Comparison of Using Combination of Acetic Acid and Hydrochloric Acid with Only Hydrochloric Acid as Matrix Pre Flush

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1 International Journal of Petroleum and Geoscience Engineering (IJPGE), 1 ISSN xxxx-xxxx Academic Research Online Publisher Research Article Comparison of Using Combination of Acetic Acid and Hydrochloric Acid with Only Hydrochloric Acid as Matrix Pre Flush Mian Umer Shafiq a, Muhannad Talib Shuker a, Aung Kyaw a a Department of Petroleum Engineering and Geosciences, Universiti Teknologi PETRONAS, Perak, Malaysia * Corresponding author. Tel.: ; address: shaheenf17@yahoo.com ARTICLE INFO Article history Received:01Feb2013 Accepted:12Feb2013 Keywords: Matrix Stimulation Permeability Mud Acid Acidizing Pre flush Sandstone A b s t r a c t Matrix acidizing is a stimulation technique commonly applied to sandstone reservoirs to remove near wellbore skin damage and improve well productivity. For any acidizing process, the selection of acid (formulation and concentration) is very important. Mud acid has been successfully used to stimulate sandstone reservoirs for a number of years. It is a mixture of hydrofluoric (HF) and hydrochloric (HCl) acids designed to dissolve clays and siliceous fines accumulated in the near-wellbore region to re-open or enlarge microscopic flow paths and thus improve formation permeability. The success of acidizing depends to a great extent upon proper acid selection and good treatment design. There are mainly three stages in acidizing process i.e., pre flush, main acidizing, after flush treatment. The results of acidizing depends on all these stages. For effective acidizing process best acid combination should be used in these stages to ensure that the permeability and porosity change is maximum. In this paper combinations of (HCl and CH 3 COOH) had been used and compared with the standard acid combination (15% HCl for the pre flush stage). As this stage is the initial stage and the main function of the acid should be to remove the carbonates present in the formation and also to remove the Na and K ions. Academic Research Online Publisher. All rights reserved. 1. Introduction Acid treatments have been applied to wells in oil and gas bearing rock formations for many years. Acidizing is probably the most widely used work over and stimulation practice in the oil industry as demonstrated by (Bay, 2005) [7]. By dissolving acid soluble components within underground rock formations, or removing material at the wellbore face, the rate of flow of oil or gas out of production wells or the rate of flow of oil-displacing fluids into injection wells may be increased. A number of different acids are used in conventional acidizing treatments, the most common are: 41 Page

2 a. Hydrochloric, HCl b. Hydrofluoric, HF c. Acetic, CH 3 COOH d. Formic, HCOOH These acids differ in their characteristics. Choice of the acid and any additives for a given situation depends on the underground reservoir characteristics and the specific intention of the treatment, for example near well bore damage removal, dissolution of scale in fractures, etc. Typically, sandstone matrix stimulation involves three stages as demonstrated by (Bay, 2005) [7]: 1. A pre flush stage to dissolve any carbonates that might be presented to displace the connate water from the acidizing zone as it contains ions of Na and K whose salts of fluosilicic acid are highly insoluble (A reaction HF and SiO 2 produce fluosilicic acid). 2. A mud acid treatment to dissolve siliceous and damaging material. 3. An after flush to restore wettability and provide rapid formation cleanup. Sandstone matrix acidizing generally consists of an HF/HCl mixture aimed at reducing nearwellbore damage by dissolving silicate minerals, such as clays and feldspars. The HF/HCl mixture is usually preceded by a preflush of HCl for two reasons. First, the HCl removes carbonate minerals, preventing the consumption of the HF acid in reactions with these minerals. Second, and more importantly, the HCl is intended to prevent precipitation of HF reaction products that could redamage the formation. 2. Theory Matrix stimulation treatments increase well productivity by pumping a specially formulated treatment fluid (frequently, but not always, an acid) which is designed to remove (normally dissolve) the formation damage. However, the keys to successful treatments demonstrated by (Shaughnessy et al, 1981) [5] are: 42 Page

