COMPLETION/STIMULATION Trends in Matrix Acidizing Curtis Crowe Jacques Masmonteil Ron Thomas Tulsa, Oklahoma, USA Eric Touboul Montrouge, France Saint-Etienne, France Faced with poor production from a high-permeability reservoir, an operator’s first thought is a matrix treatment. This commonly involves pumping acid into the near-wellbore region to dissolve formation damage and create new pathways for production. This article reviews the state of the art of matrix acidizing and discusses how technical break- throughs are helping optimize matrix acid jobs. The simple aim of matrix acidizing is to HCl into a limestone producer using arsenic improve production—reduce skin in reser- as an inhibitor. The previously dead well voir engineer parlance—by dissolving for- produced 16 barrels of oil per day, and mation damage or creating new pathways interest in acidizing was reborn. Dow within several inches to a foot or two formed a subsidiary later called Dowell to around the borehole. This is done by pump- handle the new business (next page, top). ing treatment fluid at relatively low pressure Three years later, Halliburton Oil Well to avoid fracturing the formation. Compared Cementing Co. also began providing a com- with high-pressure fracturing, matrix acidiz- mercial acidizing service. ing is a low-volume, low-budget operation. Sandstone acidizing with hydrofluoric Matrix acidizing is almost as old as oil- acid [HF]—hydrochloric acid does not react well drilling itself. A Standard Oil patent for with silicate minerals—was patented by acidizing limestone with hydrochloric acid Standard Oil company in 1933, but experi- [HCl] dates from 1896, and the technique ments in Texas the same year by an inde- was first used a year earlier by the Ohio Oil pendent discoverer of the technique caused Company. Reportedly, oil wells increased in plugging of a permeable formation. Com- production three times, and gas wells four mercial use of HF had to wait until 1940, times. Unfortunately there was a snag—the when Dowell hit on the idea of combining acid severely corroded the well casing. The it with HCl to reduce the possibility of reac- technique declined in popularity and lay tion products precipitating out of solution dormant for about 30 years. and plugging the formation. The mixture, Then in 1931, Dr. John Grebe of the Dow called mud acid, was first applied in the Chemical Company discovered that arsenic Gulf Coast to remove mudcake damage.1 inhibited the action of HCl on metal. The following year, the Michigan-based Pure Oil Company requested assistance from Dow Chemical Company to pump 500 gallons of 24 Oilfield Review nEarly acidizing operations by Dow- ell, a division of Dow Chemical established in 1932. Chemistry For help in preparation of this article, thanks to A. Matrix acidizing of carbonates and silicates Ayorinde, Ashland Oil Nigeria Ltd, Lagos, Nigeria; Jim are worlds apart.2 Carbonate rocks, com- Collins, Dowell Schlumberger, Calgary, Alberta, Canada; Harry McLeod Jr, Conoco, Houston, Texas, USA; Arthur prising predominantly limestone and Milne, Dowell Schlumberger, Dubai; Carl Montgomery, dolomite, rapidly dissolve in HCl and create ARCO Oil and Gas Co., Plano, Texas, USA; Giovanni reaction products that are readily soluble in Paccaloni, AGIP S.p.A., Milan, Italy; and Ray Tibbles, Dowell Schlumberger, Lagos, Nigeria. water: In this article, CORBAN, FoamMAT, MatCADE, MatTIME, PARAN and ProMAT are trademarks or service → marks of Dowell Schlumberger; NODAL (production CaCO3 + 2HCl CaCl2 + CO2 + H2O system analysis) and Formation MicroScanner are marks Limestone Hydrochloric Calcium Carbon Water of Schlumberger. acid chloride dioxide 1. A classic paper on sandstone acidizing: → Smith CF and Hendrickson AR: “Hydrofluoric Acid CaMg(CO3)2 + 4HCl Stimulation of Sandstone Reservoirs,” Journal of Dolomite Hydrochloric acid Petroleum Technology 17 (February 1965): 215-222. 2. For general reference: CaCl + MgCl + 2CO + 2H O . 2 2 2 2 Economides MJ and Nolte KG (eds): Reservoir Stimu- Calcium Magnesium Carbon Water lation, 2nd ed. Houston, Texas, USA: Schlumberger chloride chloride dioxide Educational Services, 1989. Acidizing: SPE Reprint Series No. 32. Richardson, Texas, USA: Society of Petroleum Engineers, 1991. The rate of dissolution is limited mainly by Schechter RS: Oil Well Stimulation. Englewood Cliffs, the speed with which acid can be delivered New Jersey, USA: Prentice Hall, 1992. to the rock surface. This results in rapid gen- eration of irregularly shaped channels, called wormholes (left). The acid increases production by creating bypasses around the damage rather than directly removing it. By comparison, the reaction between HF and sandstones is much slower. Mud nMold of wormholes created by HCl in acidizing seeks to unblock existing path- limestone from a central conduit. Acid dis- ways for production by dissolving wellbore solves the rock as soon as it reaches the damage and minerals filling the interstitial grain surface. Matrix acidizing in carbon- ates aims to create new pathways for pro- duction rather than removing damage. October 