Genel Energy surprised the market this week by downgrading the reserves in its Taq Taq field in Kurdistan. 2P recoverable reserves dropped from 683mmbbls to 356 mmbbls, with the bulk of the reduction attributed to a lower than originally estimated fracture porosity in the Cretaceous Sharanish reservoir. Yet it has taken six years and 184mmbbls of production to get to this point – so how can this be and what does it tell us about the development of fractured reservoirs?
Fractured carbonate reservoirs are known to be challenging in terms of accurate recovery prediction due to their complexity. Difficult to characterise and requiring the careful application of recovery techniques, they can nevertheless achieve recoveries that compare favourably with many conventional fields. Fractured reservoirs are divided into four groups, with those in Kurdistan typically classified as Type II, that is consisting of a low porosity and permeability matrix rock providing some storage capacity and fractures (or cracks) in the rock providing fluid flow pathways. This dual porosity system means the production characteristics are different from standard reservoirs. Initially high production can decline as major fractures are drained and flow switches to the lower porosity and permeability matrix, and excessively high production rates can change the balance of recovery mechanisms that feed the matrix oil into the fractures. Fracture networks can also be connected to aquifers, which can lead to water breakthrough especially under high production rates which is what seems to have happened at Taq Taq. In order to manage this the production rate should be controlled to limit water production and studies have shown that some of the highest recoveries are achieved from reservoirs with strong bottom water drive that are managed in this way. Horizontal wells are ideally suited to this as they can be positioned further away from the water contact and be produced under lower pressure drawdowns. In addition horizontal wells have the advantage of being able to increase the intersection with the higher producing fracture zones.
To date Taq Taq has been developed with vertical wells, though it is not clear why this was selected as the original development option. In the Shiranish, the water level has been observed to be rising faster during 2015 than originally predicted and the water level is irregular as a result of coning around individual wells.To address this Genel is planning to reduce production from the wells on the crest and add horizontal wells on the flanks of the fields, and this fits with the recommended method to manage this type of reservoir.
This new dynamic data has allowed the company and its reserves auditors to refine the reservoir model and as part of this exercise it has become clear that the fracture porosity has been over estimated, leading to a substantial reduction in recoverable reserves.
While the reserves reduction is mostly from the Shiranish, it should also be noted that the reserves from the remaining Pilaspi, Kometan and Qumchanga reservoirs also dropped significantly from initial estimates, albeit from more modest starting points. This was as a result of different issues from the Shiranish, with reduced matrix contribution for the Kometan and Qumchanga together with bypassed oil in the Qumchanga.
This only serves to highlight the complexity of this type of reservoir and the need for them to be carefully managed. Despite a host of static data in the form of logs, core and geophysical data, it has taken dynamic data gathered after years of production for Genel to adapt and change. This does not however only affect Genel and Taq Taq. Other operators in Kurdistan should take note. Each fractured reservoir responds in its own distinctive way and needs informed evaluation and planning from as early as possible in order to achieve an optimal technical and economic performance.