Reservoir monitoring for gas/condensates fields is often considered as challenging when the production reduces the reservoir pressure below the dew point. The induced compositional changes are usually thought to hamper the use of fingerprinting for allocation and reservoir monitoring. Similarly fluid composition from segregated grading gas condensates reservoirs could be significantly modified through production, thus generating fingerprinting changes.
In this paper we propose a methodology to model fingerprinting changes that are generated either by segregation or pressure changes. Gas condensate fluids' production allocation can be thus obtained using combined geochemical fingerprints and thermodynamic calculations. Results from cases showing compositional changes due to depletion and/or un-mixed zones show the high potential of this methodology.
Finally sampling issues are also important to assess since separator or multiphase flow meter sampler may not be available to carry out adequate sampling of the single phase and/or commingled flow. Therefore it is a prerequisite to define a sampling protocol that will not affect the condensate molecular composition so it remains possible to derive subsurface information. If such protocol is not applicable, the thermodynamic model should be used to evaluate the consequences of the phase split on the allocation results.
This methodology will not only provide a solution to allocate the production when production logging tools are too expensive or not applicable but also permit to better assess connectivity in the development of gas condensate fields.
Geochemical fingerprinting technology has been extensively used to assess connectivity issues and production allocation in oil fields 1–5. Utilizing this technology for gas fields is usually much more challenging due to the following reasons:
Compound or peak ratios used to derive a condensate fingerprint represent only a small fraction of the whole fluid that is mainly composed of gas components
Using the molecular composition of the gas fraction is not possible due to the large difference of the gas component thermodynamic properties and to the existence of non equilibrium situation for these components in many reservoirs, especially in deep offshore environment.
The 13C isotopic composition differences that could potentially be useful for the fingerprinting of gas/condensates are usually too small within a same reservoir/field. The gas isotopes are also often affected by in situ differentiation processes such as diffusion and biodegradation/methanogenesis.
Sampling (downhole or surface) and sub-sampling conditions can significantly affect most of the compounds concentration as well as the ratios, thus making the fingerprint non-representative of the whole fluid composition.
Recent attempts to using fingerprinting of gas/condensates for compartmentalization assessment6 and production allocation7 have identified pitfalls and potential way forward. The aim of this paper is to further address the issues and the solutions that will enable the help from the fingerprinting technology for gas production allocation. We will first present a case study where two reservoir fluids exhibit a retrograde condensation at reservoir conditions therefore the liquid condensate at surface condition evolves during depletion. Then we will discussed the case where significantly graded reservoir due to the presence of (nearly) critical fluid prior to production start can be addressed. The use of thermodynamic calculations then helps anticipating the changes that will affect a fluid fingerprint through the production.