1992 25 pore space, rather than by creating new Before acid After acid pathways. The HF reacts mainly with the associated minerals of sandstones, rather than the quartz (right). The acid reactions caused by the associated minerals—clays, feldspars and micas—can create precipi- tants that may cause plugging. Much of the design of a sandstone acid job is aimed at preventing this (see “HF Reactions in Sand- Mud acid stones,” below). The usual practice is to preflush the for- mation with HCl to dissolve associated car- bonate minerals. If these were left to react 2µ with HF, they would produce calcium fluo- ride [CaF2], which precipitates easily. Then the HF-HCl mud acid is injected. Finally, the formation is overflushed with weak HCl, hydrocarbon or ammonium chloride [NH4Cl]. This pushes reaction products far from the immediate wellbore zone so that if precipitation occurs, production is not too constricted when the well is brought back Fluoboric acid on line. Another plugging danger is from fine par- ticles, native to the sandstone, dislodged by 2µ the acid but not fully dissolved. To minimize this eventuality, Shell in 1974 proposed n lower pumping rates—less likely to dislodge Scanning electron micrographs showing pore-filling clays before and after expo- sure to both regular mud acid and fluoboric acid. In the fluoboric acid micrographs, fines—and, more important, a chemical sys- some clays, lower left, are dissolved while others, kaolinite platelets in the middle of tem that did not contain HF explicitly, the photographs, are partially fused preventing fines migration. instead creating it through a chain of reac- tions within the formation.3 In principle, this introduced a retarded acid system using flu- As HF is spent, dissolving clays and other allows greater depth of penetration and oboric acid [HBF4]. This hydrolyzes in water minerals, it is constantly replenished longer reaction times for maximum dissolu- to form HF:4 through hydrolysis from the remaining fluo- tion of fines. Since then, several other sys- boric acid. The slow rate of this conversion ↔ tems of in-situ generated—so-called HBF4 + H2O HBF3OH + HF . helps guarantee a retarded action and there- retarded—mud acid systems have been pro- Fluoboric Water Hydroxyfluoboric Hydrofluoric fore deeper HF penetration. As a bonus, the posed. Recently, Dowell Schlumberger acid acid acid fluoboric acid itself reacts with the clays and HF Reactions in Sandstones 2– 3– The reaction of hydrofluoric acid [HF] on the pure Secondary cement: Quartz, feldspars, AlF5 and AlF6 (left). The concentration of each quartz component of sandstone follows these two carbonate, quartz chert and mica. aluminum complex depends on the concentration equations: Pore-lining clays, of free fluoride ions in the dissolving solution. e.g. illite Some of these products combine with free SiO + 4HF ↔ SiF + 2H O , 2 4 2 sodium, potassium, and calcium ions to produce Quartz Acid Silicon Water tetrafluoride four compounds with varying degrees of solubility and in the spending acid: SiF + 2F– ↔ SiF 2– , 4 6 • sodium fluosilicate [Na2SiF6], Silicon hexafluoride • sodium fluoaluminate [Na3AlF6], resulting mainly in the silicon hexafluoride anion, • potassium fluosilicate [K2SiF6], 2– • calcium fluosilicate [CaSiF ]. SiF6 . Pore-filling 6 Reaction with the feldspar, chert, mica and clay clays, e.g. kaolinite Matrix treatments are always designed to prevent components of sandstones also results in this the formation of these compounds, to remove any nConstituents of sandstone, all of which are soluble in anion, but, in addition, produces a range of alu- risk of precipitation. 2+ + – HCl-HF mud acid systems. minum complexes: AlF , AlF2 , AlF3, AlF4 , 26 Oilfield Review silt, forming borosilicates that appear to help 4000 bind the fines to large grains (previous page, top). Recent treatments with fluoboric acid for Ashland Nigeria have confirmed the power of this technique (right).5 3000 All in all, sandstone acidizing poses a greater challenge than carbonate acidizing Mud acid treatment and certainly generates more than its fair share of controversy among both operators 2000 and service companies. Production, BLPD Production, Diversion 1000 A challenge that must be faced in either lithology is diversion. As acid is pumped, it flows preferentially along the most perme- Fluoboric acid treatment 0 able path into the formation. The acid opens 012 these paths up even more, and less perme- Time, yr able, damaged zones are almost guaranteed n not to receive adequate treatment. Some Production improvement in a Nigerian oil well after fluoboric acid treatment. The well was initially acidized with mud acid and technique to divert the treatment fluid produced 850 barrels of liquid per day (BLPD) with a 34% water toward more damaged formation or dam- cut. Production then declined almost to zero, most likely due to aged perforations is therefore mandatory. fines movement. After fluoboric acid treatment, production rose to There is a variety of diversion techniques 2500 BLPD, obviating the need for further acid treatments.