acidizing and fracturing treatments. Background Various chemicals and techniques have been used in the last 30 years tominimize or prevent the damaging effects of clays and fines in oil- andgas-producing formations. To understand how these work, we must understand claychemistry. A thorough discussion is beyond the scope of this paper, but severalreferences are available, ranging from brief to extensive presentations. Basically, clay surfaces of the most common clays found in oil-producingformationsi.e., smectite, kaolinite, illite, and mixed layer versions-have manynegatively charged sites. These negative charges are responsible for theirsensitivity to fluid and provide the mechanism by which most clay-stabilizingagents operate. provide the mechanism by which most clay-stabilizing agentsoperate. Clay minerals exist naturally in stacked or randomly arrangedplatelets within the pores, as either pore-lining or pore-idling plateletswithin the pores, as either pore-lining or pore-idling minerals, and aresurrounded by a saline connate-water layer. Usually Na+ or Ca++ makes up thesalt and is fixed onto the clay surfaces by electrical attraction, effectivelyneutralizing the negative charges. In this state, the clay is stable. Theintroduction of a less saline foreign fluid (e.g., oilwell treating fluids orout-of-zone produced water) can dilute the connate water and reduce its salinecontent. As the cation cloud covering the clay surface becomes more diffuse, water molecules rush in between the clay platelets, resulting in swelling(smectite clays and some mixed-layer clays) or dispersion (kaolinite, illite, chlorite, and mixed layers). This type of damage is, for the most part, irreversible and requires acid stimulation for removal. Early Clay-Stabilizing Chemicals. The first step in maintaining claystability is to ensure that the salinity of any treating solution is equal toor greater than that of the connate water surrounding the clay. However, different salts (cations) maintain stability better than others at the sameconcentration. Fig. 1 compares the relative clay-stabilizing ability of variouscations regularly used in oil- and gas-well treating solutions. These data, obtained from X-ray diffraction (XRD) analysis of smectite clay, aremeasurements of the distance, in angstroms, from the top of one clay plateletto the top of the next one stacked upon it. This distance is called the basalspacing. As the smectite clay swells (imbibes water), this distance increases. For a basal spacing of 21, the clay is considered to be dispersed. These datashow that the NH4+ and K+ cations maintain a stable clay at much lowerconcentrations than Na+. For this reason, these cations have become popularclay stabilizers and are nearly always included in aqueous treating solutions. However, because these cations can easily displace the Na + cation during thetreatment, they can also be exchanged during production back to Na+; therefore, they are not permanent. Calcium ion is also an excellent stabilizer, but it isnot widely used because of chemical incompatibility with many formation watersand chemical additives. Calcium ion will also induce some clay swelling even inhigh concentrations. Improvements in clay stabilization came with thedevelopment of "polymeric" inorganic cations such as hydroxyl aluminumand zirconium oxychloride. These chemicals consist of a complex structure ofmultiple cationic sites that are resistant to cation exchange by Na+ andtherefore are more permanent. Disadvantages of such treatments include the needto re-treat after acidizing and their restriction to non-carbonate-containingsandstone formations. Organic Clay Stabilizers. Brown's early work showed that various quaternaryorganic cationic compounds are capable of stabilizing smectite clays toexposure to fresh water. Other researchers later applied these findings tocreate a series of COP's characterized by multiple cationic sites that providemultiple points of attachment, like the inorganic polymers described points ofattachment, like the inorganic polymers described previously. These organicpolymers had the additional advantages of previously. These organic polymershad the additional advantages of being acid-resistant and placeable from acidicsolutions. They are also unaffected by carbonate content in the formation andmay be used to stabilize clays in carbonate formations. These compounds aretypically composed of long-chain organic polymers with molecular weights from5,000 to well over 1,000,000. Variations of these chemicals have beenintroduced to solve various clay- and fines-related production problems, suchas fines migration. COP Limitations. The use of COP's has been very successful in fines control, sand control, acidizing, and in some fracturing applications. However, the sizeof these molecules has restricted their use to low concentration inlow-permeability formations to prevent plugging of the small pore throats bythe polymer mass. A prevent plugging of the small pore throats by the polymermass. A commonly used COP has a molecular weight averaging ∼500,000 and acalculated average size of 1.1 m. Fig. 2 shows a pore-diameter distribution fora 10-md sandstone measured by petrographic examination. SPEPE P. 252