Water invasion law in deepsea bottom-water gas reservoirs and a water invasion risk identification method

In order to explore the strategies for water control development of deepsea bottom-water gas reservoirs, this paper carried out a testing experiment of gas production profile along horizontal well and a large-scale 3D physical simulation experiment of water invasion in bottom-water gas reservoirs, based on the geological, water body and development characteristics of LS17-2 Deepwater Gas Field in the South China Sea. Then, the factors influencing bottom water coning were quantitatively analyzed. And based on this, a water invasion risk identification method suitable for the development of deepsea bottom-water gas reservoirs was developed. And the following research results were obtained. First, water coning in the development process of a bottom-water gas reservoir is influenced by reservoir heterogeneity, production system and horizontal well toe heel effect, and the influence degree of which on water invasion decreases in turn. Second, whether gas well productivity is influenced by water invasion risk is controlled by the aspect ratio (the ratio between the horizontal length of the well control area and the avoiding distance of a horizontal well) of well block. The identification limit of water invasion risk is affected by reservoir heterogeneity. And the stronger the reservoir heterogeneity is, the smaller the safety limit of aspect ratio is. Third, when the reservoir permeability difference is 1, 10, 20 and 30, the corresponding safety limit of aspect ratio is 41.18, 21.61, 12.60 and 5.31, respectively. Fourth, based on the relation curve between permeability difference and safety limit of the aspect ratio, the areal reservoir permeability difference of Well A4H is 30 and its corresponding aspect ratio is 77.20, which is much higher than the safety limit (5.31) of the aspect ratio. It is indicated that the well has high water invasion risk, so water control development is inevitable. Finally, the following strategies were proposed for the water control development of deepsea bottom-water gas reservoirs. First, improve the toe horizontal well heel effect and weaken the influence of reservoir heterogeneity so as to restrain the unbalanced coning of bottom water. And the corresponding water control measures include connected multi-stage artificial bottom hole technology and variable-density screen technology suitable for horizontal wells. Second, adjust the production system to prevent the formation of nonuniform water invasion. And the corresponding water control measure is the periodic gas production technology. Third, build up a water blocking barrier near the bottom hole to inhibit the coning of frontal water head. And the corresponding measure is the water permeable and water blocking gravel packing technology of horizontal well. Fourth, combine these measures comprehensively to form a complementary composite water control development technology for the whole life cycle of a gas reservoir. In conclusion, the research results not only can be used to judge the water invasion risk of deepsea bottom-water gas reservoirs with the reservoir permeability difference of 1-30 and the gas recovery rate of 3%, but also can provide reference for the water control development of offshore and onshore bottom-water gas reservoirs. © 2020, Natural Gas Industry Journal Agency. All right reserved.