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

No abstract

Clay stabilizers have been used in various forms in oil well treating solutions for over 30 years. Cationic organic polymers have been the most popular type stabilizer in use recently because of their resistance to wash-off and chemical attack, ease of application, and effectiveness in moderate to high permeability formations. Recent studies investigating the effectiveness of such stabilizers in very low to low permeability sandstones have shown them to be less than 100% effective in stabilizing clays to prevent swelling and migration. Results indicate that higher molecular weight polymers can create permeability impairment when injected into these formations. These findings indicated the need for further research into clay stabilizers. This research led to the development of a novel clay stabilizing chemical for low, as well as high, permeability formations. This low permeability clay stabilizer also offers additional benefits in treating low permeability formations by enhancing fluid recovery after fracturing, and preventing the detrimental effects of fracturing fluid gel adsorption onto formation surfaces.

In the Lick Creek Field of Bradley County, Arkansas, Phillips Petroleum Company has used a single stage, nonionic polymer treatment to combat severe channeling problems in a CO2 water-alternating-gas (WAG) operation. After treating two of the 23 injection wells in the field, production has increased by 65 bbl/day. The field, consisting of 23 injection wells and 38 producers, was unitized in 1976 to initiate CO2 injection into the reservoir. Alternating CO2 and water (WAG) flooding techniques have been used. Channels had developed throughout the field and flooding was becoming uneconomical. Previous E0R treatments using an anionic polyacrylamide were successful for only a few cycles of CO2-water injection. Failure was attributed to the inability of anionic crosslinked polymer to withstand the acidic environment created by CO2. The successful treatments were composed of single stages of injection of a low viscosity monomer solution containing an organic crosslinker. The rigid polymer gel which is formed in-situ to block high permeability channels has remained stable in the CO2 environment. Laboratory studies dealing with the CO2 resistance of the polymer, tracer surveys, job design considerations, and field results are presented.

Lost return problems have been typically solved with lost circulation pills containing plugging materials like mica, cellophane, and walnut hulls. However, should lost returns occur across potential producing formations, use of such materials may result in permanent damage to these zones. Dislodgment of the plugging material can result in resumption of lost returns. This paper describes the laboratory development of a foamed fluid system that can function to solve the problem of lost circulation. Foaming offers the operator an option to prepare a fluid with densities below that obtainable even with water. Additionally, this system possesses acid solubility, thereby minimizing damage to potential producing formations. Laboratory slurry property data such as thickening time, compressive strength, and acid reaction rates are provided. In addition to lost circulation, the foamed acid soluble cement system has application in both oil and gas wells as a temporary plugging material. Potential field applications of acid soluble cement in these areas are discussed.