Well Production Optimization under the Scale Effect and CO2-WAG Injection in a Carbonate Model of the Brazilian Pre-Salt

Abstract

Scale represents a risk to the profitability of projects in the oil exploration and production industry. The occurrence of the phenomenon has the potential to cause severe damage to producer and injector wells, subsea lines, and surface systems, commonly requiring the cleaning and replacement of these elements and resulting in lost profit because of reduced production efficiency, need for unscheduled maintenance stoppages, and, in the most serious cases, permanent losses of wells, lines, and equipment. The problem becomes even more severe in projects that use the injection of reactive fluids into the reservoir, as is the case with carbon dioxide-water alternating gas (CO2-WAG) injection. To avoid such an occurrence, scale inhibitors are used, which are continuously dosed at various points in the production flow, from subsea chemical injection mandrel, commonly positioned at the ends of the production columns, wet Christmas tree valves, and at various positions in the plant process on the surface. However, the regions upstream of the chemical injection mandrels, such as the perforations zone and the reservoir itself, are normally not protected by continuous-dosing inhibitors and are therefore critical for the occurrence of the phenomenon. These critical points require removal operations when the accumulation of scale is significant to economically justify a treatment. To prevent the recurrence of scale in the short term, the cleaning operation is commonly associated with the squeeze of inhibitors into the porous medium. The effect of this inhibition, however, is temporary, as the inhibitor adhered to the rock is released with the water production, until it ceases to be effective after a determined accumulated production of this fluid, requiring the evaluation of a new application of the product. This work proposes a workflow for optimizing cleaning treatments associated with scale inhibitor injection, as well as CO2-WAG fluid exchanges, aimed at protecting the reservoir and well producer perforations, and seeking to optimize the project’s net present value (NPV) in operations carried out by means of rigs or remotely by the stationary production unit (SPU). To this end, a study was carried out by coupling producer scale and inhibition proxies to a flow simulation with a reactive transport model, representative of some fields of the Brazilian pre-salt. The method developed, in addition to providing greater robustness to the predicted production curve by considering the occurrence of scale in the producer perforations, can identify optimal time windows for the treatment, even if they are in periods where the inhibition is no longer effective. The results obtained in the study for a single producing well indicate a substantial gain in the NPV with the use of the proposed methodology, around 13 million USD in relation to the proposal of treatments always carried out in a preventive way. The application also indicated that it is more advantageous to carry out remote operations through the SPU, even with lower cleaning efficiency when compared with rig treatments, with a gain of about 12 million USD.