Potential and mechanism of improving sweep efficiency by ion-matched suspension in carbonate reservoir

Improvement of water flooding sweep efficiency can significantly enhance the development of carbonate reservoirs. In this paper, an ion-matched suspension was obtained and studied with stereoscopic microscopy and a laser particle size analyzer, based on considering the ion composition of formation water. The suspended calcium sulfate particles are in a critical saturation state, most of which are approximately 10 μm in size, and the maximum size can reach 50 μm after standing due to flocculation and aggregation. Experiments on static properties of suspension show that, due to the “retrograde solubility” of calcium sulfate and the change of hydrate, its particles disappear slowly when the temperature is less than 45 ℃ and precipitate rapidly when the temperature is 90 ℃. The concentration and particle size of calcium sulfate particles increased significantly with a change of salinity and ion composition. In the parallel core experiment with a permeability ratio of 4.7, the ion-matched suspension increased the flow distribution ratio of the lower permeability core by up to 9.8% and the ultimate recovery factor by approximately 4.3% of original oil-in-place (OOIP). The influence of temperature, pressure, and salinity, as well as the ion exchange between the injected water and the formation water, causes the saturated calcium sulfate in the suspension to periodically dissolve and precipitate, resulting in pressure fluctuations. Pressure fluctuations complicate the dissolution/precipitation characteristics of calcium sulfate and eventually lead to altering the flow distribution ratio in parallel models in the late stage of water flooding. The unstable ion-matched suspension containing saturated calcium sulfate is a new attempt based on material science to the field of petroleum production and has promising potential. It is more effective, reliable, and environmentally friendly than the uncontrolled in-situ particles produced in conventional under-saturated ion-matched water flooding.