The sampling of oil and gas condensate reservoirs require that representative fluid samples be removed by either surface or subsurface sampling techniques. This paper briefly reviews both of these techniques and discusses their relative merits. Several practical examples are provided that demonstrate the utility of an equation of state model to verify the quality of separator samples to be used in a recombination. In situations where free gas has been entrained with the separator samples, the equation of state model can frequently be used to synthesize an appropriate gas to be used in a recombination.
Obtaining representative reservoir fluid samples has become of increasing importance in the development and exploitation of oil and gas condensate reservoirs. This is especially true of reservoirs where extensive computer simulations are used to scope out developmental strategies or where enhanced oil recovery options are investigated, Often times these decisions are based on properties measured on relatively small fluid volumes produced from the reservoir at one point in time. Therefore it is imperative that the fluid samples used to make these decisions closely match the characteristic properties of the reservoir fluids at actual reservoir conditions.
Representative fluid samples can usually be obtained from producing reservoirs at either surface or subsurface locations. Surface samples are removed at either the separator or at the wellhead, with the associated gas and liquid subsequently recombined in proportions to represent the actual reservoir fluid. Subsurface samples are removed from within the wellbore at actual reservoir conditions using bottom hole sampling tools and techniques. The suitability of the particular sampling technique will depend on a large number of factors which may include economic considerations such as the cost of sampling and associated loss of production, the type of surface facilities that are available, the fluid volumes that will be required and the type of reservoir and fluid to be sampled.
The sampling technique employed can be of particular importance in saturated oil or gas condensate reservoirs where the possibility of entrainment of disassociated phases decreases the likelihood of obtaining a truly representative fluid. In some of these situations, various techniques can be employed to compensate for entrainment of these dissociated phases. The focus of this paper is aimed at determining the techniques used to recombine the separator samples to represent oil and gas condensate systems along with many situations where they have been depleted into the two-phase region. A brief review of those situations where bottom hole sampling will more likely provide a representative sample will also be discussed.
A thorough review of the equipment and techniques used to obtain these different types of fluid samples is outside the scope of this discussion and individuals who are interested in more extensive information on sampling procedures should refer to the cited literature (1,2,3) and information available from equipment vendors and service companies that specialize in sampling. However, a brief review of the most common sampling techniques will be useful to establish the fundamental principles that will discussed later in this paper