Impact on Scale Management of the Engineered Depressurisation of Waterflooded Reservoirs: Risk Assessment Principles and Case Study

Abstract Increased oil and particularly gas production may be achieved in waterflooded reservoirs by stopping further water injection, and depressurising the reservoir to release solution gas.Pressure depletion may be accelerated by back producing injected brines.However, there is the possibility that these brines may cause formation damage by mobilising fines or deposition of inorganic scales.Scale deposition in production wells may also occur as a result of pressure depletion, with calcite scales being precipitated when the system drops below the CO2 bubble point pressure.This paper discusses the assessment and prediction of scale related formation damage problems that are likely to occur during depressurisation of a case study field.The potential for the specific problem arises from the formation of barium sulphate scale as a result of mixing of injected and formation brines during production.Data used in this study includes well brine chemistries and an existing finite difference reservoir simulation model of the field depressurisation, which was used to calculate the mixing of injected and formation brines, and the movement of the mixing and temperature fronts during waterflooding and subsequent depressurisation. This study has determined that the behaviour of the scaling potential for each well in this field is different.Also, the degree of scaling, both deep within the reservoir where it does the least damage, and around the wellbore (for both injectors and producers) where it may adversely affect production, can be predicted by detailed modelling using both conventional and reaction-flow simulations.Former injectors converted to water production or infill wells drilled in the aquifer for pressure depletion may experience an increase in the scaling potential that significantly impacts the economics of the project because of the need for extensive prevention (inhibition) treatments.The increased scaling potential in these wells is a result of the dynamics of brine mixing in the reservoir, the lowering of reservoir temperature in the vicinity of injection wells during waterflooding, and the large volumes of water that require to be produced to achieve depressurisation.The magnitude of the scaling problem and the economic impact are lower for the production wells due to lower water production rates and higher temperatures.