(451a) Experimental Study of Retention Mechanism of Two Different Types of Scale Inhibitors in Carbonate Formations

When oil reservoirs produce water, then one of the potential production problems is the formation and deposition of mineral scales such as barium sulphate and calcium carbonate. One of the most effective measures for preventing the formation of mineral scale in the oilfield is through the application of chemical scale inhibitors (SI). These inhibitors are usually applied by injecting them into the near-well reservoir formation in a procedure referred to as a "squeeze" treatment. Once these SI chemicals are placed into the formation, they may interact with the formation minerals through mechanisms, such as adsorption or precipitation. It is these interactions that affect the SI retention within the reservoir and the subsequent release of SI in the squeeze treatment.

Since over 50% of the worldâ??s oil production takes place from carbonate formations, especially in the Middle East, understanding the chemistry behind the mode of retention of scale inhibitors within carbonate formations is an important issue. Our experimental results show that different adsorption/precipitation regimes are clearly demonstrated in carbonate mineral substrates for the scale inhibitors, DETPMP and VS-Co. Precipitation is more dominant for DETPMP/carbonate retention while pure adsorption is more dominant for VS-Co (but both adsorption and precipitation are observed for both of these SIs). The rather different behaviour of DETPMP and VS-Co may be ascribed to different functional groups having different strengths of SI â?? Ca binding. Coupled adsorption/precipitation experiments were carried out using two different size fractions of Moroccan calcite (315-500 µm and 125-250 µm) to study the effect of particle size on apparent adsorption. The reason that particle size was investigated was that the SI/Ca complex which precipitates may form a â??skinâ? on the surface, which could affect further SI-Ca interaction, as a â??surface poisoningâ?. In fact, our results using both ESEM/EDAX and direct particle size analysis (PSA) clearly show that no (or very little) surface deposition or coating around calcite grains occurred for either of these SIs. Essentially, the results for both particle sizes were the same for each of the SIs. The DETPMP retention showed coupled adsorption/precipitation behaviour which was predominantly precipitation (at [DETPMP] > 100ppm) and this was quantitatively almost the same for both calcite particle sizes. Likewise, for VS-Co the results were quantitatively almost the same for both particle sizes and retention was predominantly by adsorption up to [VS-Co] ~ 3000ppm (although precipitation at higher [VS-Co] was observed). Both sets of adsorption/precipitation experimental results for DETPMP and VS-Co on each calcite particle size fraction were modelled using a model developed previously and quantitative matches to the data were obtained. This was carried out by matching the results for one (m/V) ratio* and then predicting the results for the other (m/V) ratio (* m/V is the ratio of mass of mineral substrate to the volume of SI solution in the â??apparent adsorptionâ? experiments).

As there is some debate about the common retention mechanism of phosphonate and particularly polymeric scale inhibitors and also the effect of particle size on apparent adsorption of these SIs in carbonate systems, understanding the surface chemistry of the retention mode of scale inhibitors and skin formed by precipitation of a SI/Ca complex around the calcite grains and consequently surface poisoning, by performing static adsorption/ compatibility tests, ESEM/EDAX and Particle Size Analysis, can help production engineers to design improved inhibitor squeeze treatments. Subsequently, longer squeeze lifetimes and improved efficiency of SI deployment in carbonate reservoirs could be achieved.

Keywords: Scale Inhibitors, Squeeze Treatment, Retention, Adsorption, Precipitation