Optimizing Sandstone Acidization
- Eduardo Ponce da Motta (Petrobras R and D Center) | Benjamin Plavnik (Petrobras R and D Center) | R.S. Schechter (U. of Texas)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1992
- Document Type
- Journal Paper
- 149 - 153
- 1992. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 3.3.1 Production Logging, 1.6.9 Coring, Fishing, 2.4.3 Sand/Solids Control, 2.2.2 Perforating, 2.4.5 Gravel pack design & evaluation, 1.8 Formation Damage, 3.2.4 Acidising, 4.2.3 Materials and Corrosion, 1.10 Drilling Equipment, 1.2.3 Rock properties
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The treatment of sandstone formations with mixtures of HF and HCl to removeformation damage is a widely practiced stimulation method; however, littleattention normally is given to process optimization. This paper presents aprocedure design that minimizes the total acid volume paper presents aprocedure design that minimizes the total acid volume requires to remove damagefrom a fixed-zone, near-wellbore region. To account for differences information mineralogy, a single laboratory coreflood experiment with field coresis necessary but more are recommended. The optimization method relies onscaling this experiment to a radial-flow geometry, where acid flux rates maydiffer substantially from those used in the single core test. Changing theinjection rate is shown to result in a minimum acid volume at an optimuminjection rate, and this optimum is found for a specific example.
Matrix acid treatments to remove near-wellbore damage within sandstoneformations is an old, but still widely used, procedure. If applied in damagedwells, excellent results can be obtained. Despite the considerable progressreported on understanding the mechanisms underlying sandstone progress reportedon understanding the mechanisms underlying sandstone acidizing, surprisinglylittle design precedes an actual treatment, even though improved results may bepossible from a minimum amount of laboratory work. The purpose of this paper isto present a design procedure that defines the optimum rate and total acidvolume and that also decline at the maximum depth of acid penetration attainedby the treatment. Of course, many other factors must be specified to completethe design. Corrosion inhibitors, chelating agents, diverting agents, andsurfactants are sometimes required, but these additives are not discussed here.Williams et al. recommended that the acid be applied at the maximum possiblerate and that as much acid be used as can be economically possible rate andthat as much acid be used as can be economically justified. Hill et al.modified this strategy by noting that it applies only to wells with deep damageand that there is an optimal policy that depends on the depth of the damagedzone. They development crude "design charts" to guide the choice ofacid volume and injection rate. These charts, however, do not recognize thedifferences in sandstone mineralogy and are based on experiments carried outwith Berea sandstone cores, which are unlikely to be representative of theformation to be treated. In this paper, the basis for an optimum acid-injectionrate is examined and a fundamental approach for its determination found. Thestrategy proposed here will require at least one laboratory acidization of afield core at reservoir temperature. Futhermore, the effluent acidconcentration must be measured in addition to the permeability increase as afunction of PV's of acid injected, which is now routinely recorded. Theeffluent acid concentration is difficults to measure and is therefore seldomreported. This difficulty remains to be resolved, but this paper is intended toshow the need for this sort of data. Ion-specific electrodes have been used tomeasure the activity of F-, but this approach is difficult because other ionspresent in the solution tend to interfere. A simple, but very time-consuming,approach is to measure the amount of glass that can be dissolved by a fixedvolume of effluent. We used a combination of these two methods and found it notto be truly satisfying. Therefore, a substantial need exists for an analyticaltechnique that provides a measure of the F- concentration in a solutioncontaining dissolved silicates. Once an analytical technique is available andfound suitable for routine analyses, then the approach proposed here will proveuseful. The analysis begins with the model proposed here will prove useful. Theanalysis begins with the model developed by Hekim and Fogler.
Flow and Reaction In Linear Systems
As HF enters a sandstone core, almost all the minerals present begin todissolve, but at different rates depending on the intrinsic rates ofheterogeneous reactions and the exposed surface area. the reacting mineral canroughly be divided into two distinct categories: slow and fast reacting.Detrital quartz and clay fragments tend to react at a slower overall rate,whereas feldspars, authigenic clays, and amorphous silica tend to react at afaster rate. The difference in overall reaction rates between these twocategories is almost one order of magnitude.
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