Maintaining Injectivity of Disposal Wells: From Water Quality to Formation Permeability
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An extensive laboratory study was carried out with two objectives: to evaluate the effect of water quality on injectivity of disposal wells with reservoir core plugs and to restore injectivity of damaged wells. In this paper, water-quality guidelines to minimize or prevent formation damage are recommended. On the basis of laboratory work, a novel chemical treatment was successfully applied to restore injectivity of several damaged disposal wells. This novel treatment reduced the long operation time and cost of a typical treatment practice while effectively stimulating the well.
Effect of Water Quality and Formation Permeability on Injectivity
Water quality has a major influence on the injectivity of injection and disposal wells. Poor injection or disposal-water quality can compromise the effective injectivity of even high-quality sandstone or carbonate formations. Source water used for injection often contains solids, which can reduce permeability of the formation around the wellbore.
Solids-particle damage depends on particle size of the solids, oil present in the injected water, and the average pore-throat diameter of the formation. If the particles are larger than the average pore-throat diameter of the formation, then the particles cannot penetrate the pores. As a result, an external filter cake with permeability lower than that of the formation will form (Fig. 1a above). Another type of injectivity impairment occurs when the size of the particles present in the injected water is smaller than the average pore-throat diameter of the formation. These particles will invade the formation and bridge at some pores (Fig. 1b). As solids concentration in the injection water is increased, the rate of permeability decline becomes greater. Obviously, if the size of the particles is significantly smaller than the average pore-throat diameter of the formation (Fig. 1c), then the particles will flow through the formation without causing any damage. As a result, there will be no loss of injectivity for a long period of time.
Stimulation of Water-Disposal Wells
In carbonate formations, hydrochloric acid (HCl) (up to 28 wt%), formic acid (up to 10 wt%), acetic acid (up to 10 wt%), and combinations of these acids can be used to remove formation damage and enhance well injectivity. The acids are used to dissolve the inorganic scale and bypass the organic scales by creating wormholes. In sandstone formations, where the formation permeability cannot be stimulated and the ultimate target is to restore original formation permeability, a two-stage treatment is commonly used to remove damage from disposal wells.
Results and Discussions
Disposal-Water Characterization. As a key issue in identifying the quality of disposal water, geochemical analyses were conducted for disposal-water samples collected at different times. The total dissolved solids (TDS) in these samples ranged from approximately 157,000 to approximately 172,000 mg/L. The average TDS value for these samples is 165,635 mg/L, indicating that formation water was diluted by the injected water. The pH values of these samples are within the neutral region and range from 6.3 to 7.3.
Total suspended solids (TSS), which is an important parameter in characterization of disposal water, was measured for six samples collected in May/June of 2014 and February/March of 2015. The results reveal the considerable variations in produced-water quality in terms of TSS. Particle-size distributions for suspended solids in a disposal-water sample displayed a mean particle size of nearly 18 μm, while 90% of the particles had a size of 51.52 μm or less and 10% had a size of 3.54 μm or less. These results suggest the need to implement a filtration process to reduce the amount of particles and the average particle size and thus minimize injectivity loss.
Oil-content measurement for disposal-water samples collected from Well GOSP-B during the period from January to March 2015 revealed an average oil content value of 31.7 ppm with few points above 50 ppm (the maximum limit). The results indicate a good control of oil in disposal water with respect to the designated limit. This limit should be reduced to minimize formation damage in disposal wells.
Effect of TSS. The coreflood results indicate that the extent of damage in core permeability is a function of disposal-water quality (in terms of TSS) and core initial permeability. The results reveal that disposal water, even with high TSS, will have less effect on high-permeability formations. Poor-quality injected water will have a greater effect on low-permeability formations. The higher the TSS is in disposal water, the greater the damage is to the core permeability.
Effect of Filtration on Permeability Impairment of Disposal Water. A core plug representing the formation of interest was used to assess the effect of filtration of disposal water on core permeability. The results revealed that, at low filtration sizes, the extent of damage will be less; hence, there is less reduction in the formation permeability. These results suggest that a good filtration system will minimize formation damage associated with disposal water.
