Is the word prepararei the same in Brazilian and European Portuguese?

No.

While the spelling, meaning, and grammar of "prepararei" are identical in both variants, there is a significant difference in typical usage. In Brazilian Portuguese, the futuro do presente (simple future) is considered very formal, literary, or even archaic, and is rarely used in daily conversation. Brazilians almost always replace it with the periphrastic future (vou preparar). In Continental Portuguese, however, the simple future is much more common and is a natural, standard way to express the future in both formal and informal contexts.

Brazilian Portuguese Examples

1. Eu vou preparar o jantar para a família. (I will prepare dinner for the family.)
2. Vou preparar uma surpresa para o meu namorado. (I will prepare a surprise for my boyfriend.)
3. Eu vou preparar a mala para a viagem de amanhã. (I will prepare the suitcase for tomorrow's trip.)
4. Vou preparar o café assim que eu acordar. (I will prepare the coffee as soon as I wake up.)
5. Eu vou preparar toda a papelada necessária. (I will prepare all the necessary paperwork.)

Continental Portuguese Examples

1. Prepararei o relatório para a reunião de amanhã. (I will prepare the report for tomorrow's meeting.)
2. Prepararei a mesa para o almoço de domingo. (I will prepare the table for Sunday's lunch.)
3. Prepararei tudo conforme o planeado. (I will prepare everything as planned.)
4. Prepararei o terreno para a nova construção. (I will prepare the ground for the new construction.)
5. Prepararei a apresentação com muita antecedência. (I will prepare the presentation well in advance.)