A sweet solution

A SWEET SOLUTIONOLUTION

Tom Swanson, Solugen, USA, discusses a novel oxidant and sugar derivative, designed to improve saltwater injection challenges.

Oil and gas operations rely on the ability to until the chloride levels are beyond reuse purposes. process produced water economically and Commercial saltwater disposal systems in Oklahoma responsibly. Some of the common methods play a critical role in accepting used produced water for produced water handling involve reuse in oil and from centralised locations and utilising methods of gas operations and, when required, at the end of the sterilisation and pre-treatment before injecting into lifecycle is disposal. The oil and gas industry utilises deep wells. a network of salt-water disposal wells which are regulated by state and federal agencies and are Saltwater disposal designed to inject spent produced water deep Saltwater disposal (SWD) operators are faced into the earth, beyond usable water tables. with a myriad of challenges, including the Oklahoma’s Scoop and Stack Basin, precipitation of metal salts, which can US, produces large amounts of be found at high concentrations water from high water cut wells. in the saltwater and known This produced water is to precipitate and block used in oil and gas flowlines and tubing processes systems.

In some SWD operations, water is prepared for injection through a process of forced precipitation which involves the use of oxidisers. The oxidisers chemically convert the metal salts into precipitating oxides and hydroxides which then can be separated from the water and disposed of in environmentally acceptable landfills. The remaining water is then conditioned with additives to prevent the residual dissolved solids from forming scale within the tubing or reservoir to which they are injected. Scale formation can reduce the life of the saltwater injection well and add higher operational costs to the overall process to remediate damage or, in extreme cases, damage the injection well beyond economical repair. Although oxidiser precipitation is a common practice, it is almost never a complete process due to atmospheric exposure, incomplete mixing, and inadequate chemical concentrations to match the volume and chemical makeup of the arriving water from various sources.

Table 1. Ionic makeup and water analysis and scale prediction

Brine composition

pH 6.7 Calcium (Ca), mg/L 523

Temperature 125˚F Iron (Fe), mg/L 18

Specific gravity 1.025 Magnesium (Mg), mg/L 1

HCO 3 , mg/L 891 Sodium (Na), mg/L 10 911

Chlorides - CL, mg/L 17 000 Barium (Ba), mg/L 6

Sulfate - SO 4 , mg/L 49 Strontium (Sr), mg/L 73

Probable mineral composition

Calcium bicarbonate mg/L 1184 Magnesium bicarbonate mg/L 540

Calcium sulfate mg/L 69 Magnesium sulfate mg/L 0

Calcium chloride mg/L 584 Magnesium chloride mg/L 0

Figure 1. The furthest left corrosion coupon is the BioChelant TM tested in neat form vs other chelants.

Table 2. Injection performance data

Product PPM Injection pressure (psi) Hours

Hydrogen peroxide combined with corrosion and scale inhibitor

Biochelant and oxidiser combination product

Biochelant and oxidiser combination product

Biochelant and oxidiser combination product 250 1300 24

150 1250 48

200 1125 72

300 1100 96

Enzymatic oxidation

To address the issue of oxidised induced scales, advances have been made in bio-chemical manufacturing with enzymes and renewable organic feedstocks. These new bioreactors use corn sugar and enzymes in a process called enzymatic oxidation. The process starts with a renewable feedstock, such as corn sugar, which is enzymatically converted into oxidised sugar with a byproduct of hydrogen peroxide. This chemical reaction is unique in that the oxidised sugar is co-existing with the oxidiser in a chemically stable form making its use as a single product viable. Economics are favourable for this combination chemistry due to the low cost of the available feedstock and manufacturing process. This unique process is also carbon negative compared to other chemical manufacturing processes.

The oxidised sugar has benefits in oil and gas as it has exceptional performance in solubilising metal ions, thus preventing them from forming precipitating scale in the salt-water disposal operations. Additionally, the oxidised sugar can be deployed with an oxidiser in one product thus reducing the costs of maintaining and acquiring additional chemical injection systems. Furthermore, the required concentrations are lower than traditional chemistries due to the non-neutralising eff ect of the oxidiser on the secondary product.

Case study

An opportunity to test this new chemistry in the Scoop and Stack Basin in Oklahoma was of interest due to the known challenges of injection pressures due to scale and the wide use of oxidisers. The candidate facility was at a saltwater disposal facility processing and disposing of approximately 30 000 bpd of water. Water was being trucked into the facility and pumped to an open earthen lined pit for processing. During the transfer, hydrogen peroxide or peracetic acid was applied to neutralise hydrogen sulfide gas and begin the oxidation of metals from the water. Throughout the continuous process, the calculated retention time in the pit for the water was measured at over 24 hours. Aft er treatment in the pit, water was transferred to a series of serge tanks where corrosion and scale inhibitor were injected and finally the saltwater was then pumped into the disposal well.

