Method of sealing an annular space in a wellbore

A method of sealing a space in a wellbore formed in an earth formation, the method comprising:

• inserting a plurality of swelleable particles into said space, the particles being susceptible to swelling upon contact with a selected fluid;
• and
• inducing said selected fluid to contact the swelleable particles whereby the swelleable particles swell so as to form a body of swollen particles in said space,
• wherein the step of inserting the particles into said space comprises pumping the swelleable particles in a stream of carrier fluid into the space;
• wherein said plurality of swelleable particles comprises at least one of a first group of particles susceptible to swelling upon contact with hydrocarbon fluid, and at least one of a second group of particles susceptible to swelling upon contact with water;
• and
• wherein said plurality of swelleable particles includes particles which are susceptible to swelling upon contact with water from the earth formation and which include a matrix material provided with a compound soluble in said formation water, wherein the matrix material substantially prevents or restricts migration of the compound out of the swelleable particle and allows migration of said formation water into the swelleable particle so as to induce swelling of the swelleable particle upon migration of said formation water into the swelleable particle.