Effect of PVT properties variation with depth in hydrocarbon reservoirs

Gravity segregation in thick reservoirs, in particular, affects the distribution of hydrocarbon components in a fluid column. Given sufficient time, heavy molecules will migrate towards the bottom of the hydrocarbon column to form a mat of viscous oil. Compositional grading can be in the origin of miscalculation of reserves. In fields producing volatile oils with heavy ends, in particular, the oil formation volume factor (FVF) can vary significantly with depth. That can lead to overestimation or underestimation of the reserves depending upon fluid sampling depth. For that reason, subsurface sampling depth is critical and fluid sampling from different depths only can be considered as representative sampling. In this paper, assessment of compositional grading was explored using variation of reservoir fluid properties with depth. Different hydrocarbon column thickness have been tested to prove that grading will be enhanced for reservoir thickness exceeding 164 feet.