Modelling the Impact of Diesel vs Water Overflush Fluids on Scale Squeeze Treatment Lives Using a Two-Phase Near-Wellbore Simulator

Abstract

This paper describes the development of a two-phase near-wellbore simulator to predict the impact on squeeze lifetime of the overflush fluid type. In the past, a single aqueous phase model was used for both brine and hydrocarbon overflush treatments. The new model enables a more accurate description of the displacement process if a hydrocarbon fluid (eg diesel) is used, and the impact on inhibitor transport through the formation and retention onto the rock matrix. Field data is presented, with various treatments where either seawater or diesel are applied as displacement fluids being considered. In each case, the well was treated with the same aqueous scale inhibitor. The initial squeeze treatments used a diesel overflush. However, subsequent treatments utilised the same inhibitor but with seawater as the overflush fluid. It is clear from the field returns that the use of seawater rather than marine diesel improved chemical placement and extended treatment life. The theory behind this phenomenon is outlined, so allowing for more accurate treatment designs. The process followed involved first deriving an isotherm using a single-phase squeeze model based on the water overflush treatment. This is the established conventional approach used in many hundreds of cases worldwide. This isotherm was then used to model the same treatment using the new two-phase model, which accounts for saturation changes during the treatment. A good match was achieved using the isotherm, giving confidence that the two models agree for purely aqueous treatments. A diesel overflush treatment was then simulated using the two phase model and the same isotherm, and again a good match was achieved. However, modelling the diesel overflush treatment in the single-phase model required a different isotherm to achieve the match. This clearly indicates that diesel overflush treatments may be accurately modelled using the two-phase model. Additional sensitivity calculations were performed to investigate the impact of splitting the overflush volume into separate diesel and water stages to improve well clean up while reducing the logistic challenges associated with pumping large volumes of diesel.. Introduction Scale formation represents a very significant flow assurance challenge to the oil and gas industry. Scale formation is due to the precipitation of inorganic minerals, such as the carbonate and sulphate scales caused by pressure drop and brine mixing respectively. The most common method to prevent scale formation is to use a scale inhibitor (SI) squeeze treatment, which is a well established procedure in onshore and offshore oil production facilities 1–11. In general, a scale inhibitor squeeze treatment comprises the injection of SI, normally as an aqueous solution, usually preceded by a smaller preflush, and followed by a larger overflush stage. Finally, there will often be a shut-in stage, usually lasting from 6 to 24 hours, that allows time for the scale inhibitor chemical to interact with the reservoir rock. The treatment is then complete, and the well is brought back on to production. This process is illustrated in Figure 1. The main purpose of this paper is to present results of a modelling study for a field case where the well was treated with the same aqueous inhibitor on a number of occasions, but where the initial squeeze treatments used a diesel overflush whereas the more recent treatment used a seawater overflush. Results using a single-phase model (SQUEEZE V) are compared with a two-phase model (SQUEEZE VI). The underlying principles of overflush design were presented in a previous study 12. In this paper modelling tools are used to explore in greater depth the key parameters pertinent to the overflush stage, and to throw some light on the differences between using single and two-phase models for these types of calculation. In addition, sensitivity studies investigating the impact of the initial water saturation in each layer, and the consequence of splitting the overflush into separate diesel and water stages are presented. The conclusion drawn is that this type of study should be performed using a two-phase flow simulator.

Proactive Scale Mitigation Strategies for Simple to Complex Multilateral Producers in a Saudi Arabian Carbonate Field

“…The simulations indicated a best treatment life for 100% water and worst for 100% diesel, and a significantly improved life for split overflush compared to that of 100% diesel. In addition, water followed by diesel is the preferred sequence in the overflush 18,19 . This could be due to the fact that the initial water injection during the overflush causes desorption of the chemical near the wellbore and displaces it further away from the wellbore, as does the following diesel overflush stage displaces the aqueous chemical slug further into the reservoir.…”

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“…In addition, we reduced the aqueous phase in the SIS to minimize the adverse aqueous-based injection effects and water-sensitive effects for the producers. Sensitivity calculations were performed to investigate the impact of splitting the overflush volume into separate diesel and water stages to improve well cleanup while reducing the logistic challenges associated with pumping large volumes of diesel 20 . Consequently, water followed by diesel was preferred to diesel alone as the overflush, and that was implemented for our recent SIS jobs.…”