Simulation and Performance of Immiscible WAG Pilots in Mauddud Reservoir Using Three Phase Relative Permeability with Hysteresis
Authors: AL-Muftah Ali , BuAli Yusuf , Mahmoud Ahmed , AlGhadhban Hamed 
Published: March 2019

Abstract
The Bahrain Field, being the first oil discovery in the gulf region in 1932, is now in a mature stage of development. Crestal gas injection in the Mauddud reservoir has continued to be the strongest driving mechanism since 1938. Over the last five years, gas injection and fluid production rates have grown three folds with expanded drilling, workovers, and high volume lift activities. However, there are significant opportunities to increase oil production and optimize gas injection.
An Immiscible-Water-Alternating-Gas injection (IWAG) process was carried out on two composite samples extracted from the Mauddud reservoir of the Bahrain Field. The resulting production and pressure profiles were history matched by using hysteresis and three-phase relative permeability modeling options. Representative relative permeability and capillary pressure curves with the associated hysteresis and three- phase relative permeability parameters were obtained by history matching the experimental IWAG flood results. The history match was carried out by generating the hysteresis parameters and relative permeability curve sets. Experimental results, including two-phase water/gas flood steady state and unsteady state results, were honored to the degree possible. In both composite samples, the IWAG process showed incremental recovery compared to the base case water and gas injection cases. The incremental recovery obtained (above 10% PV) was largely due to the reduction of gas relative permeability during three-phase flow. A maximum trapped gas saturation of 23% was used to history match the core-flood results.
A sector model of the Mauddud reservoir was run using the relative permeability and hysteresis model parameters obtained from the history matching of the composite core-floods. A water and gas flood base case was run and compared to the IWAG sequence. The IWAG process showed incremental recovery compared to the base case water injection. In the up-dip pattern where the water saturation is low, IWAG recovers 3% more than base case gas injection, while gas injection recovers 5% more than the IWAG sequence in the down-dip pattern where water saturation is higher.
The objective of introducing the Immiscible Water Alternating Gas process (IWAG) in Mauddud was to reduce gas production by controlling the mobility during the three-phase flow. Incremental oil, compared with gas and water injection was also to be evaluated. Three IWAG pilots were introduced after an extensive study on optimum locations. Two inverted 5-spot patterns and one line drive pattern were selected; each pattern is around 40 acre spacing, targeting Mauddud B interval. The original Water Alternating Gas (WAG) ratio was designed to be 1:3 (Water: Gas) and the WAG period was originally designed to be from three to six months based on simulation work. WAG ratio and duration optimization were subject to performance. After one year of cyclic injection, both inverted 5-spot patterns showed lack of response to the WAG cycles. In one of the two latter patterns, the water cycles critically affected oil production. In the line drive pattern, the WAG cycles initially showed a favorable response. After one year of injection, water and gas overcame oil production, leading to higher oil decline and the termination of the pilot due to confinement and operational issues.