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Matrix acidizing refers to one of two stimulation processes in which acid is injected into the well penetrating the rock pores at pressures below fracture pressure. Acidizing is used to either stimulate a well to improve flow or to remove damage. During matrix acidizing the acids dissolve the sediments and mud solids within the pores that are inhibiting the permeability of the rock. This process enlarges the natural pores of the reservoir which stimulates the flow of hydrocarbons. Effective acidizing is guided by practical limits in volumes and types of acid and procedures so as to achieve an optimum removal of the formation damage around the wellbore.
Two basic acidizing treatments
Acidizing is used to either stimulate a well to greater than ideal matrix reservoir flow or to remove damage. These are two distinct and different purposes, the field applications and results of which are often merged or confused. Basically, there are two types of acid treatments that are related to injection rates and pressures. Injection rates resulting in pressures below fracture pressure are termed "matrix acidizing," while those above fracture pressure are termed "fracture acidizing."
Fig. 1 shows the increase in pressure linearly with rate until parting pressure is attained, at which time rate can continue to increase with little change in pressure above parting pressure. Matrix acidizing is used primarily for damage removal, while fracture acidizing is used to enlarge the effective wellbore by creating an acid-etched fracture deep into the wellbore for relatively low-permeability formations to improve well productivity several-fold.
Fig. 1—Matrix acidizing injection rates below fracturing pressure.
Acidizing to remove damage
A matrix treatment restores permeability by removing damage around the wellbore, thus improving productivity in both sandstone and carbonate wells. Although the acid systems used in sandstone and carbonate differ, the same practices apply to both. In the absence of damage, the large volume of acid that is required to improve the formation permeability in the vicinity of the wellbore may not justify the small incremental increase in production, especially in sandstone. In carbonate rock, hydrochloric acid enlarges the wellbore or tends to bypass damage by forming wormholes. The permeability increase is much larger in carbonate than in sandstone. The effect of damage on well productivity and flow is illustrated in Figs. 2 and 3.
Severe damage (kD/k less than 0.2) is usually close to the wellbore, within 12 in., as in Fig. 2. More moderate damage (kD/k greater than 0.2) may occur much deeper (3 ft from the wellbore or more), as described in Fig. 3. Oilwell flow behavior is greatly affected by the geometry of radial flow into the wellbore; 25% of the pressure drop takes place within 3 ft of the wellbore if no damage is present, as shown in Fig. 4.  Because of the small flow area, any damage to the formation at that point may account for most of the total pressure drop (drawdown) during production and, thereby, dominate well performance.
Fig. 2—Effect of damage on well productivity-shallow damage.
Fig. 3—Effect of damage zone on flow-deep damage.
Fig. 4—Pressure distribution around a well.
Acidizing to enhance productivity
Matrix acidizing is applied primarily to remove damage caused by drilling, completion, and workover fluids and solids precipitated from produced water or oil (i.e., scale or paraffin). Removal of severe plugging in carbonate or sandstone can result in very large increases in well productivity. On the other hand, if there is no damage, a matrix treatment seldom increases natural production more than 50%, depending on the size of the treatment and the penetration depth of live acid, as demonstrated in Fig. 5. 
Fig. 5—Effects of acidizing an undamaged well.
Wormholes are small, continuous channels formed by acid preferentially enlarging pores in carbonate, usually around 2 to 5 mm in diameter. In radial flow, wormholes form a dendritic pattern, like the roots of a tree. Gdanski developed a practical model for wormholing during matrix acidizing in carbonates, which shows that practical limits for effective penetration of hydrochloric (HCl) acid varies from about 1 to 5 ft. Penetration is limited by injection rate and volume. The maximum rate allowed is a function of the carbonate permeability. Radial penetration is so limited in low-permeability carbonate that it is a better candidate for fracture acidizing.
Improper or poorly executed acid treatments
When there is no damage present, improper or poorly executed acid treatments can reduce the natural formation permeability and reduce well productivity, as in new wells with low reservoir permeability. Gidley presented the results of an extensive statistical review of one company's acidizing success in sandstone reservoirs in the U.S. He found that only 54% of 507 wells increased in production following hydrofluoric (HF) acid stimulation. More recently, Nitters et al. stated that past programs resulted in only 25% success.
Evaluation and quality control
Where better evaluation and quality control have been implemented, the percentage of successful treatments has improved to 75 to 90%. Such a program was developed by Brannon et al.,  who successfully acidized 35 of 37 wells (95% success) for an average production increase of 343 BOPD. Other areas and formations still suffer from poor acidizing responses, which implies that opportunities for technology development still exist.
- McLeod, H.O. 1986. Matrix Acidizing to Improve Well Performance. Short Course Manual. Richardson, Texas: SPE.
- Earlougher, R.C. Jr. 1977. Advances in Well Test Analysis, 5, 8, and 14. Richardson, Texas: Monograph Series, SPE.
- Gdanski, R. 1999. A Fundamentally New Model of Acid Wormholing in Carbonates. Presented at the SPE European Formation Damage Conference, The Hague, Netherlands, 31 May-1 June 1999. SPE-54719-MS. http://dx.doi.org/10.2118/54719-MS.
- Gidley, J.L. 1985. Acidizing Sandstone Formations: A Detailed Examination of Recent Experience. Presented at the SPE Annual Technical Conference and Exhibition, Las Vegas, Nevada, 22-26 September 1985. SPE-14164-MS. http://dx.doi.org/10.2118/14164-MS.
- Nitters, G., Roodhart, L., Jongma, H. et al. 2000. Structured Approach to Advanced Candidate Selection and Treatment Design of Stimulation Treatments. Presented at the SPE Annual Technical Conference and Exhibition, Dallas, Texas, 1–4 October. SPE-63179-MS. http://dx.doi.org/10.2118/63179-MS.
- Brannon, D.H., Netters, C.K., and Grimmer, P.J. 1987. Matrix Acidizing Design and Quality- Control Techniques Prove Successful in Main Pass Area Sandstone (includes associated papers 17274 and 17466 ). J Pet Technol 39 (8): 931-942. SPE-14827-PA. http://dx.doi.org/10.2118/14827-PA.
Noteworthy papers in OnePetro
Paccaloni, G. and Tambini, M. 1993. Advances in Matrix Stimulation Technology. J Pet Technol 45 (3): 256-263. SPE-20623-PA. http://dx.doi.org/10.2118/20623-PA
SPE short course titled "Matrix Acidizing to Improve Well Performance"