The GOGD recovery mechanism in fractured reservoirs consists of taking advantage of gravity to develop the oil stored in the matrix through the management of the position of an oil rim within the fractures, where producers are placed. Gas oil gravity drainage is an effective method to increase ultimate recovery but it is a slow process. After 50 years into the development of a giant oil field in Oman, a new strategy for placement of additional GOGD wells is proposed, with the objective to optimize the performance of specific layers and achieve top quartile performance.
The understanding of a complex fractured field is based on a thorough data analysis phase with a focus on the total picture, the conceptual interpretation of 50 years of history, the successes and the failures, to locate the remaining oil1 . The data itself reveals the fluxes of the fluids within the reservoir and the physics of the recovery mechanisms taking place at all scales (mega-scale, meso-scale and micro-scale). The insights on reservoir plumbing have enabled the identification of in-fill opportunities through a better definition of faults & fracture maps and an updated fluid-fill concept (for matrix & fractures) explaining post-production contacts variability.
We have described a new well placement strategy to target parallel GOGD processes happening across scales, and specifically within the background fracture network confined between fault corridors and mechanical shale barriers, in what we call "matrix blocks". Conceptual box models have been constructed to prove feasibility of the new development strategy through dynamic simulation.
The infill development has been phased according to the ranked risk assessment of every specific location and the value of information it can provide for successive phases targeting smaller matrix blocks or more uncertain remaining oil.
Standard GOGD practice in intensively fractured reservoirs is to manage the final position of the fracture oil rim, gas and water contact within the fractures, through managed gas injection and potentially aquifer pump off wells. We hereby propose the infill drilling of additional shorter horizontal wells avoiding major fracture corridors, placed above shales at the bottom of geological sequences to maximise gravity head and fluid hold up at discrete matrix blocks. Such wells are not at field wide final oil rim depth and they do not intend to target matrix sweet spots but rather accelerate the recovery of unfaulted stacked units.