A continual conundrum for comingled production wells is back allocation, a process by which oil and gas production is apportioned to the contributing wells. The task becomes even more problematic when the wells produce from multiple zones as the production allocation needs to be extended to the individual zones. Simplistic methods (e.g. an even split factor between the wells or a split factor based on the ratio of the permeabilities or the permeability/thickness products of the producing zones, etc.) are bound to lead to erroneous results on the performance of the wells/zones, and more important, the estimation of the remaining reserves. To date, several investigators have presented their solutions on how to tackle the unwieldy problem of back allocation putting the main weight on satisfying the mass balance and honoring the (down-hole) pressure measurements. In all these efforts, the connection of the reservoir and well with the surface pipeline network is ignored. Downstream of the wellhead choke lies a whole pipe network that takes the production to the separator(s) before routing it to the process facility. Imposing a specific choke opening and separator pressure makes the subsurface system consistent with the pressure constraints and alters the wells’ IPR/VLP operating point, the multiphase flow regime in the wells and the degree to which phenomena such as condensate banking, coning, sand production, etc. appear exacerbated or mitigated.
Our work addresses the contribution of the surface network in defining the well and zone production. More precisely, the pressure constraints stemming from the surface pipe system in conjunction with the total measured production play a pivotal role in determining the wells’ production in the initial steps of back allocation. Furthermore, once the well production is derived, the back-allocated production is based on the identification of the individual zone’s IPR. The latter is generated from a number of techniques such as multi-layered testing but most commonly from short production periods when solely a specific zone is producing (a common operating practice for e.g. well test purposes, high water cut mitigation effort, well workovers, etc.). The form of the IPR used becomes generic and has a particular form which incorporates the effect of the zone permeability, skin, the fluid viscosity, etc and, hence, no additional well test interpretation is necessary. Our method employs minimal information of the production system: total (comingled) production rate and choke pressure measurements. An optimizer is employed to identify the choke opening that brings about the recorded pressures. Critical parts in the developed method are the selection of the pipe pressure drop correlation as well as the PVT characterization of the fluid (for multiphase flow). A field example is given and prediction results generated by the developed back allocation algorithm are compared against field measurements. The proposed technique is a holistic approach as it engulfs all three domains of upstream production (surface network, well and reservoir) and provides a reliable input for the remaining reserves.
Even more important, a definitive description of the system and the individual contributions and back-allocation may lead to multi-well, multi-zonal optimization for overall improvement of the system performance.