The disposal sources of water contained high concentrations of prevalent metals which were predicted to form scales of calcium, magnesium salts without further oxidation and iron oxides/hydroxides when hydrogen peroxide or peracetic acid were deployed as the primary oxidiser (Table 1). Furthermore, the pH of the source water was near neutral (7) and therefore posed additional risks to the solubility of salts in general.

The operator of this facility noted that injection pressures were increasing over time, which increased concern that scale was forming in the injection tubing. The primary injection of hydrogen peroxide (34%) averaged 200 – 300 ppm depending on the concentration of hydrogen sulfide and content of the metals in the arriving water. The scale inhibitor deployed on this location was a phosphonate and the corrosion inhibitor, was a quaternary amine at a total combined dose rate of 30 ppm based on the total volume of water. There were two injection sites for both products since they could not be combined due to chemical compatibility.

Prior to recommending changes, an investigation was performed regarding the performance of the scale inhibitors in the presence of the oxidiser. A water sample was obtained and tested on a Millipore TM filtration apparatus at 20 psig, 0.45 micron filter paper, and 1 l of water. Various dosage rates were evaluated with similar results. The filters were evaluated with wet chemistry techniques and it was determined that there was appreciable iron, calcium, and magnesium-based oxides and hydroxides present. Due to the fact these scales were present and water analysis predictions indicated that these scales were expected

to form, the next phase of the project was to look at oxidiser compatible scale and corrosion inhibitors as suitable replacements.

In this process, oxidisers are required to sterilise the water and neutralise hydrogen sulfide. Therefore, attention was turned to available oxidiser compatible sequestrants (also known as biochelants from the process and feedstock from where they are derived). These biochelants are the oxidised sugars from the process previously described to produce hydrogen peroxide. The unique property of biochelants, is they have binding sites for metals and create a water soluble, not precipitating form which is suited for these types of applications. A second function of the biochelant is its ability to inhibit corrosion in oxidised water. The biochelant’s carbon backbone contains a series of hydroxyl groups which when applied in aqueous systems prevent oxidation at the metal surface due to concentration gradients at the metal surface. Additional laboratory testing on these biochelants were conducted in high concentrations at 150˚F for 72 hours and the corrosion rate was calculated at less than 2 mpy (Figure 1). As noted in the carbon steel coupons, there was no signs of pitting or surface deformation when compared to oxidise water without biochelant present.

Based on this knowledge, a formulated hydrogen peroxide and biochelant blend were prepared for testing and evaluated using Millipore filtration. Post filtration of the treated water, it was noted that the filters were absent from metal oxides and hydroxides which remained soluble and passed through the filter material, therefore it was deemed as a viable test chemical for this system. The formulated product referred to as an oxidising biochelant and was used to replace the 34% peroxide and multifunctional corrosion and scale inhibitor at similar dosage rates to compare diff erential injection pressures over time.

Stay informed

It was noted that aft er 24 hours the injection pressures began to reduce which was in alignment with the filter’s absence of iron precipitant. Overall, the diff erential pressure was reduced by 175 psi, allowing additional water to be injected without concern regarding the integrity of the tubing cleanliness (Table 2). The reversal of pressure under similar conditions was a key factor in assuring that scale formation was not attributing to the potential early failure of the injection well.

Post test data indicated that an optimum dose rate would be pre-determined by the ionic makeup of the source water and presence of hydrogen sulfide with this combination product. The prevailing component was hydrogen peroxide with secondary component of the biochelant which required lower overall concentrations. Additional optimisation steps were taken to further refine the application, but the concept of combining an oxidiser and scale/corrosion inhibitor proved to be successful as a combined product with performance and economic advantages.

Conclusion

Biochelants derived from oxidised sugars are an economical solution for salt-water disposal systems, which are plagued by metal scales as a result of the oxidation process. These biochlelants can be produced from a renewable source and off er economical solutions for oil and gas operations where low environmental risk is needed and to maintain flow assurance in salt-water disposal systems. Future areas of interest for produced water in the future are the re-purposing of water beyond disposal where additives will need to be non-toxic and have a low environmental impact for future considerations.

Keep up to date with us to hear the latest in upstream oil and gas news   www.oilfi eldtechnology.com