3 1. The identification of a suitable candidate well which is capable of a greater hydrocarbon production rate. 2. The selection of the optimum type of treatment fluid for the removal of the formation damage. 3. The design of the operational aspects of the treatment such that the required economic criteria are successfully achieved. Sandstone is primarily composed of Silica and Silicate minerals, including quartz, various forms of clays, feldspars and in rare cases zeolites. Formation damage is often caused by siliceous materials, such as drilling solids, formation fines and swollen and migrated clays. Sandstone Acidizing is used to stimulate the true permeability of sandstone formations. The fluids are pumped into the porosity of the rock at below the fracturing pressure and the acid reacts with a large portion of the formation. The well known, steady state, radial-flow equation describes the well inflow as demonstrated by (Muecke, 1982); [4]: Q o = K o h (P e - P wf ) (1) Where, 141.2Bo {ln (r e /r w ) +S} Q o = well oil production rate (STB / day) K o = formation permeability to oil (md) h = P e = reservoir thickness (ft) reservoir pressure (psi) P wf = wellbore flowing bottom hole pressure (psi) = oil viscosity (cp) B o = oil formation volume factor (reservoir bbl / STB) r e = r w = S = well drainage radius (ft) well radius (ft) skin (dimensionless) 43 Page

4 An increased well inflow (Q), or well stimulation, can be achieved by: Increasing the factor (kh). Decreasing one of the factors S or re/rw or. During the first part of the acidization, the average permeability of the core remains essentially constant, or it may even decrease somewhat. The loss in permeability has been noted by several investigators such as (Smith and Hendrickson, 1965) [6]; and is attributed to: Interaction of the acid with clays and other fines, causing them to move and resulting in partial plugging. The reprecipitation of acid-mineral plugging The reprecipitation of acid-mineral reaction products. Through experimental laboratory work, three important concepts of sandstone acidizing have emerged demonstrated by (Shaughnessy et al, 1981) [5]: 1. Hydrofluoric acid (HF) begins to increase rock permeability as soon as it is injected, provided the rock has been preflushed properly with HCl to remove carbonates. 2. Silica precipitation from spent hydrofluoric acid can be very damaging to rock permeability. 3. Sandstone formations containing iron-bearing minerals can be acidized successfully. These three concepts are important to the successful design of hydrofluoric acid stimulation treatments in the field. This phenomenon is not limited to HF attack; it has also been found that permeability reduction can be induced by acids containing no hydrogen fluoride, provided the matrix cementing material is acid soluble and can be contacted by the acid. Sandstone formations containing calcite often exhibit reduced permeability in the first stages of acid contact even with dilute hydrochloric acid. 44 Page

5 2.1. The importance of pre flush In general, HF follows the same reaction paths as HCL. It will react with limestone and dolomite with speed and ease. Thin sections of acidized cores show the reaction of HF with limestone or calcite is faster than its reaction with either clay or sand. When HF reacts with calcite (CaCO 3 ) theoretically, calcium fluoride (CaF 2 ) is precipitated, and has been blamed as a major cause of reduced permeability. On the other hand, ph and pressure such as that encountered in an underground formation under acid treatment definitely retard CaF 2 formation, so the whole question of CaF 2 deposition in wells is a subject for study as demonstrated by (Smith and Hendrickson, 1965); [6]. The removal of damage results from dissolution of clay by reaction with the hydrofluoric acid: 36 HF + Al 2 Si 4 O 10 (OH) 2 4H 2 SiF H 2 O + 2H 3 AlF (2) The acid will also react with sand and other siliceous minerals: 6 HF + SiO 2 H 2 SiF 6 + 2H 2 O (3) These reactions, though they appear simple, are really very complex. Further reactions may take place that produce insoluble reaction products. It is for this reason that excess hydrochloric acid should be maintained in the mixture and dilute hydrochloric acid solutions are used as preflushes ahead of the mixed acids Pre-Flush Characteristics There are mainly 2 points which are necessary to be effective pre-flush as demonstrated by (Muecke, 1982) [4]; React with the carbonates present in the formation to avoid their reaction with hydrofluoric acid. Maintain a low ph to avoid deposition of calcium fluoride. In past, Regular acid (10 percent HCl) when used in the proper volume can satisfy all three requirements. 45 Page

