Pressure transient analysis for wells drilled into vertical beads-on-string caves in fracture-caved carbonate reservoirs: Field cases in Shunbei Oilfield

Tarim Basin, Western China has newly discovered fracture-caved carbonate reservoirs formed by large-scale fault tectonic activity and multiple-stage karstification. The large karst caves interconnecting with tectonic fractures in series to form vertical beads-on-string structure is one of the most common patterns. Moreover, the situations of wells drilled into caves often occur during drilling. The complex geological and engineering factors cause that traditional continuum-flow models are difficult to match the recorded pressure data. To present a novel well test model to reasonably estimate reservoir parameters, this work first presents a multi-cave multi-fracture series connection physical model by simplifying complex vertical beads-on-string structure. Four kinds of mediums are considered in physical model, including large caves, small vugs, high-angle tectonic fractures and rock matrix. Thereafter, the multi-cave-multi-region model for wells drilled into vertical beads-on-string caves is developed, among of which the flow in fracture regions obeys the Darcy's law, while the flow in large caves is assumed to obey free flow. Particularly, gravity effect is considered in fracture regions because the fluids flow along vertical structure. Then we analyze the typical flow regimes and conduct sensitivity analysis on crucial parameters. Results indicate that (a) the new cave storage regimes, characterized by unit-slope pressure-derivative curve, are recognized. This may help identify large karst caves in field case analysis; (b) The typical feature of vertical beads-on-string structure on type curves is that cave storage regimes and linear flow regimes alternately appear; (c) large cave and vug can be distinguished by the slope of pressure derivative on type curves; (d) gravity effect could lead to unit-slope pressure and pressure derivative curves, which is traditional recognized as the presence of a closed boundary when it may not be the case. Finally, three field cases from Shunbei Oilfield in which it is difficult to achieve good fitting quality using traditional models are interpreted to verify the practicability and feasibility of the proposed model.