Heavy Components Control Reservoir Fluid Behavior

Abstract

The five reservoir fluids (black oils, volatile oils, retrograde gas-condensates, wet gases, and dry gases) are defined because production of each fluid requires different engineering techniques. The fluid type must be determined very early in the life of a reservoir (often before sampling or initial production) because fluid type is the critical factor in many of the decisions that must be made about producing the fluid from the reservoir. This paper summarizes the guidelines for determining fluid type from field data. If any of these three properties fails to meet the criteria described, the test fails and a representative sample of the reservoir fluid must be examined in a laboratory to establish fluid type. It also shows the expected results of laboratory analysis of the fluids. The paper also summarizes schematically the trends of producing GOR and stock-tank liquid gravity. These fluid types are defined for engineering purposes. They should not be confused with reservoir fluid descriptions defined by legal agencies that regulate the petroleum industry. Regulatory definitions of oil, crude oil, condensate, gas, natural gas, casing-head gas, etc., are not related to these engineering definitions and, in fact, often contradict them.