Abstract: Cas injection aimns at reducing interfacial lension between oil and injected gas, forming miscible or partial misciblc, and producing in-situ remained crude oil. However, such a gas injection process tends to changing thermody namic conditions in the petroleum system. incInd-ing the systemn' s conuposilion. temperature and pressure, possibly giving rise lo precipitation of organie solid materials. This paper begins with assumptious of conventional multi composition model, supplemenls hypotheses required inn precipitation of organic solid materials, finally. de-velops a multi-conposition mathematic model with 3D and 4 Phase (oil, gas, waler and organic solids) by using Darey 's law and moles conscrva-lion theorem and a numerical model with explicit compnsition and saluration as well as implicit pressure by using finite difference metlhod.

Key words: 3D and 4 Phases, multi-composition, mathematic mortel, numcrical model, gas injection, organic solid