| Popis: |
Many oil fields in operation today have major scale control problems, and new insight and know-how in this area are important in order to maximize production at minimal operational costs. The models on mineral precipitation available today are based mainly on thermodynamic data found in the literature. The impact of scale precipitation on well performance, however, is not being accurately modeled due to lack of data on precipitation kinetics. It is therefore important to gain a proper understanding of the kinetics of scale formation and its detrimental effects on porosity and permeability in the near well bore region. A series of core flooding experiments were conducted at 80°C to investigate CaCO3 scale formation. This paper focuses mainly on the CaCO3 precipitation kinetics data obtained at various flow rates, supersaturation ratios (SR) and substrates. However, permeability and porosity alteration due to precipitation during flow are also presented and discussed. Kinetic data were derived from both continuous pH measurements and determination of the remaining calcium concentration in the effluent. The precipitation rate was very dependent on the amount of CaCO3 already precipitated. The combination of residence time and duration of the experiment determined the effluent SR. After a steady state precipitation rate was reached in a Lochaline sandstone, the SR of the effluent changed from the initial 3.00 to ~1.55 and ~1.15 for core residence times of 1 and 4.5 minutes, respectively. With initial SR = 2.00 the corresponding SRs were ~1.25 and ~1.13 for residence times of 1 and 4 minutes, respectively. A permeability-porosity profile was determined showing that the permeability changed rapidly early in the experiment and was followed by a long period with relatively little change as a function of porosity decrease. Towards the end of the experiment a dramatic decrease in permeability, typical to tube blocking test behavior, was observed. Visualization experiments using two dimensional glass porous medium models showed that when crystals had been formed by nucleation, further precipitation occurred preferably on existing crystals. |
