It is becoming increasingly common to commingle two or more separate gas reservoirs in a single wellbore, especially in stacked-sand environments such as those frequently found in the Asia-Pacific region. Providing certain conditions are met, the total gas initially in place (GIIP) of the commingled reservoirs can be estimated using conventional material balance techniques. However, allocation of total GIIP to individual reservoirs presents a significant challenge. This paper describes how production logging (PL) results can be used, in conjunction with suitable dynamic modeling techniques, to define GIIP on a zone-by-zone basis.

The Selective Inflow Performance (SIP) technique for estimating pressures and deliverability of individual zones using suitable PL data is well established. Combining SIP results with other well information (including initial reservoir pressures and total well production) allows allocation of production to individual zones and calculation of their associated GIIP. This is done using a commingled well model (CWM) that matches both the initial well conditions and those obtained from the SIP results at the time of the PL. The paper sets out the minimum features of such a CWM.

The ability to allocate production to an individual zone, and to estimate its associated GIIP, presents opportunities to make better reservoir management decisions than is possible when only total-well information is available. Such decisions, including the location and timing of infill or step-out drilling, or whether to target well intervention at particular zones, may have large financial repercussions.

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