Effect of Mixed Filtered Disposal Water and Groundwater on Core Permeability. To assess the effect of mixing filtered disposal water and groundwater on reservoir core permeability, a core representing the formation of interest was used. This core plug has an air permeability of 6.77 md and a porosity of 25.3%. The initial brine permeability of this core plug was 9.65 md. The results showed no damage and suggest that groundwater is compatible with the disposal water.
Disposal-Water-Quality Guidelines. Laboratory results shed light on required disposal-water quality in terms of suspended solids, oil content, and some operation conditions. To prevent or minimize formation damage and thus maintain injectivity of disposal wells, the following guidelines are recommended:
- Total suspended material should not exceed 50 ppm and mean particle size should not exceed 5 μm.
- Oil content should not exceed 20 ppm.
- To prevent formation damage caused by particle settling, disposal water should be injected continuously into the disposal wells.
- In the case of the well being shut in, flow it back before putting it into injection.
Characterization of Disposal Well A Sludge Sample. Different analyses conducted on the sludge sample obtained from Well A include thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and environmental scanning-electron microscopy (ESEM). The Well A sludge sample was subjected to TGA before and after washing with toluene, indicating that more than 46 wt% of the sludge was hydrocarbon. Xylene extraction was able to dissolve more than 60 wt% of the sludge material, and asphaltene content of the organic material varied up to 23 wt%. The inorganic solids remaining after extraction are mainly water and halite. The inorganic-solid XRD and ESEM analyses showed that the sludge is mainly sodium chloride and iron-corrosion products, including iron sulfides and oxides.
Solubility Tests of Well A Sludge Sample. The sample was examined in different solvents as a function of time and temperature. Its solubility in a two-stage system (solvent and then acid) was examined at reservoir temperature (160°F). Up to 88 wt% of the damaging material can be dissolved with Solvent A followed by 10 wt% formic acid. Similarly, 84 wt% could be dissolved with Solvent A and then 10 wt% HCl.
The solubility tests evaluating dissolving sludge material in disposal flow lines were conducted at 120°F. The results showed that the emulsified 15 wt% HCl/solvent system exhibited the highest dissolution power. Increasing the soaking time to 3 and 4 hours increased the solubility slightly (95.6 and 96.96 wt%, respectively).
Treatment of the Well A Disposal-Water Flowlines. The sample collected from the Well A disposal-water flowline was found to be sludgy material that was composed of nearly 60 wt% organic material and 40 wt% inorganic material. The organic part can be dissolved with solvents, and the inorganic part is composed of acid-soluble compounds. To remove the sludge material from Field A disposal-water pipelines in a single-stage treatment, an emulsified acid/solvent system should be used. In this regard, different acid/solvent systems, including organic and mineral acids, were evaluated. Among the emulsified acid/solvent systems, the emulsified 15 wt% HCl acid with asphaltene-specific solvent/paraffin-specific solvent/surfactant system exhibited the highest dissolution power and was able to dissolve nearly 97 wt% of the sludge sample after 4 hours. A suggested disposal-water-treatment process is described in the complete paper.
On the basis of laboratory-optimized recipes, a chemical-treatment campaign on Field A disposal wells was safely and successfully implemented in 2015. A gain in injection capacity greater than 50,000 BWPD was achieved after stimulation treatments of four disposal wells in Field A.
The matrix treatment involved squeezing 260 bbl of solvent system into the formation at 1 to 2 bbl/min while reciprocating the coiled tubing (CT) across the perforated region. The solvent system was allowed to soak for 8 to 12 hours. While reciprocating the CT across the perforated liner, 65 bbl of the organic acid was squeezed into the formation. The solvent treatment was displaced from the CT with fresh water mixed with a hydrogen sulfide scavenger. The solvent system was allowed to soak for 4 hours. The well was flowed back by use of nitrogen lift until returns were solids-free.
The optimized stimulation treatments were effective in reducing treatment cost and time with a significant increase in well injectivity. The post-treatment injectivity tests showed a gain in injectivity of four treated wells with nearly 60,000 BWPD.
Maintaining Injectivity of Disposal Wells: From Water Quality to Formation Permeability
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