Study Of Horizontal Well Productivity In Low Permeability Oil Reservoir With Deformational Media

The main problem of low permeability reservoir with deformational media are as follows:(1) Unconsolidated, low compaction, and strong stress sensitivity. Conventional well-testing method based on Darcy model ignored the impact of non-Darcy flow and stress sensitivity, resulting in large deviations between interpreted and real formation parameters.(2) Relatively higher initial production capacity, with productivity diminishing fast. Therefore, it is necessary to study the stress sensitivity and seepage characteristics of low permeability reservoirs, and establish a suitable low permeability reservoir well-test interpretation and productivity evaluation method to guide the development of a comprehensive and efficient field development.Through literature research, two models considering both threshold pressure gradient and stress sensitivity effect under closed or constant pressure boundary conditions for vertical and horizontal well were established, and the influence of wellbore storage effect and skin factor were introduced. Next, the established model was numerically solved by applying multi-parameter test interpretation method based on genetic algorithms, then typical pressure and pressure differential coefficient curves were drawn, the impact of different parameters on the testing curve were also analyzed.Next, vertical well productivity was studied first, and it is found that with the increase of pressure drawdown, productivity of nonlinear plus pressure-sensitive model tends to overlap with that of threshold pressure plus pressure-sensitive model, and therefore, considering the features of high production and pressure drawdown in low permeability reservoirs at sea, the quasi-linear model can be used to describe the percolation behavior.In addition, by simplifying the 3-D problem into 2-D problem, and based on the assumption of single phase non-Darcy flow, one mathematical model of unsteady flow for horizontal well was established, and then by employing successive displacement method, one new unsteady production forecast method considering both threshold pressure gradient and stress sensitivity effect was put forward. What’s more, the impact of threshold pressure gradient, permeability modulus, effective horizontal length, net oil pay thickness and total compressibility on horizontal well productivity were studied.Based on the above research results, the low permeability reservoir well-test interpretation and productivity evaluation system software were compiled. Finally, an example confirmation of one horizontal well in Wenchang 13-2 Oilfield was given to validate the feasibility and effectiveness of this model. Relative to threshold pressure gradient model, the calculated results of threshold pressure gradient and pressure-sensitive model were more consistent with the daily oil output of that horizontal well, so this method can be applied to predict horizontal well productivity effectively in low permeability oil reservoirs of offshore oilfield.