This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 215035, “Novel Ways of Optimizing Production of Gas Wells Suffering from Downhole Salt Deposition,” by Mathijs R. Janzen, Erik Hornstra, SPE, and Bert Doldersum, TotalEnergies. The paper has not been peer reviewed.
Downhole salt precipitation is an increasing issue in mature gas fields of the North Sea. Two novel solutions have been conceptualized and applied, the first being the installation of velocity strings. A case study is presented wherein four velocity strings were set, leading to a strong increase in platform flow rate and the extension of asset life. The major advantage of this solution is that it is relatively simple and extremely robust and does not involve increased operating expense (OPEX). Compensation of evaporation by downhole supplementation of fresh water also is a method of mitigation discussed in the complete paper.
This paper describes two successful methods of increasing production performance for gas wells suffering from salt depositional issues. Enhancement studies performed by the operator first identified that velocity strings (installation of smaller-diameter tubulars), normally used for liquid-loading issues, can be applied to limit salt precipitation successfully.
Following the wider application of this principle over the portfolio of the operator, the need was identified for a different solution for high-quality reservoirs. Evaporation calculations revealed that, instead of limiting drawdown, compensation of evaporation by downhole supplementation of fresh water could be feasible. Capillary strings have been used temporarily for cleanout of halite, but now permanent application with permanent continuous downhole water injection has been proven on a several-weeks-long pilot test on one of the operators’ producers.
Diagnostics. Before deciding on which countermeasure to deploy, salt precipitation must be confirmed. The operator can follow several steps, the first being the close monitoring of the day-to-day performance of the well. Following salt deposition, the inflow performance relation (IPR) often degrades so that the well moves into a liquid-loading regime. Therefore, salt precipitation and liquid-loading issues often are seen together, rendering diagnostics purely based on flow and flowing wellhead pressure more troublesome.
In addition, topside salinities should be evaluated. In cases of downhole salt precipitation, measured salinities often are highly variable. In cases of high formation-water production, they are continuously high, but no salt issues are encountered. In cases of downhole salt deposition, the water produced to the topsides often has low salinity. After condensation water fallback or water washes, the water produced when flowing again has much higher salinity because it has dissolved the downhole salt to varying degrees.
An important third step in diagnostics is entering the well to perform data acquisition. This operator usually takes a downhole sample and, preferably, creates a downhole video. Fig. 1 shows a downhole image of severely affected tubing in one of the operator’s wells, together with two images of the salt brought up to surface.
Furthermore, evaporation calculations are conducted to form a salinity profile along the wellbore together with a maximum solubility curve.
