Common Misinterpretations of Gas Condensate Reservoirs

摘要

Geopressured gas reservoirs materialize as a result of a variety of factors, some of which include the rapid compaction of sand/shale sequences and the uplift of sediment layers by a salt intrusion. These gas reservoirs are commonly characterized by a significant drop in formation compressibility owing to the collapse of the rock matrix, and are notorious in the drilling and production industries for the various problems they cause. This paper reviews the retrograde gas condensate production data presented in SPE-2938 (Duggan, 1972) with modern analysis tools to show that gas condensate banking can cause classic material balance analysis to mimic the shape of a geopressured gas reservoir. The condensation of liquid hydrocarbons below the dew-point in a gas reservoir, and the onset of water influx from a leaky fault can complicate the material balance analysis of these reservoirs and may lead to engineers mistakenly classifying them for what they are not. Failure to properly classify a gas reservoir may lead to an incorrect estimation of original gas in place (OGIP). Accurate determination of initial gas in place is of utmost importance in the process of estimating gas reserves. A lower estimation of OGIP or not accounting for condensate banking in low permeability zones may lead to early abandonment of a gas well or significant loss of reserves for an operating company. Results show that proper consideration of the two-phase Z-factor in the material balance calculations calculated via the constant volume depletion test (CVD) leads to a more accurate determination of gas reserves, in addition, the change in formation compressibility is shown to be consolidated sandstone as opposed to geopressured rock matrix. This paper aims to demonstrate a simplified procedure for the flow-after-flow well test of determining the effect of condensate drop- out within the vicinity of the wellbore as the flowing bottom-hole pressure (BHP) drops below the dew-point pressure. Two-phase pseudo pressure was calculated from a constant composition expansion (CCE) using Whitson’s Method (Whitson, 1983) for condensate banking and measured gas composition. The classical Anderson “L” gas reservoir of the Mobil-David field in South Texas (Duggan, 1972) was chosen as a case study. Implementation of the proposed procedure will serve as an aid to gas reservoir engineers in properly classifying potential gas condensate reservoirs.