Implications of Brine Mixing in the Reservoir for Scale Management in the Alba Field

Abstract

This paper brings the discussion on brine mixing initiated at the 1999 SPE Symposium on Oilfield Scale full circle, and suggests that different scaling regimes may exist in any given reservoir that should impact the scale management strategy. The initial paper by White et al. (1999) identified lower than expected barium levels in many wells in the Alba field, and raised the question of where scale deposition is occurring. A follow-up paper at the same meeting the following year by Mackay and Sorbie (2000) identified from a theoretical standpoint where brine mixing should be expected, and suggested that significant scale deposition may occur deep within the reservoir, particularly where a large proportion of injected water sweeps through the aquifer. The current paper seeks to apply that theoretical analysis to the Alba field, on a well-by-well basis, to establish whether diagnostic tests can identify which wells should be treated conventionally, and which require special attention. Firstly, production data and produced brine compositions are analysed to identify any recurring patterns. Wells are then classified and grouped according to various types based on this analysis. Well properties such as location within the reservoir and orientation to the flood front are then compared to identify whether they can be used to give a similar grouping of wells. Three principal zones are identified, with position relative to the injection wells and the aquifer identified as key parameters. Secondly, the flow patterns around each well are studied using the existing reservoir flow model. Modelling the dynamic mixing patterns throws further light on the differences between the zones, with scale dropout predicted in the aquifer in some areas, and in the oil-leg in others. Recommendations are made regarding the treatment of existing wells and the optimum positioning, from a scale prevention perspective, of any new wells. Introduction Sulphate scale deposition is a common problem in reservoirs where injected seawater mixes with aquifer brines1,2. For reasons that will be explained in the next section, the problem is most severe in and around the production well bores, and can cause considerable disruption to hydrocarbon production after water breakthrough. To deal with the problem two general tasks must be performed. Firstly, the quantity and type of scale must be identified, together with the location and timing of the deposition. Secondly, a suitable removal and/or prevention strategy must be designed and implemented. Both of these tasks require reservoir and laboratory data, and field experience is also a vital component in ensuring successful treatment. Both tasks are routinely assisted by application of appropriate modelling tools3. The use of reservoir simulators to predict the timing and location of water breakthrough, and the application of scale prediction codes to identify the scaling regimes, have both been discussed previously4–9. In addition, models are routinely used in chemical scale inhibitor selection studies and to optimize inhibitor squeeze treatments10. The topic of this paper is not so much the design of scale prevention treatments, although the findings have a bearing on this, but on identifying scale deposition regimes throughout an actual reservoir system. To do this produced brine chemistry data and field fluid flow modelling have been analysed on a well-by-well and field-wide basis for the Alba Field in the North Sea. The evidence of both the produced brine chemistries and the flow calculations is that scale is depositing deep within the reservoir. However, the extent and impact of this deposition varies throughout the reservoir. Three regimes are identified, where the nature of the scaling problem at individual production wells can be related to the flow characteristics in that locality. Important factors are shown to be the proximity of injection wells and the proximity and extent of the aquifer.

Impact of Reservoir Reactions on Thermodynamic Scale Predictions

“…Although in the literature there are not many publications that discuss reservoir chemical reactions, it is quite easy to find examples of fields where the produced brine has concentrations of barium below the dilution line [1][2][3][4][5]8 (the concentration expected if no barium consumption occurs in the reservoir). In fact, some publications use the barium consumption to obtain a better prediction of the seawater content in the produced brine 5,7 .…”

The Optimisation of a Scale Management and Monitoring Program for During the Production-Decline Phase of the Life Cycle

“…This is best illustrated by plotting barium concentration vs. seawater fraction in the produced water. A typical example from the literature 4,5,11,12,15,19 is shown in Fig. 2, where it is clear that the measured barium concentrations for all wells in this field fall considerably below where they would be expected if dilution was the only factor, despite the fact that all the wells in this field had been under active scale management by squeeze treatments.…”

“…This in situ stripping mechanism has been proposed, studied and validated by comparing model results with field data for a large number of fields, and results have been published extensively 4,5,[11][12][13][14][15][16][17][18] . In most of the systems described in these publications there has been an excess of sulphate ions in the injection brine relative to the concentration of barium ions in the formation brines.…”

Comparison of Streamline Reservoir Simulation of Brine Flow, Mixing and Barium Sulphate Induced Formation Damage with Observed Produced Water Chemical Data to Aid Scale Management

“…However, the role of the reservoir was totally neglected in all of thermodynamic modelling work which is based on the fundamental assumption that the potential scale precipitation will be due to pure mixing of formation and injected water in the production well. However, it has been reported in some recent publications that both of brine mixing and geochemical reactions take place within the reservoir and the chemical composition of produced water would then be altered by them before arriving at producers (Paulo et al, 2001;McCartney et al, 2005;Houston et al, 2006;Mackay et al, 2006;Gomes et al, 2012;Fu et al, 2012). This paper reveals some fluid/fluid and fluid/rock interactions occurring in the reservoir through combining produced water chemical compositional data with geochemical and reservoir simulation model.…”

“…However, geochemical data was only regarded as natural non-reacting ions and the possible geochemical reactions involving them were not considered in these publications, because they normally used a conventional reservoir simulator, such as ECLIPSE, that does not have a chemical reaction model, and so only the flow transport and brine mixing were calculated. Some applications were demonstrated by Delshad et al, (2003) using UTCHEM, a three-dimensional reservoir simulator, for studying the brine mixing and transport of barium and sulphate ions and barium sulphate scale precipitation taking place within the reservoir and Paulo et al (2001) and Mackay (2003Mackay ( , 2014 applied the flow and reaction simulator to model barite precipitation. However, unfortunately, there were no observed produced water chemical data presented to compare with simulation results in these modelling studies.…”