A Sensitivity Study on the Main Factors Affecting a Polymeric RPM Treatment in the Near-Wellbore Region of a Mature Oil-Producing Well

Abstract

Abstract This paper describes a sensitivity study on the main factors affecting a polymeric Relative Permeability Modifier (RPM) treatment in the near wellbore region of a mature oil producing well. The study is divided into several parts where various factors, which affect the application of RPM technology in a chosen field base case well, are studied. These factors include the effects of instantaneous vs. kinetic adsorption for the treatment and the further influence of treatment properties, reservoir fluid properties and the reservoir formation. From the sensitivity study, we can conclude that the most influential factors in the treatment response, i.e. the water cut reduction, are the combination of polymer adsorption type (kinetic or equilibrium) with method of application of the resistance factors (threshold or variable), resistance factor ratio, reservoir fluid properties and reservoir layout. On the other hand, the polymer viscosifying effect is not such an influencing factor, and neither are the shear stress, salinity, hardness and pH that subsequently affect this property. The sensitivity study was carried out using a model capable of describing RPM treatments in the near wellbore region. The model is a radial, isothermal, two-phase, multicomponent and multi-layer mathematical model. The model considers the immiscible displacement of the oil and water phases along with polymer transport in the aqueous phase. The various phenomena that describe polymer flow through a porous medium are also considered and modelled. For each of the various physical phenomena (e.g. adsorption, fluid rheology, resistance factor, etc.), we must then make decisions about its behaviour, which is simulated by choosing certain models and the parameters in these models. For multi-layer systems, the model considers no vertical communication between layers, thus the pressure drop across the various layers remains equal. The model includes kinetic adsorption and the application of resistance factors based on a polymer adsorption threshold, which are not normally available in other models. The model has been verified by comparison with both analytical solutions and Eclipse 100 results and novel observations of kinetic and equilibrium adsorption in radial coordinates are described. Introduction Water management is becoming an increasingly important cost issue as oilfields around the world become more mature and produce increasing quantities of water. The financial impact of handling such large volumes of water can be illustrated using a typical North Sea field of 50 wells, with each well producing 5000bbl of water per day. If the cost of treating each barrel was $0.50, the daily water handling cost for the oilfield would equate to $125k, ($45.6m per annum). The problem of a single zone producing 100% water with all other, non-communicating zones producing oil, has been identified by many in the industry as being essentially solved (∼ 80% success rate) under certain circumstances. The water-producing zone can be mechanically isolated and the relevant gel treatment injected to completely block off the zone. Although this is a relatively simple case, it still requires extremely good diagnosis of the position of the water zone and a considerable amount of mechanical intervention to isolate it. Failure to achieve either of these criteria can lead to treatment misplacement and damage to the surrounding oil producing zones. Although the problem and the treatment described above are relatively simple, the preferred option would still be through the use of a "bullhead" chemical treatment; i.e. in an application forcibly pumping treatment fluids into a formation. One such treatment might be a so-called Relatively Permeability Modifier (RPM). Treatments of this type include solutions based on water-soluble polymer systems, which may include a cross-linker. This kind of treatment is the simplest and cheapest type of water control treatment as there is no need for zone isolation. If the RPM material works correctly, then the impact on the oil zone is minimal. However, it should also be possible to clean up the treatment if the oil-producing zones are damaged.