BALAKANY IX & X INTEGRATED STUDIES SHALLOW WATER GUNASHLI, CASPIAN SEA

Bakhtiyar Bagirov^1a, Elshad Aleskerov^1b, Sonya Jafarova^1c, Kevin Sylvester², Dave Waldren^3
1a SOCAR RMC (Reservoir Modelling Centre) – Senior Reservoir Engineer, Moscow Avenue, Baku, Azerbaijan
^1b SOCAR RMC (Reservoir Modelling Centre) – Senior Petrophysicist, Moscow Avenue, Baku, Azerbaijan
^1c SOCAR RMC (Reservoir Modelling Centre) – Chief Geologist, Moscow Avenue, Baku, Azerbaijan
² BP/AIOC – Development Geologist Consultant with Pinnacle Energy Limited, Guildford, Surrey, U.K.
³ BP/AIOC – Reservoir Engineer Consultant with PCT Ltd, Cobham, Surrey, U.K.

Some 100 kms east-southeast of Baku in the Caspian Sea lays a cluster of discoveries known collectively as the Azeri, Chirag and Gunashli (ACG) field. Together these are estimated to contain 5.4 billion barrels of recoverable reserves and are being developed under a 25-year PSA known as the “Contract of the Century” between an international alliance composed of 9 oil companies in the Azerbaijan International Operating Company (AIOC) and the State Oil Company of Azerbaijan Republic (SOCAR). Adjacent to and in some respects contiguous to the northwest of the ACG field is the Shallow Water Gunashli (SWG) field which is owned and operated by SOCAR and has been in production since 1980. Chirag, the first of the ACG fields to be developed, has been in production since 1997 with Azeri (West & East) 1st oil due 2005 (Phase II) and Deep Water Gunashli (DWG) 1st oil due 2007/08 (Phase III).

AIOC has a business need to understand the secondary reservoirs in ACG in order to fully optimise reservoir development plans. The adjacent and strategic location of the producing SWG field to ACG with some 223+ well penetrations provides a unique opportunity to undertake various joint subsurface studies with SOCAR in order to better understand and predict the dynamic behaviour of certain secondary reservoirs deemed materially important to Phase III development (DWG + West Chirag). Having progressed through the “Appraisal” and “Select” stage windows of pre-development, the Phase III project is currently within the “Define” stage window with project sanction expected in 3Q 2004.

In order to utilize and leverage the SWG well data on the Phase III project, BP/AIOC initially contracted the Reservoir Modelling Centre (RMC) of SOCAR and started a series of joint subsurface studies on the secondary reservoirs of Balakhany VII, VIII, IX and X in 2001-2002. Later in 2002, the third joint study, “Balakhany IX & X Integrated Studies”, was started with the primary objective to specifically evaluate the production potential for these two reservoirs as if they had been developed initially under a Phase III type scenario (i.e. 12 producers + 6 injectors), thus providing a reserves assurance and validation test to developing these same reservoirs in the adjacent DWG field.

More specifically, the “Balakhany IX & X Integrated Studies” required the preparation of static geological models for each the IX and X reservoirs and their subsequent use as input to building their respective dynamic simulation models. This was achieved through the traditional method of a “hand contoured” (deterministic) geological mapping approach versus a “computer interpolation” derived mapping style. This “hands on” geological method leads to highlighting real depositional trends and incorporates geological meaning which in the case of the Balakhany reservoir sequence mapping, presents a real and recognizable channel architecture emerging that is reflective of the palaeogeographic setting at the time of its deposition. Incorporating the historical performance data (by wells in SWG) from these Balakhany reservoirs with the geological property maps resulted in the confident build of two robust and verifiable dynamic reservoir models for history matching and prediction purposes.

Initially only 96 wells (Balakhany IX) were accessed for interpretation but later, up to 174 wells were incorporated into these Balakhany studies. The resultant net effect with increased well control was dramatic, particularly in that the Balakhany IX & X are typically lower net-to-gross reservoir systems (20-64% range) compared to the more prolific and widespread Pereriv B and D reservoirs sequences (> 75% N/G). From a suite of geological “hand contoured” maps that included gross and net sand isochores, isopachs, sandstone/shale ratio, net-to-gross and porosity maps for each reservoir sub-layer, the following conclusions and observations were noted:

1. Channel width is key to resolving a match between mapped STOIIP to observed reservoir pore volume
2. Channel positions approximate mapped faults at depth (i.e. subtle structural control to deposition?)
3. Channel “stacking” is coincident to mapped faults in “crunch zones”
4. Lateral shifting or channel switching/avulsion is evident
5. Multiple channel strike directions (NW-SE, N-S and NE-SW) are present
6. Composite channel widths range from less than 100m to 600m across and are a direct function of N/G percentages

All these observational characteristics have defined the geological framework or architecture for the Balakhany IX & X depositional sequence in SWG and what is likely to be typical in neighbouring DWG.

The Balakhany IX reservoir model (Eclipse programme) history match was very good when using the deterministic maps (based on only 96 wells), requiring only a minimum correction factor of 2 for a dynamic STOIIP match. However, when using the computer derived interpolation maps, the match was very poor, requiring a correction factor of between 4 to 5 to match the dynamic STOIIP.

For the Balakhany X reservoir model (VIP programme) history match, the well control was significantly increased to 174 wells, which in itself inevitably improved the channel margin mapping definition (using sst/sh ratio of 0.7) and geological confidence. As a result, the model history match was excellent requiring no correction factor adjustment to the measured dynamic STOIIP. This is in stark contrast to the earlier interpolation map input model of 2000 where its STOIIP value was approximately 4 times larger and hence required a significant STOIIP reduction to match actual field STOIIP.

In addition to improved property mapping, better history matching in Balakhany IX & X was in part achieved through the invention and use of a new geologically derived deterministic map referred to as, “Interface Transmissibility Map”. Basically, this map shows by way of succeeding reservoir layers, areas where vertical transmissibility (pressure and fluids are transmissible) zones or “chimneys” exist. This commonly occurs with/between vertically successive and overlapping channel sand bodies that are in vertical pressure communication (direct contact and petrophysically exceed 10% porosity). For the first time, a set of geologically derived, deterministic maps provided an approximate 3-dimensional mapped boundary limits (lateral and vertical) representation and perspective on the Balakhany IX & X reservoir channel architecture for direct input and control into the dynamic models.

In Balakhany X model, prediction runs were based on two different water flood development case plans. One case being a peripheral water flood plan and the second being a full field pattern water flood development. Model simulation resulted in the pattern flood (33.1% oil recovery) being more efficient than the peripheral flood (28.3% oil recovery).

The utilisation and leveraging of SWG well data to traditional geological mapping methods (“hand contoured”) and building dynamic reservoir models has proved to be a very reliable tool in providing reserves assurance and validation for the Balakhany (IX & X) secondary reservoirs development in DWG. Results of this work will assist in the planning and optimisation for DWG development well locations, all of which will help to deliver as efficiently and cost effectively possible, these significant secondary reservoir reserves in Phase III development.