6 3. Experimental test We have used the core samples having permeability less than 100 md as matrix acidizing is done to increase the permeability, if permeability is already high then matrix acidizing is not done. Acidizing has been performed with different concentrations of HCl and with combination of HCl and CH 3 COOH. For the purpose of acidizing, we have to do the saturation of the core sample for some time under vacuum conditions to speed up the process. Desiccators are used in order to create the vacuum and saturation time is 4 hours to ensure that maximum acid should enter into the core sample. The core is dried before and after the acidizing process for 24 hours at 80 o C. The size of the core is 3 inch in length and 1.5 inch in Diameter. The total volume used is 175 ml which includes both acids and distilled water. The combination used is 2.5% CH 3 COOH and 7.5% HCl Acid Volume Calculations Acid volume calculations have been made using the formula M 1 V 1 = M 2 V (4) For Example: If combination is 2.5% CH 3 COOH: 7.5%HCl, the calculations are like that: Concentration of acids is already given by supplier; HCl is 37% CH 3 COOH is 100% For HCl For CH 3 COOH M 1 V 1 = M 2 V 2 M 1 V 1 = M 2 V 2 (37) V 1 = (7.5) (175) (100) V 1 = (2.5) (175) V 1 = 35.5 ml V 1 = 4.4 ml 46 Page

7 As, there is one liter of solution, the remaining volume will be filled in by the distilled water. Volume of water = = ml If combination is 10% HCl, the calculations are like that: M 1 V 1 = M 2 V 2 (37) V 1 = (10) (175) V 1 = 47.3 ml And the water is = ml Table.1: Final properties after experiments. Acid Combination Final Porosity % change Final k % change 10 % HCl % % 7.5 % HCl % CH 3 COOH % % 4. Results and discussion Fig. 1. Cores after acidizing 4.1. Initial properties of the core Porosity = 11.5, Permeability = md 4.2. Final properties of the core The final values (shown in table 1) given are the average values of the experiments performed. 10% HCl is used commonly as preflush, and in some research papers both hydrochloric and acetic acid has been used but in stages. 47 Page

8 The hydrochloric acid has been used to dissolve the carbonates while acetic acid is added in small amount to control the speed of reaction to allow the maximum acid to penetrate the core sample, basically acetic acid acts as a ph buffer solution. It controls the ph of the solution to change at a slow rate so that maximum acid can react with the formation and maximum penetration should be achieved. Figure # 1 shows the core samples after the acidizing 4.3. Colour change test After the acidizing process the core is put in the oven for 24 hours to evaporate any acid or liquid present inside the pores of the sample. After the evaporation, the products formed due to the reaction remain there in the sample which causes the change in the colour of the sample (Fig #1). This change in color of sample will tell us that how much is the penetration of the acid in the core sample. There will be a change in the colour, wherever the acid reacts inside the sample. The core on the right (Fig #1) is one reacted with the 7.5 % HCl % CH 3 COOH, and the other on the left is reacted with 10% HCl. From figure it is clear that the penetration of the right sample is much as compare to the left one. Acid entered completely in about half of the sample in both the cases. But it is clear that there are many spots in the centre part of the sample where there is a change in the colour which means that acid entered that part also. So, we can say that acid penetrated completely. But as you can see, the amount acid penetration is more clear and visible on right core which means that this acid reacts with most of the matrix as compared to HCl, that why the permeability increase is more. 48 Page

9 5. Conclusions i. The results show that combination of hydrochloric and acetic acid can be used as a ii. preflush in sandstone acidizing. This combination has the potential to be used as a preflush acid to dissolve carbonates and other materials before the main acidizing treatment, which is responsible for the precipitation during acidizing. References 1. His CD, Bryant SL. Experimental Validation of Sandstone Acidization Models. SPE International Symposium on Oilfield Chemistry, New Orleans, Louisiana, 2-5 March Motta PD, Plavnik E, Benjamin. Optimizing Sandstone Acidizing. Journal of SPE Reservor Engineering 1992; 7(1): Farley JT, Miller BM, Schoettle V. Design Criteria for Matrix Stimulation with Hydrochloric- Hydrofluoric Acid. Journal of Petroleum Technology 1970;22(4): Muecke, Thomas W. Principles of Acid Stimulation. International Petroleum Exhibition and Technical Symposium Beijing, China March Shaughnessy, Christopher M, Kunze, Kenneth R. Understanding Sandstone Acidizing Leads to Improved Field Practices. Journal of Petroleum Technology 1981;33(7): Smith CF, Hendrickson AR, Dowell D. Hydrofluoric Acid Stimulation of Sandstone Reservoirs. Journal of Petroleum Technology 1965;17(2): Sharaf M, Sarwat AEE, Osama AR. A Novel Approach to Acidizing of Carbonates: Case History From the Gulf of Suez in Egypt. SPE Annual Technical Conference and Exhibition, Dallas, Texas 1-4 October Page