Accurate Insitu Compositions in Petroleum Reservoirs

Abstract

Abstract This paper describes experimental procedures for determining accurate estimates of original insitu reservoir oil and gas compositions. The proposed equilibrium contact miring (ECM) method can use samples which are clearly not representative of insitu fluids (e.g. due to near wellbore 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. The Peng-Robinsen (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. Introduction Historically, the only acceptable method for determining initial reservoir compositions has been to directly obtain bottomhole or recombined separator samples which truly represent insitu compositions. Sampling procedures have been developed to assist in obtaining insitu-representative samples, but for reservoirs that are initially saturated or only slightly undersaturated, it may be impossible to obtain such samples. When flowing bottomhole pressure drops below the reservoir fluid's saturation pressure, multiphase behavior near the wellbore may result in mixtures flowing into the wellbore which are clearly not insitu representative. When reliable insitu-representative samples can not (or have not) been obtained early in the life of a reservoir, considerable uncertainty in initial hydrocarbons (oil and gas) in place may exist. One consequence is that process facilities may need to be overdesigned to account for these uncertainties. Accurate insitu-representative samples are particularly important for gas condensate reservoirs where significant income may come from processed LPGs, NOLs, and stabilized condensate. Obtaining accurate insitu oil composition early in the life of a reservoir is not usually a problem even when flowing bottomhole pressure drops below the original bubblepoint Separator samples can be recombined in a ratio (not necessarily the same as measured during sampling) that yields a bubblepoint pressure equal to the reservoir pressure at the gas-oil contact (GOC). This approach generally works well, mainly because separator gas and separator oil compositions are relatively insensitive to multiphase effects near the wellbore. A problem in many older oil reservoirs is that samples were not collected initially (e.g. many West Texas CO2 candidate reservoirs). No generally-accepted procedure has been published for determining the initial oil compositions in depleted reservoirs. Usually the only alternative is to recombine currently-producing separator oil and separator gas samples in a ratio that yields the initial reservoir bubblepoint pressure. P. 239^