A relationship has been established in the laboratory between rate of water injection into a simulated fracture-matrix type reservoir model and rate of imbibition. In a series of experiments involving linear countercurrent flow, it was discovered that the higher the water injection rate, the higher the imbibition rate and the greater the indicated ultimate oil recovery.
The model consists of a horizontal slab of Berea sandstone having approximate dimensions of 6 in. × 6 in. × 112 in. sealed on all sides except for opposite 6 in. × 11 in. ends. Plates were placed across the exposed ends with a 0.001-in. to 0.002-in. clearance. Water was injected into the simulated fractures between the plates and the exposed sand faces.
Repeated runs, made using one side of the matrix block and then the opposite side, indicate that the water injection rate (same time duration):imbibition rate correlation is well defined. Two additional factors that dictate the nature of the injection rate-imbibition response are temperature and the condition of the sand face exposed to imbibition.
The results of the experimental work suggest that, under proper conditions, increased water injection rates may lead to higher production rates and oil recoveries in production rates and oil recoveries in reservoirs having fracture-matrix type porosity.
Production of oil and gas from fractured reservoirs is common. Such reservoirs occur in various parts of the world. The host rock may be sandstone, siltstone, shale, limestone, or dolomite. Many of the reservoirs exhibit low values for permeability within the formation blocks themselves (called matrix permeability) and yield oil and gas at commercial rates due primarily to the existence of an elaborate primarily to the existence of an elaborate fracture system in the producing zone. Wells completed in low-permeability fractured reservoirs may flow initially at fairly high rates as the oil originally residing in the fractures moves rapidly to the wellbores. The movement of the oil from the matrix blocks to the fractures does not keep pace with the flow of oil from the fracture system to the wells, however, and the wells experience sharp decline in production rate. Early in the life of the pool, the wells may settle down at very low rates as the slow process of oil migration from the matrix blocks process of oil migration from the matrix blocks to the fractures and then eventually to the wells begins. The ultimate recovery from reservoirs of this type when subjected to solution gas drive is also very low.
