A detailed study of the scale inhibitor phase envelope of PPCA in the context of precipitation squeeze treatments
Farooqui, Nazia Mubeen
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Scale formation in the oilfield is considered to be one of the major problems associated with oil and gas production. This mineral scaling problem relates directly to the water produced in hydrocarbon production and the worldwide industry produces more than 10 times the volume of water than oil. Barium sulphate and calcium carbonate scales are the most common mineral scales found in oilfield operations. The formation of these scales may result in the blockage of tubulars or safety valves, the failure of ESPs (electrical submersible pumps) and in the blockage of rock pores (in the near wellbore formation), which will greatly reduce the well production. The most effective way to prevent the scale formation is to treat the near wellbore region of the producer wells with chemical scale inhibitors (SI). Chemical scale inhibitors are widely applied in the oil industry to prevent downhole scale formation in so-called squeeze treatments. A successful squeeze treatment can be defined by two attributes; one it must prevent scale crystal growth at sub-stoichiometric concentrations between 2–20 ppm and it must interact with the formation in such a way to give low concentration returns to provide longer squeeze lifetime, typically in the range of 3-12 months. Scale inhibitors are generally either phosphonate species or they are polymeric and both of these SIs can be applied as adsorption or precipitation squeeze treatments, depending on the mechanism of SI retention in the formation. Phosphino Poly-Carboxylic Acid (PPCA) is a well-known industry standard polymeric scale inhibitor which is often applied in precipitation squeeze treatments. In these, the PPCA forms a sparingly soluble complex with calcium ions, denoted PPCA_Ca. The research on PPCA precipitation processes described in this thesis aims to fully develop its potential to provide reliable and long-lived squeeze lifetimes. The objectives of this research are as follows: 1. To develop the full understanding of the Phase Envelop of PPCA. 2. To define the important role of the molecular weight (MW) and molecular weight distribution (MWD) of PPCA in the return curve in precipitation squeeze treatments. 3. To understand the dynamics of PPCA precipitation treatments in sand-pack floods carried out over a range of flow rates. 4. The development of a retention model based on the MWD of the precipitate/supernatant/stock for polymeric scale inhibitors. Static and dynamic lab tests were carried out at realistic reservoir conditions in order to understand the phase behaviour of PPCA, the solubility of the precipitated PPCA_Ca complex and why/how the precipitated polymeric species performed better than the stock. All of the parameters governing the phase behavior have been studied and reviewed as they relate to the MWD of PPCA in precipitation or phase separation squeeze treatments. These processes rely upon the interaction of the inhibitors with metal cations such as calcium (Ca2+), pH and temperature. The non-equilibrium dynamic sand-pack floods suggest that the reduced flow rate leads to higher effluent concentrations and vice-versa. The static and dynamic results from this work will be used to develop improved models of coupled adsorption/precipitation and inhibition efficiency (IE) for polymeric scale inhibitors. These models will be incorporated into future field squeeze design models for adsorption/precipitation (Γ/Π) processes using polymeric scale inhibitors such as PPCA. It is believed that these results are the most detailed to be published in the literature on the PPCA system applied as a precipitation processes and that they are of particular significance and application for all polymeric scale inhibitor precipitation squeeze treatments.