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Immiscible Water-Alternating-Gas (WAG) Study and a Probabilistic Approach for Forecasting Full Field Scale

Mohd. Raziff, Hikma Qadreney

Engineering Honours Degree, 2006

University of Adelaide

Executive Summary

X field is situated about 130 km from Y Crude Oil Terminal (YCOT) with an average water depth of 76 m. X structures are East West trending anticline with an area size of about 11 km by 3.5 km. The field is dissected by numerous normal faults with assumption that they are sealing. The reservoirs within the X field comprise a series of sands deposited in fluvial channel, distributary mouth bar and tidal environments.

The HQ fault block has been developed through a total of 6 wells and production commenced in March 1991. The fault block was initially developed by natural depletion. However, falling reservoir pressures and decreasing production rates led to the implementation of a peripheral water injection scheme through downdip wells A29 and A31 in 1996. A Water-Alternating-Gas (WAG) scheme was initiated in August 2002 in an attempt to improve recovery from the E12/13 and E14 reservoirs within the fault block and also to evaluate the technique as a method for future EOR on the greater X field and potentially other Malaysian fields.

The primary objective of the study was to evaluate the impact of the WAG scheme to date and for future reservoir management purposes. Hence, this project analyzed results from the optimizing WAG injection pattern and cycles for selected field. A limited number of prediction runs has been conducted as part of this study. These simulations indicate the following anticipated ultimate recovery to 1st January 2020 for the E12/13 and E14 sands in the HQ fault block.

Run Case

Cumalative Oil Produced,Np(MMSTB)

Recovery Factor(%)

Water Injection



WAG Injection



Gas Injection



In this simulation study, wells B05L, B16, and A02L were targeted as producers to capture mobilized oil from WAG process. From these initial runs it can be concluded that the WAG scheme is the best option since the WAG pilot at E12/13/14 zones in HQ block was predicted to recover 1.06 MMSTB, which is equivalent to 4% of incremental oil recovery over the base case scenario (Water Injection Case) by year 2020. This would be accomplished by injecting 4.0 MMscf/d of hydrocarbon gas alternately with water injection of 3500 stb/d with three month cycle in tandem with continuous water and gas injection in well A31L and A14L respectively.

Sufficient pilot data have been collected to enable preliminary assessment of X Field-wide WAG Implementation. The assessment approach is highly dependent on producing pilot responses from WAG injectors guided by dynamic modeling assessment. Data obtained from pilot was recorded and analyzed and later scaled up to Field-wide level using statistical method to estimate the incremental oil recovery. The incremental oil will be recovered through existing drainage points in X field. The analysis concludes that, Field-wide WAG Implementation in X field area will yield an incremental reserve of 16 MMSTB at the most likely case (P50). Based on the positive impact of WAG scheme and the potential reserves estimated, it is highly recommended that Field-wide WAG should be implemented.

Australian School of Petroleum



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