Accurate Insitu Compositions in Petroleum Reservoirs

Abstract This paper describes experimental procedures for determining accurate estimates of original insitu reservoir oil and gas compositions. The proposed equilibrium contact mixing (ECM) method can use samples which are clearly not representative of insitu fluids (e.g. due to nearwellbore multiphase behavior, reservoir depletion, or separator sampling problems). ECM procedures are recommended for saturated, undersaturated, and depleted reservoirs. Examples are given for reservoir fluids ranging from very lean- gas/black-oil systems to highly volatile gas/oil systems. Furthermore, it is shown that the proposed ECM method can be used to obtain depth-weighted average insitu compositions in reservoirs with gravity- induced vertical compositional gradients.1 The Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state (EOS) are used in calculations, with extensive characterization of the C7+ fractions. Static PVT experiments and radial 1D/2D compositional simulations of typical fluid-sampling conditions are used to verify the proposed methods. Partly due to the success of the ECM method, the traditional definition of a "representative" sample is reconsidered, and a more general definition is recommended. The general definition ("reservoir-representative") is any sample produced from a reservoir, where the measured composition and PVT properties are of good quality. The traditional definition ("insitu representative") is a special case where the sample represents an insitu reservoir composition at a specific depth (or an average composition for a depth interval). Separator sampling of gas condensate and volatile oil reservoirs is widely used. We present an analysis of traditional separator sampling methods, potential errors in separator sampling, and a critical evaluation of the "isokinetic" sampling method. Isokinetic sampling is currently used to sample separator gas streams when separator liquid "carryover" is suspected. Problems with the isokinetic method are discussed, and we suggest field and laboratory measurements which are needed to confirm the validity of isokinetic sampling.